Experimental aspects of high energy neutrino physics

EXPERIMENTAL ASPECTS OF HIGH NEUTRINO PHYSICS

B.C. BARISH Cal~forniaInstitute of Technology, Pasadena, Calif 91125, U.S.A.

ii

NORTH-HOLLAND PUBLISHING COMPANY - AMSTERDAM

PHYSICS REPORTS (Section C of Physics Letters) 39, No.4(1978) 279-360. NORTH-HOLLAND PUBLISHING COMPANY

EXPERIMENTAL ASPECTS OF HIGH ENERGY NEUTRINO PHYSICS*

B.C. BARISH Cal)!ornia Institute of Technology, Pasadena, Cal)f 91125,

U.S.A.

Received 19 October 1977

Contents: 1. Introduction 2. Experimental methods 2.1. Beams 2.2. Fermilab 15 ft. bubble chamber 2.3. HPWF counter facility 2.4. Caltech—Fermilab counter facility 3. Charged current reactions 3.1. General phenomenology 3.2. Low energy charged current results (E~,< 10 GeV) 3.3. High energy charged current results 3.4. Experimental tests of sum rules 4 Neutral current results 4.1. Existence of neutral currents 4.2. Pure leptonic neutral current reactions 4.3. Semi-leptonic neutral current reactions — Exclusive channels

281 281 283 286 288 290 292 292 295 297 308 312 312 320

4.4. Semi-leptonic neutral current reactions—Inclusive channels 5. Direct production of new particles 5.1. Search for direct production of W-bosons by neutrinos 5.2. Search for gauge-theory type heavy leptons 5.3. Charm baryon candidate 5.4. Di-lepton events 5.5. Trimuon events 6. Future prospects 6.1. CERN—SPS facilities 6.2. CERN—SPS initial results 6.3. Fermilab facilities References

328 335 335 336 338 341 350 353 353 355 358 359

324

Abstract: Neutrino physics from the viewpoint of the high energy results from Fermilab are discussed. The high energy experiments are described and their results are discussed. The status of both the charged current and neutral current reactions are reviewed in detail. Also, the important observations of topologies indicating new particle production by neutrinos are covered. Comparisons between results of different experiments are made where relevant.

Single orders for this issue PHYSICS REPORT (Section C of PHYSICS LETTERS) 39, No. 4(1978) 279—360. Copies of this issue may be obtained at the price given below. All orders should be sent directly to the Publisher. Orders must be accompanied by check. Single issue price Dfl. 36.00, postage included.

* Work supported by the U.S. Energy Research and Development Administration. Prepared under Contract EY-76-C03-0068 for the San Francisco Operations Office.

B.C. Barish, Experimental aspects of high energy neutrino physics

281

1. Introduction Large scale neutrino experiments at high energies have now been in operation for several years at Fermilab. In constructing the experimental areas at Fermilab, special emphasis was placed on the neutrino area. There was every expectation that this would be a rich field to pursue with a new very high energy proton synchrotron. This review is written about the time the “first wave” of experiments have been completed at Fermilab. Over the next few years, the new facilities at the CERN—SPS and new and upgraded detectors and beams at Fermilab will be used for deeper and more quantitative investigations of high energy neutrino interactions. The “first wave” experiments have revealed for us a large variety of new and fundamental physics. The spectacular success of the quark-parton model in giving a qualitatively correct picture of high energy neutrino collisions, observations of the neutral current interactions, the discovery of dimuon events (probably the first direct observation of charm), and the recent observation of trimuon events represent just a few highlights. I have “keyed” this review to these and other physics results reported from the Fermilab experiments and have selectively included results from other laboratories where they are complementary and help put the physics in perspective. I have made no attempt to make a balanced review of all experimental neutrino physics.* The physics of high energy neutrino interactions can really be divided into three distinct physics areas. I have done that in sections 3, 4 and 5. In section 3, I cover the charged current reaction, which involve investigations of the underlying structure of hadrons using neutrinos as a point-like simple probe. This is quite analogous to studies of deep inelastic scattering with electrons. In section 4, I discuss the neutral current interaction. The discovery of neutral currents stands as one of the most exciting and important developments in recent times. The understanding of the detailed structure of this interaction is just beginning and to a large extent remains a challenge for the future. In section 5, I discuss direct particle production by neutrinos. Since neutrinos interact weakly, it is possible to produce many new objects efficiently in neutrino collisions (e.g. charm can be produced singly, instead of in pairs). The results of dileptons and trilepton observations are reviewed in this section. Finally, this article contains short descriptions of the primary experimental tools used in these investigations and a short review of some of the initial CERN—SPS results.

2. Experimental methods In this section I describe the Fermilab neutrino beams and detectors as they have been used to obtain the data reviewed in this report. I should point out that there are many improvements presently being made to these detectors; an improved dichromatic beam is now under construction; and an experiment designed to detect v,~—escattering is being installed. All of these represent bright prospects for the future of neutrino physics at Fermilab. In addition, the elaborate facilities just coming into operation at CERN—SPS will.. soon be making great contributions to this field. I will not describe these various developments in this section and instead will concentrate on the detectors and beam responsible for the data reviewed in this report. * For a review with emphasis on the CERN Gargamelle resultssee “Neutrino physics with Gargamelle” by P. Musset and J.P. Vialle. Physics Reports.

282

B.C. Barish, Experimental aspects of high energy neutrino physics

~—3O’

LAB ‘C---~ N

~

LAB

I

I

r°~°”~,, ~ co0~o0o0~

~

1000’

EARTH MUON ABSORBER

kI~looo~, ~Io WI

~r~~I0oo/

°TT H~I

-.

I

fl~fl~ I ~f~I ~

~

NEUTRINO ~TARGET 200 TARGET TUBE (FOCUSING DEVICE)

Fig. 2.1. Schematic view oflayout ofneutrino area at Fermilab. Protons up to 500 GeV are brought from the accelerator to the v-target. a fucusing device is inserted in the next 200 ft. followed by the decay region. Experimental stations are used at the Wonder building (part way down the shield) and Labs C, E, and the 15 ft. v-building bubble chamber at the end of the shield.

Fig. 2.1 shows a general schematic layout of the “neutrino area”. Simply, the main features are a target/focusing system region, a 375 meter decay pipe, a dirt and iron shield of 1000 meters, and four experimental buildings. Halfway down the shield (called the berm) is the Wonder building, which housed the original Caltech—Fermilab detector and now is being used for the v~,—eexperiment, Lab-E is for the new Caltech—Fermilab facility, next is the 15 ft. bubble chamber building, and finally the HPWF facility in Lab-C. In addition, the Fermilab muon beam uses the same decay pipe and runs along one side of the berm, and a hadron beam for the bubble chambers and other uses runs down the other side of the berm. -~

B.C. Barish, Experimental aspects of high energy neutrino physics

283

2.1. Beams Typically, the Fermilab accelerator produces (1—2) x 1013 protons/pulse with a 10 sec repetition rate at 400 GeV. The proton beam is extracted from the machine and is split to the three experimental areas (meson, neutrino and proton). The fraction of the beam delivered to each area can be varied. The beam can be slow resonant extracted (1 msec—1 sec) from the machine and/or fast extracted (e.g. 27 Ausec for 1 turn extraction). For neutrino running the whole spectrum of spill conditions has been used. For horn running fast extraction has been used, but it is planned to pulse the horn for 1 msec in the future. For the HPWF experiment and for Caltech—Fermilab neutral current studies, 1 msec spill has been preferred, while for normalized running in the dichromatic beam, where accurate flux monitoring is required, the long spill (-~1 sec) has been employed. In the neutrino area, the proton beam is brought into a target hall, which is an enclosure covered by an earth shield of about 30-foot thickness. Inside the target hall are a set of railroad tracks (30” gauge) which pass through the hall and into a 200-foot extension called the target tube. The hall, itself, is used for beam transport and focusing magnets as well as beam instrumentation. The secondary beams are formed in the 200-foot target tube. Beam elements are mounted on bedplates over these tracks. Each bedplate is 20’ long and the target tube can hold as many as ten bedplates. The secondary beam elements are then just mounted on these railcars and the “train” is rolled into the tube. All beamline equipment is prealigned before rolling into the tube. There are four different target loads a horn beam, a quadrupole triplet beam, an unfocused sign-selected beam, and a dichromatic beam (table 2.1). The horn beam is designed to give maximum v-flux and operates fast spill primarily for the 15’ bubble chamber. The quadrupole triplet beam is peaked at higher neutrino energies and operates either short or long spill. The signselected beam is a simple unfocused beam with dipoles to select the sign of the hadrons and thereby give a relatively pure v or ~ beam. Finally, the dichromatic beam has the lowest flux, but provides the most information about the incident neutrino or antineutrino spectrum, and can be instrumented in a rather straight-forward manner to yield absolute v-flux measurements. —.-

—

Table 2.1 Neutrino area beams Beam

Particle

Yield for 1013 incident protons (300 GeV) (examples-depending on tune)

Broad band neutrino

v

Narrow band neutrino

v

Triplet train

v + ~tt

3 x I0~° v peaked at 15 GeV -~. I interaction/30 tons oftarget 2 x l0iOt~peaked at 15GeV 1 interaction/120 tons of target 4 x l0~v peaked at 40 GeV 3 x l0~v peaked at 105 GeV I interaction/900 tons oftarget 1.6 x 108 .t peaked at 40 GeV 7 x 106 g peaked at 105 GeV —. I interaction/6000 tons of target 1.5 x l0~peaked at 40 GeV I interaction/400 tons of target

284

B.C. Barish, Experimental aspects of high energy neutrino physics

2.1.1. Broad-band beams The most intense broad-band beam is a Horn focused neutrino beam [2.1]. The focusing elements in this beam have the feature of having a large geometric acceptance and are highly achromatic focusing devices. A pulsed current goes down an inner conductor and back on an outer conductor producing a toroidal magnetic field. The horns are shaped to produce the desired focusing of hadrons of one charge. The Fermilab focusing system consists of three horns constructed as two physical units referred to as a double horn. The horns are connected in series by a high current transmission line. The horns are connected to the pulsed power supply through coaxial cables, a polarity reversal switch, and a transmission line. At present, the quarter cycle time of the power supply (115 ~isec)necessitates using short spill operation from the accelerator (20—60 /lsec). Improvements are planned to lengthen this to 1 msec which will reduce problems in the 15 ft. bubble chamber and EM! caused by residual muon background coming out of the shield. Also, the counter experiments will become compatible with horn running. The second, broad-band focusing device consists of a quadrupole triplet. This beam can run long spill and is used both for the bubble chamber and electronic experiments. The triplet is not as achromatic as the horn system and is optimized around a particular energy. For example, in fig. 2.2 the spectrum is shown for a 200 GeV focusing condition using 400 GeV incident protons. The high energy flux is quite comparable to the horn flux, but the beam is much “harder” and has considerably fewer low energy neutrinos. The third broad band system is a “sign-selected unfocused beam” [2.2]. This beam has no focusing device and therefore has lower flux than either the quadrupole or horn beam. The beam consists of a pair of dipoles used only to select the sign of the parent hadron traversing the decay region. This scheme produces a reasonably pure v beam for positive hadrons and an enhanced ~ beam when set to negative hadrons. The beam is simple to operate and has been used as a backup for the horn system. A comparison of the fluxes from all three broad band systems is shown in fig. 2.2. ‘-.~

g __________________________________________________________

tO’

I

F

I

r~.. w

8

tO

iO~

0

\HORN BEAM

r~

40

80

BARE TARGET BEAM 120 60 200 v ENERGY GeV

240

280

Fig. 2.2. Neutrino broad-band spectrum for horn, quadrupole, and bare target beams.

B.C. Barish, Experimental aspects of high energy neutrino physics

285

2.1.2. Narrow band beam The reasons for building a dichromatic neutrino beam are rather obvious. Basically, the goal is to make the incident neutrino more of an “observable” (Op, E~known), in order to facilitate analysis of observed v-interactions. The directions of incoming neutrinos are well defined from geometry for all focusing devices, but something special has to be done in order to have neutrinos of known energy and type (v or ~) incident on the apparatus. This can partially be accomplished by developing a dichromatic v-beam [2.3], which has two well defined energy bands for neutrinos or antineutrinos creating a two-fold ambiguity in E5. The principle of a dichromatic neutrino beam is by now well known. Very simply, the idea is to make a sign and momentum selection of secondary hadrons (e.g. it’s and K’s). Most of the pions and kaons decay via two body decay channels and therefore make unique energy neutrinos for a given decay angle. The original Caltech—Fermilab detector subtended 1 mrad and thereby provides an angle cut-off that selects two bands of neutrinos one from pion decay (E~-~ 0.4 E,1) and one from kaon decay (E5 0.9 EK). This concept is illustrated in fig. 2.3, in which the two body kinematics for the neutrino energy versus decay angle for 160 GeV it and K-decay are shown. In actuality, the spectra are broadened due to the finite transverse size and angular dispersion of the hadron beam in the decay pipe as well as the spread in hadron momenta accepted. The initial narrow band beam used at Fermilab was designed to maximize flux by accepting as large a momentum bite as possible and still resolve the v~and VK peaks. In order to obtain a truly dichromatic beam, a much smaller momentum bite is necessary. Now that the proton intensity is improved and detectors are larger, it is becoming possible to employ a true dichromatic beam. The CERN—SPS beam recently commissioned and the Fermilab beam, under construction, have much better energy resolution and reduced broad band background. A sample spectrum from the present narrow-band beam at Fermilab [2.4] is shown in fig. 2.4. The actual resolution is better than is indicated since the radial information of where the neutrino interacts in the apparatus is also useful. As can be seen in fig. 2.3, the correlation of E8 and O~can reduce the spread. For the CERN—SPS beam, where the apparatus subtends a much larger angle —

DICHROMATIC BEAM

~

\

(SCHEME) 2

8

NOTE: TRANSVERSE SCALE EXPANDED

—~

375 meters

~—

DECAY

\

KINEMATICS \~—..-

(mmd)

.

I meter

DECAY REGION

SHIELD.

7r—~/.Li,

E~dI6OGeV

-

50

100

E~(GeV)

:--

DETECTOR

NEUTRINOS

ACCEPTED SPECTRUM

i~v

(

150

‘\

50

100

150

E~ Fig. 2.3. Scheme for producing the dichromatic neutrino beam.

286

B.C. Barish, Experimental aspects of high energy neutrino physics IC :

I

I

I

I

=

BEAM SET FOR POSITIVE HADRONS

-

vFROMir”

-

IN DECAY PIPE 1.0:~

I—

vFROMK’ INDECAY PIPE

-

-

>

-

~/~FROM

-

.

a)

-

0.0.

-.

‘ ‘ ‘

o 0

I

I

25

50

I

75 100 E~(GeV)

I

l25

150

175

Fig. 2.4. Neutrino spectra from the dichromatic neutrino beam.

from the decay pipe, radial information will be used extensively. In fig. 2.4 the antineutrino contamination in the neutrino beam is also shown. The broad-band background comes from decays of hadrons before momentum and therefore sign-selection is made. In order to minimize this background, the distances before sign selection are kept short and the beam is pointed at an angle (—.~6 mrad) with respect to the detector. This sign selection is particularly important in neutral current studies where the type of neutrino cannot be determined, a posteriori. 2.2. Fermilab 15ft. bubble chamber The Fermilab 15 ft. chamber (fig. 2.5) has been constructed from spherical shapes in order to minimize the chamber and vacuum tank wall thickness [2.5]. The optical system is bright-field illumination with Scothlite, fisheye optics, 108°lens and 70 mm film. The piston is fiberglass (six feet in diameter) with pressure assisted lip seal rings. The large superconducting magnet gives 30 kg at the center of the chamber and has 400 megajoules stored energy. The chamber, itself, consists of a 12.5 ft. sphere plus a snout extending the length to 15 ft. at the center of the neutrino (or hadron) beam. The chamber can be filled with a useful volume of -~ 1 ton hydrogen, or 2 tons deuterium, or 20 tons of neon. So far the chamber has been used with hydrogen and neon—hydrogen mixtures for major neutrino runs at Fermilab. This bare chamber has been useful for qualitative studies of neutrino interactions at high energy,

B.C. Barish, Experimental aspects of high energy neutrino physics

--:~t.~.~

2.50 NEUTRINO HADRON~

~

-

-

-

2.5°

-

-

0

~108°

I

..

I~[ JI~ NEUTRINO & HADRON BEAM DIRECTION -

287

~

CHAMBER LIQUID ~

~_.

~

SCOTCHLITE

I

0

5FT.

Fig. 2.5. Fermilab IS It. bubble chamber.

search for jie events, etc. In addition, the chamber has been equipped with an “External Muon Identifier” [2.6] (EMI) for identifying final state muons in neutrino events. A first stage EM! has been operational for some time, and an improved version with better coverage should be available in the near future. A schematic view of the bubble chamber and EM! with a simulated interaction is shown in fig. 2.6. The EM! detector plane consists of an array of 24 multiwire proportional chambers, each with a sensitive area of 1 meter square. They are arranged three high and eight wide and cover 135°azimuthally and 45°vertically. The geometric efficiency for 300 GeV proton induced -~

288

B.C. Barish, Experimental aspects of high energy neutrino physics

EMI ,)Proportional Chambers

~+

________

//

~Vacuum Tank Bubble Chamber

Coils

Fig. 2.6. A simulated neutrino interaction, v + p -. ~ + p + ir~,in the 15 ft. bubble chamber. The g produced passes easily through the superconducting magnet coils and hits the EMI plane along the trajectory observed in the chamber. The ir~ hadron interacts in the zinc between the magnet coils, and does not appear in the region expected by extrapolating its track to the EMI. The proportional chambers are mounted directly on the vacuum tank to maximize the solid-angle coverage.

single horn running has been estimated to be 87 % for muons from charged current events. In order to use the 15 ft. bubble chamber and EMI together, it is necessary to match bubble chamber tracks to “hits” in the EM!. To accomplish this, bubble chamber tracks are extrapolated through the absorber to the proportional chamber, including effects from energy loss and the magnetic field. A circle is predicted for the particle to hit, where the size of the circle depends on measurement errors, multiple Coulomb scattering, etc. Hadrons usually undergo strong interactions in the zinc, coils, etc. and either are completely absorbed, will scatter out of the circle, or produce hadron cascades. Thus they are identifiable by the fact that they produce no particles in the circle or many particles in the region of the circle. On the other hand, muons, since they do not interact strongly, usually fall within the circle. 2.3. HPWF counter facility The HPWF (Harvard—Pennsylvania—Wisconsin—Fermilab) counter facility [2.7] has been built in Lab-C at Fermilab. A view of the original detector with a neutrino induced dimuon event superposed is shown in fig. 2.7. The apparatus is a separated function device with a target-detector for producing neutrino interactions and measuring hadronic energy, followed by a separate 11’ 5” diameter iron toroidal muon spectrometer for determining muon momenta. The target-calorimeter consists of 16 segments, each 18” thick, viewed by six 5” phototubes on each side. Four segments are bolted together and structurally held by 3” thick honeycomb panels

B.C. Barish, Experimental aspects of high energy neutrino physics

SC1 SC2 SC3 ENERGY DEPOSITION

SC4

SC5 SC6 SC7

289

SC8

~

B~E~A~ A

°5L ID

I

HADRON CALORIMETER

METERS

/

/

B C MUON SPECTROMETER

D ,uT

FILTER

Fig. 2.7. The original HPWF counter facility with a dimuon event superposed.

on the ends. A wide gap spark chamber is inserted between each package of4 segments. To optimize light collection efficiency for each segment, the inner surfaces of the containers have been coated with 0.001” thick teflon (n = 1.33). This gives a total internal reflection angle of 25° when the vessels are filled with Nuclear Enterprise (NE235A) mineral oil base liquid scintillator (n = 1.47). The HPWF target-detector is essentially pure liquid scintillator, and has the nice feature of being a totally active detector. This allows the possibility of detecting slow recoils, which can be very helpful in separating out such processes as quasi-elastic scattering, v~,+ n p + p. The main deficiency of the detector is that both the radiation length (53 cm) and absorption length (84 cm) of liquid scintillators are relatively large. This means that hadronic cascades are very penetrating in the device creating a leakage problem out the rear. This is most severe at large hadron energies and requires making a sizable correction in order to determine the hadron energy. The response of this calorimeter has been calibrated in a hadron beam and was measured to be nearly linear to 150 GeV with an r.m.s. resolution of 10—12 % from 20—150 GeV. Following the target-calorimeter is a large toroidal magnet spectrometer. A schematic view of the magnet and details of one module showing associated spark chambers, etc. is shown in fig. 2.8. Each toroid is wound with water cooled copper coils run with 67000 amp-turns. This yields a circular field about the axis of the toroid with -~ 21 kG near the center and 16.5 kG at the outer radii. The particles are tracked using wide gap spark chambers (5 cm), viewed optically and recorded on film. The r.m.s. momentum resolution, including multiple scattering errors, radiative effects, etc., varies from 10 to 15 % for muon momenta between 5 and 150 GeV/c. For recent dimuon and trimuon experiments (see sections 5.4 and 5.5) an upgraded version of the detector has been used. The new detector has three separate target-detectors and two large iron toroidal spectrometers. This detector (NEULAND) has three sections of Fe target ~ 8 g/cm3, total mass 250 metric tons), 12 sections of pure liquid scintillator calorimeter (—.~ 0.8 g/cm3, total mass 45 metric tons), and 10 sections of Fe calorimeter (-..~ 3 g/cm3, total mass 90 metric tons). Wide gap spark chambers are inserted in the liquid scintillator and iron calorimeters to provide position information. This calorimeter-target section is followed by an 8 m diameter magnetic muon spectrometer, also using wide gap optical chambers and capable of measuring the properties of muons < 600 mrad for E,~> 3 GeV. Smaller angle muons also traverse the 4 m diameter spectrometer (from the original detector) which follows this large magnet. The momentum resolution is -~ 15 % for muons of 100—200 GeV/c. Special triggers have been incorporated using —+

‘~

-

290

B.C. Barish, Experimental aspects of high energy neutrino physics

/

-“~

/

‘\~“...,

~—WIDEGAP SPARK CHAMBER 0’ X 0’ X 4’ (one of five)

MAGNETIC MUON SPECTROMETER COILS (12 per module) (0)

3/32’ GAPS

HALL PROBES

(b) Fig. 2.8. 11.5 ft. diameter toroidal magnet spectrometer used in the original HPWF detector.

various scintillator hodoscopes to enrich the sample of multimuon events, relative to ordinary neutrino interactions. 2.4. Caltech—Fermilab counterfacility The original Caltech—Fermilab apparatus was housed in the so-called “Wonder building” located about halfway down the berm and adjacent to the Muon laboratory. All the data reported in this review was taken in that location. An upgraded version of the detector is presently being installed at the end of the berm in front of the 15’ bubble chamber in Lab-E. A schematic view of the original apparatus [2.9] is shown in fig. 2.9. It is also a separated function apparatus, in that the target and muon spectrometer are separated. The target-calorimeter is a sandwich of scintillation counters between iron plates. The incoming neutrino scatters off nucleons in the plates, producing hadrons which in turn set off hadronic (and electromagnetic) cascades by subsequent interactions. The hadronic shower is contained in about 1.5 meters of Fe. The total amount of light detected by scintillation counters sampling the shower every 10 cm is approximately proportional to the hadronic energy released in the interaction. This has been checked experimentally by calibrating a calorimeter [2.10] of similar granularity in a hadron beam whose energy is known. Fig. 2.10 shows the pulse height response versus several hadron energies for this test calorimeter. The pulse height grows almost linearly with

B.C. Barish, Experimental aspects of high energy neutrino physics

291

I~ EVENTS ENERGY DEPOSITED IN CALORIMETER

SCINTILLATION

COUNTERS ,FIRED

,

II

35 30 25 20

15

10

5

II

I’

/

I1II~JL.UIIYWIII

~

STEEL TARGET (5 x 5 area 50 long)

SPARK CHAMBERS

~~

MAGNET Hadron energy 9GeV Muon energy 30GeV Fig. 2.9. The Caltech—Fermilab counter facility with a neutrino charged current event superposed (v + N

—~ g

~

0

20

40 60

80

120 140 160 180 200 220 240 ENERGY (GEV)

100

.0

4” STEEL SPACING

0 Fig. 2.10.

Energy dependence of mean pulse

I0O height and

of resolution for 4”

-

1000 Fe calorimeter.

+ hadrons).

292

B.C. Barish, Experimental aspects ot high energy neutrino physics

hadron energy and the resolution varies approximately as 1.105/,~/~. This means that the resolution improves as the fraction of the energy in the hadron system increases. The physical dimensions of the target-calorimeter is 5’ x 5’ x 48’. It is composed of 70 Fe slabs (5’ x 5’ x 4”), liquid scintillators inserted between each slab, and wire spark chambers inserted every second slab. The whole target weighs about 140 tons of which about ~ is inside a useful fiducial volume. This target-detector is followed by a relatively small, 5’ diameter x 8’ long Fe toroidal magnet for determining muon momenta. The muon energy is determined by the radial deflection in the toroidal spectrometer magnet. The momentum resolution of this spectrometer is about 21 %. The experiment is triggered in two ways: (1) a muon traverses the iron spectrometer 100 mrad); or (2) there is a significant deposition of energy into the hadron system (Eh ~ 12 GeV). The combination of the two triggers covers essentially the complete kinematics for charged current studies. Also, the hadron trigger is used for studying neutral currents beyond a minimum inelasticity. (°M

~

3. Charged current reactions 3.1. General phenomenology The most common interaction for high energy neutrinos is the charged current reactions: and ~+N—~p~+X. These reactions are analogous to the deep inelastic processes with incident electrons. v-reaction

2,v

Q

c-reaction

<

>

(~,p

hadrons

hadrons

In the laboratory frame, v

=

=

E

—

E’

=

4EE’ sin2(O/2)

N Q2

=

—q2

Virtual W-boson or photon energy or Energy transfer to nucleon system Invariant 4-momentum transfer.

In both cases, studies of the differential cross sections can be used to reveal the underlying structure of the hadrons. As is well known, the electron scattering data exhibit, at least approximately, Bjorken scaling behavior [3.1]. That is, the structure functions, which describe the

B.C. Barish, Experimental aspects of high energy neutrino physics

293

hadronic structure at the bottom vertex, are functions of the dimensionless scaling variable, x = Q2/2Mv. This scaling behavior, discovered several years ago, inspired the development of the constituent or parton models for the underlying structure of hadrons. If one makes this same scaling hypothesis for neutrino scattering, then, in the scaling limit: E 2, v ~ v/Q2 finite, the differential scattering can be written for neutrinos and 5 ~ Q as follows: antineutrinos —~

—+

d2

2MEV {(1

55,e dxdy

— y)F

2(xF~’~(x)) ±y(1 2(x)~’~ +y

G

—

y/2)xF 3(x)~}

(3.1)

it

2/2Mv, and the inelasticity y = v/E where the scaling variable x = Q 5 = (E5 E~)/E5.For electron scattering, there are two structure functions, while for neutrino scattering, the third structure function, F3(x), represents the V,A interference term and changes sign going from neutrino to antineutrino. If the above expressions are integrated over both x and y, the total cross section is obtained, —

G~MEV Jd {~F2(x)+ ~(F1(x) ±Fs(x))}.

=

(3.2)

a number-slope So, if scaling holds, the v and li cross sections grow linearly with energy E5 and the slopes of the rising cross section are determined by the integrals over the structure functions. The usual local current—current weak interaction theory predicts that neutrino—lepton cross sections at high energy rises linearly with laboratory energy. Now as we have just shown, if, in addition, the deep inelastic structure functions scale in the dimensionless scaling variable, x, the neutrino—nucleon cross section must rise linearly with laboratory energy as well. The behavior of the total neutrino (antineutrino) charged cross section, cr5(o~),on nucleons therefore provides simultaneously a directly interpretable check of both (a) weak interaction theory and (b) hadronic scaling. In the constituent models, under the assumption of scattering off point-like objects (quarks?), the cross 2a~ sections G2ME are often written in an 2alternative notation: d = [q(x) + Z~(x)(1 y) + k(x)(1 y)] (3.3) dxdy it

_____

—

,42 V u 0

dxdy

2A4L? 1ViL~,

~

=

—

[q(x) + q(x)(1

— y)

—

2

+ k(x)(1

—

y)].

(3.4)

In this case, q(x) represents the scattering off spin ~ fermions, ~(x) the scattering off spin ~ antifermions, and k(x) the scalar or non-spin ~ scattering. In terms of the other notation, k

=

F 2

— 2xF1

(3.5)

q

=

1(2xF1 + xF3)

(3.6)

q

=

~2xF1

— xF3).

(3.7)

If the scattering is off spin -j- constituents, an additional relation further simplifies the form of

294

B.C. Barish, Experimental aspects of high energy neutrino physics

the scattering. This is the Callan—Gross relation, 2xF1 = F2 in eq. (3.1), or alternately, k = 0 in eqs. (3.3) and (3.4). The resulting distributions, after making all the assumptions summarized in table 3.1, depend only on the “type” of projectile and “type” of target. In terms of the scaling variables x and y, target

~

projectile

q

_

1

(1

2

v (1

—

y)2

dxdy

=

G2ME

y)

-

2~(x)]

5 [q(x) + (1 do~ G2ME = [~(x) + (1 dxdy it

1

— y) —

y)2q(x)].

(3.8) (3.9)

Integrating over x,

do5

=

dy

5

it

doV

=

2Q]

G2ME [Q

(3.10)

+ (1

—

y)

(1

—

y)2Q].

G2MEV[Q +

(3.11)

These are the formulas for the expected y-distributions which we will compare with the data. The total cross sections are obtained by integrating over y, G2MEV~Q

S

cTV

=

~~Q]

(3.12)

+~

(3.13)

G2ME 5~

Table 3.1 Major assumptions needed in order to reach simple forms of eqs. (3.8) and (3.9) Assumptions

— Charge symmetry

q. k

weak decays

= q-

k.. .4 A2

=

2

High mass propagator

Q

Scaling

q(v, Q2) = q(x)

etc... for

Q2 k,

Callan—Gross relation

F 2

1 GeV2 4 GeV2

>

W2 Spin 1/2

Evidence (low energy)

>

=

=

0

Gargamelle v-experiments (e.g. Adler sum rule) Search for W, scaling in vN (fit to propagator ~ 25 GeV) SLAC eN good to I0% also high energy pN “- 10°/a For Q2 < 1 GeV2 large deviation Independent of W SLAC eN violations

2xF1

— known 10°/a

May be as large as 20% at small x

B.C. Barish, Experimental aspects of high energy neutrino physics

295

It is convenient to define a parameter which gives the fraction of antiquark component

~/(Q +

X

(3.14)

~)

or alternately, in the other notation the amount of V—A interference B

=

JxF3dx/5F2 dx.

(3.15)

The relation between ~ and B is =

~(1

—

B).

(3.16)

Then, in the simplest picture where we have dominant quarks in the nucleon (o ~ 0 and B 1), we expect = 2~+1 (3.17)

~

~

~

(3.18) (3.19)

2.

~ (1

— y)

I will now review the experimental situation with respect to these expectations first at low energies and then discuss in some detail the results at high energies. 3.2. Low energy charged current results (E 5 < 10 GeV) Cross sections have been obtained from the CERN—Gargamelle heavy liquid chamber using the CERN—PS wide band beam [3.2]. The total cross sections for freon (neutron/proton = 1.19) are shown in fig. 3.1. Note the approximate as predicted behavior 2 of these linear data isrise rather low for -~scaling 0.8 GeV2 at E (eq. (3.2) or (3.12) and (3.13)). The average q GeV, 2 > 1, W2 5> 4 4GeV2). so Also, most events are below scaling cuts applied at SLAC for ep scattering (q the ratio of the antineutrino to neutrino cross sections is quite close to the predictions of the simple quark-parton model (see fig. 3.2). The difference from ~ can be interpreted as representing a small antiquark component. This means that in terms of y-distributions, the picture that emerges from the CERN—Gargamelle experiment becomes —i

Simple

Actual

_~HI yEh/Ep

yEh/E

0

296

B.C. Barish, Experimental a.spect.s of high energy neutrino physics I

I

I

I

I

I

I

I

I

I

NEUTRINO AND ANTINEUTRINO TOTAL CROSS-SECTIONS

12

EVENTS E>2GeV 17001

-

-

2490v =

0.74 X 10-38 E (GeV)

/

NEUT~~4

-

-

38E

b

o-~=O.28XI0

h ~

-

ANTINEUTRINO

2I

__ -

-4

-

~ -~

-

12

14

16

18

20

E GeV Fig. 3.1. Total v. ~ycross-sections from CERN—Gargamelle experiments.

0.8

-

RATIO

~

I

0~’

I

I

I

4

I

I

I

-

10

E 1, GeV Fig. 3.2. Ratio of antineutrino/neutrino total cross sections reported from the CERN-~GargamelIeexperiment. The best fit yields = 0.39 ±0.02.

B.C. Barish, Experimental aspects of high energy neutrino physics

297

The parameter ~ = 0.06, giving °~/o~ = (2ci + 1)/(3 — 2x) = 0.39. So, there seems to be evidence for a small q q sea in addition to the “valence” quarks. In principle, the x-dependence of the two structure functions q(x) and ~(x) can be separated by comparing neutrino and antineutrino scattering. Integrating eqs. (3.8) and (3.9) over y and taking the ratio, one obtains doV(x) = ~(x) ,+ ~q(x) do’~(x) q(x) + ~(x)~

(3 20)

Therefore, the ratio ~/q versus x can be determined by measuring the ratio doV/dov as a function of the variable x. This type of analysis has been performed for the CERN—Gargamelle data [3.3] and the x-distributions of the structure functions q(x) and ~(x) determined this way are shown in fig. 3.3. All these low energy results are consistent with a picture where the scattering off a nucleon is mainly from “valence” quarks (q) distributed over a fairly wide range of x, plus a rather small “sea” of quark—antiquark pairs (~) concentrated mainly at small x. Other features, sum rules, etc., are discussed in section 3.4 after reviewing the experimental data at Fermilab energies to compare with this picture. 3.3. High energy charged current results There are many possible physics mechanisms that could significantly alter the behavior of neutrino and antineutrino scatterihg compared to the low energy results and the simple quarkparton model. One important possibility that could significantly modify the behavior at high energies would be the onset of non-scaling effects. In the scaling model treated so far, we have basically assumed the impulse approximation, while we know that at some level we cannot ignore the interaction between quarks in the target. These effects have been calculated in asymptotically free field theories and Q2 dependent non-scaling terms appear.

1.5

q,q vs x

GARGAMELLE

‘1.0

Fig. 3.3. q(x) and ~(x) versus x determined by comparing cross sections for v and V data. The solid curve is a phenomenological fit to the distributions.

versus x after

integrating over y for the Gargamelle

298

B.C. Barish, Experimental aspects of high energy neutrino physics

~Impulse approximation’

Asymptotically free field theories”

The structure function q(x) actually becomes q(x, Q2) including these corrections which have terms like 1/ln (Q2/A2). In terms of the physics, the basic effect of including these terms is to change the fraction of sea/valence. This comes about because the q ~ pairs in the sea are typically at smaller Q2 than the valence quarks. As a result, one expects ~V/~v to rise with energy (since ~ is increasing). Asymptotically, a,, = for E,, —÷ x. Also, by increasing the fraction of “sea”, the antineutrino y-distribution is predicted to become flatter (or the mean value, v, somewhat larger as E~increases). Again, asymptotically both neutrinos and antineutrinos will become do/dy c’~ 1 + (1 — y)2. Another possible effect on the charged current interaction as we study the reaction at high energies could be from new particle production, which could also modify the behavior. For example, theories have been proposed and predictions made for classes of models with V + A coupling and new quarks [3.4]. If a new doublet

(~)

exists, a new diagram for antineutrino

(but with no analog for neutrino) interactions would be the following

This new diagram has the property that it introduces a new energy-dependent flat component for antineutrinos. This, again, has the effect that both °~/o,, and v increase with E~. Qualitatively, this effect is the same as from asymptotic freedom corrections and distinguishing between them requires a quantitative study. We will discuss one such phenomenological analysis after presenting and discussing the data. 3.3.1. Previous high energy results

First, I will summarize the experimental situation at high energies prior to recent cross section results from the Caltech—Fermilab group [3.5], then I will review these new results. (Relevant results on these same questions have very recently been presented from the CERN—SPS experiments [3.6], and I briefly give these results in section 6.1.) A “so-called” y-anomaly was reported some time ago by the HPWF group [3.7] for antineutrino scattering at high energies. Actually, defining what is anomalous, coupled with apparent differences in experiments, has made the situation somewhat confusing. At least qualitatively, the experimental data have been in agree-

B.C. Barish, Experimental aspects of high energy neutrino physics I

140

-

120

-

00

-

I

299

I

E~IO-3O0eV ALL X 326 Events -B 0.8 ~J2~

40-

20

C 0

0.2

0.4

I

0.6 yr o/ E

I

0.8

Fig. 3.4. The y-distribution in the 10—30 GeV energy range for antineutrinos from an chamber (FIMS). The best value for the antiquark component is a = 0.12 ±0.05.

.0

experiment in

the Fermilab 15 ft. hydrogen bubble

ment, although the only data with enough statistical significance and energy coverage to be useful in making quantitative conclusions have been from the HPWF experiment. Basically, there is evidence for a substantial non (1 — y)2 component in antineutrino distributions at high energies. Fig. 3.4 shows the y-distributions in the 10—30 GeV range from a Fermilab 15 ft. hydrogen bubble chamber experiment [3.8] and fig. 3.5 shows the data from the HPWF group [3.9] who have presented the strongest arguments for an anomaly. One way to parameterize the non-(1 y)2 character of the antineutrino distributions is to determine the parameter c~or B of eqs. (3.14)—(3.16) for various experiments. Fig. 3.6 shows such a compilation where measured ~ or B has been plotted versus Er. It seems from these data that the character of the distributions has changed somewhat at high energies compared to CERN— Gargamelle but the quantitative energy dependence is unclear. Actually, the only points with any real statistical significance are the HPWF data, which give ~ = 0.03 ±0.03 for 10 < E 30 GeV —

and ~

=

0.28 ~

for E~>50 GeV.

Considering the apparent flattening of the antineutrino y-distributions (as predicted in several models considered above), it is of great interest to determine whether the ratio o~/o,,also rises. In the parameterization we have used, that would just be a mathematical consequence of the larger value of ~ (see eq. (3.17)). However, it should be emphasized that there is a physics assumption of “charge symmetry” in this parameterization. A priori, we cannot tell whether what has been observed implies possibility A or B below.

300

B.C. Barish, Experimental aspects of high energy neutrino physics

_

o~

(A)

(B)

In the first figure (A), charge symmetry has been assumed to be valid at small values of y (that is, d5/dyIy~ø= do/dy~~0). This would imply a growing °~/°,, ratio, while in the second figure (B) I have hypothesized an effective violation of charge symmetry or an experimental problem that has redistributed antineutrino events (but not creating excess events). IHPWF DATA (a) NEUTRINOS IO
(b) NEUTRINOS E>5OGeV

I:o~~~90.__‘.0

o0!5I,0

Ic) ANTINEUTRINOS 10 ‘~ E ~ 30 GeV -

Z150~

(d) ANTINEUTRINOS E > 50 GeV

B~’~O.95±O.I a~.03

H~I:o-~

I

BE=O.45+OI5 —0.10

75

__ —

Fig. 3.5. The “anamalous” y-distributions reported the HPWF group. Note distributions that for 10 ~ have E ~ a30substantial GeV the antineutrinodistributions 2 behavior, while for E >by50Gev the antineutrino non (1 y)2 component. are consistent within Parameterization a (1 termsy)of an antiquark component yields a = 0.28 ±0.06. (The shaded regions of the histogram are where the geometric acceptance ofthe experiment is poor and should be ignored.)

—

B.C. Barish, Experimental aspects of high energy neutrino physics

I

301

I

-GGM £-FIMS-15BC S

a 0.5

-

Q —

vs. E~,

__________

5-CALTECHFERM LAB

O.4~

-

-

a

-

+1 50

I

I

100

150

E~-GeV Fig. 3.6. A compilation of the reported values of antiquark component a versus E, from all experiments.

The experimental situation on whether charge symmetry is valid at small y has been very confused. Earlier, there had been some reports of a charge symmetry violation in this kinematic region for data from the HPWF experiment [3.10]. These results have been discussed extensively [3.11] but have never been published and I will not discuss those results here. In the next section, a rather sensitive test from the Caltech—Fermilab experiment [3.12] is discussed. It gives equal cross sections at y = 0 for neutrino and antineutrino, within errors (—‘ 5 °/j. The HPWF group [3.13] have also reported results on the ratio 0V/0i~ versus E. These data show a sharply rising ratio °~/o~ versus energy. The data taken in a broad band beam have been analyzed assuming charge symmetry to hold in a particular region [3.14] and normalized neutrino and antineutrino rates “internal” to the data in that region. These results are shown in fig. 3.7 and indicate a rapidly rising °V/°,, ratio from 20—80 GeV. The HPWF data have also been analyzed by normalizing “externally” by using calculated quadrupole beam fluxes. These calculated spectra are shown in fig. 3.8. Unfortunately, the fluxes are dropping rapidly in the region from 50—100 GeV making this method difficult. They again conclude, from their best knowledge of these fluxes, that the 5V/05 ratio demonstrates a rapid rise from 40—100 GeV confirming the other method. The only data that could be compared previously, were published cross sections [3.15] normalized to measured fluxes in the dichromatic beam. These data are shown for comparison, but it should be noted that the point at 112 GeV is based on only 11 i-events. This dramatic rise in the o~/°,, ratio reported by the HPWF group, along with the y-anomaly data have led to considerable conjecture about possible new phenomena at high energies. For example, Barnett [3.4] has done detailed calculations of the behavior, including HPWF experimental cuts, of <~> and °~/°,, under different model assumptions. Fig. 3.10 shows the results of his calculation in comparison to the data. He concludes that the four quark model alone cannot explain either the reported rise in or o~/o,,.Even including asymptotic freedom effects, he finds that the experimental effects are too large. However, if something new, like production of a b-quark at high energies is included the results could be explained.

302

B.C. Barish, Experimental aspects of high energy neutrino physics I

I

I

I

I

I

~ CROSS-SECTION RATIO, INTERNAL FLUX NORMALIZATION

IS

.4

I~-I.2-S4~-I.6 ~I4

2.2

-

Wmox 1GeV) HPWF DATA

.2

-

.1 1.0

-

I

bO.8-

-

:~

R r 0.33

0.2-

-

0

0

20

40

I 60 E GeV

I 80

I 00

I

120

Fig. 3.7. Rising cross-section ratio from HPWF experiment, using charge symmetry in the invariant mass ranges shown to obtain relative normalization.

It is, of course, extremely important to determine whether there are new quarks needed beyond the standard four quarks. The HPWF data indicated that more new quarks are needed, but the new data from the Caltech—Fermilab experiment, discussed in the next section, and the initial CERN—SPS results (section 6.1) are in conflict with this conclusion. 10-2

___________________________________________________ I

I

I

I

I

(I)

P(7r)\

Li_ a I0_

6

0

\v(ir) S

00

\ I

200

\

\l

300

E~(GeV) Fig. 3.8. Quadrupole beam spectra used for }-IPWF v,

it

cross-section ratio determination.

B.C. Barish, Experimental aspects of high energy neutrino physics

~ CERN-GARGAMELLE I ~ HPWF NEW DATA

I RATIO OF TOTAL CROSS-SECTI ONS R”o’(r)h(i’) I EXTERNAL FLUX MEASUREMENTS

0.8 i CALTECH DATA

o

O

I 20

I 40

I 60

303

I 80

I 110

I 120

E GeV Fig. 3.9. Ratio of antineutrino—neutrino total cross-sections, using external flux measurements. HPWF results use calculated quadrupole flux spectra (fig. 3.8). The previously published Caltech—Fermilab results, normalized to measured fluxes from the dichromatic beam, are also shown.

FITS BY BARNETT-~+N-ø~÷÷~J

0.5

-

b-quark mb=5 GeV

-

0.3 ~

O

0.8

-

0.2

-

~::

50

100

50

200

freedor~c~rr

E~(GeV) o 0

50

100

150

200

E1~(GeV) Fig. 3.10. Fits by Barnett to the reported <~>and o,,/c7~results from the HPWF experiment. He finds that something new like production of a b-quark is needed to quantitatively reproduce the striking energy dependence of the data.

304

B.C. Barish, Experimental aspects of high energy neutrino physics

3.3.2. Recent Caltech—Fermilab cross section results Recently, externally normalized cross section results [3.16] have been reported from the Caltech—Fermilab experiment in the energy range 45 to 200 GeV. The data were taken using the Fermilab narrow band beam. Instrumentation in that beam was used to determine absolute flux normalizations for neutrinos and antineutrinos (fig. 3.11). Ionization chambers were used to monitor the flux of secondary particles and a gas ~erenkov counter system at the end of the decay region to determine the fraction of pions, kaons, and protons in the beam. Two body kinematics, using the known lifetimes of the pion and kaon, then determine the neutrino fluxes and spectrum. Wide band sources of v(1J)-interactions occurring from decays of hadrons upstream of the decay region have been removed from the data empirically. This was accomplished by running the experiment for a fraction of the time with the momentum slit closed, allowing directly measurement of this background. The measured v-spectrum in the apparatus for a hadron beam setting of + 190 GeV is shown in fig. 3.12. The characteristic two peak spectrum resulting from the dichromatic beam is evident. The predicted spectra from the beam for v5 and VK (resulting from the decays) of pions and kaons, smeared by the experimental resolutions are also shown. Good agreement is obtained both in shape and position for these peaks. This provides a good check on both the energy calibrations and knowledge of the resolutions in the apparatus. The positions of the peaks agree to about ±2 ~ at all hadron energies. Also, in order the measure y-distributions the relative calibration of hadron energy (Eh) and muon energy (En) must be correct. This has been checked by seeing if the measured peaks from the beam agree separately with events where most energy is carried by muons (y ~ 0) or a significant amount by hadrons (y-large). In both cases, there is no shift within the statistical accuracy of the comparison. The remaining problem in determining cross sections is the separation of v~and VK events in the overlap region (90—120 GeV in fig. 3.12). This has been done statistically using the radial position of the interaction in the apparatus and the measured variables as well as the overall energy in assigning a probability for v,~or VK. A test of charge symmetry [3.12] at high energies has been reported from the Caltech—Fermilab experiment using this normalized data. From eq. (3.1), we see that 2ME JF~(x)dx. =

~j5vV~

=

(3.21)

G

Charge symmetry invariance in this region near y = 0 on an isoscalar target predicts F(x) = F~(x), implying that the cross sections are equal (o~= o~)and rise linearly with E. This small-y cross section, where little energy is transferred to the nucleon, is relatively insensitive to hadronic thresholds. Deviations from equality at fixed energy are expected to be smaller than sin2O~-~ 0.05, where 8~is the Cabibbo angle. For the analysis of the Caltech—Fermilab data [3.12], all events with y <0.2 were used, with a small correction based on the shape of the y-distribution applied to extrapolate the data to y = 0. In practice, small deviations from either equality at fixed E and/or linearity versus E might be expected due to (a) scale breaking at low or high Q2, (b) threshold effects due to charmed-particle production, (c) neutron—proton excess in the iron target, or (d) some fraction of the cross section that is energy independent (e.g., quasi-elastic). Such effects should be of order 5 % or less over the energy range of this experiment. Much larger deviations from equality, or substantial energy dependence, would indicate that important modifications to

B.C. Barish, Experimental aspects of high energy neutrino physics SECONDARY BEAM TRANSPORT

AND PRIMARY DUMP

TARGET

~\

PRIMARY

I

~

SECONDARY

~///_

NEUTRINO

,,,,,,,,,,, .~.‘~ ,.~. / ~/~/çION CHAMBERS-~”.~~~ ~“,/f,/A~,;~\’~ ~ _________________________

__________ ~11_.: ______ 0 ~ ~ CONDART~~ ~ 4—TOm $s

35Om

305

~“~‘~EARTH~’ “

‘/SHIELq,’7

-, ~

DETECTOR

;4/~ _______

_____

500m

P1+0

Fig. 3.11. Scheme for normalizing dichromatic fluxes at Fermilab. Secondary hadron intensity is monitored with ion chambers in the decay region and the fraction of it/K/p are determined in ~erenkov counter system at end of decay region. Decay kinematics of it and K then determine v-spectra in the apparatus.

the V—A structure of charge-current weak interactions are necessary. As mentioned previously, some interpretations [3.10, 3.11] of existing data from the HPWF experiment have suggested such anomalous behavior.

Fig. 3.13 shows the measured data for o0/E versus E for both neutrinos and antineutrinos. The data have been fit to the form (o0/E)5,~= a,,,v + bV,VE

(3.22)

I

800

I

I

NEUTRINOS

-

+ 600

i,,r

+190Gev SETTING

5)

0

-ç I

Z40011j > w

II

C,)

/

I

I

-

A

IOO

200 ESIIM (GeV)

300

Fig. 3.12. Observed v-total energy spectrum from + 190 GeV hadrons in the Caltech—Fermilab apparatus. The solid curves are the expected v~and VK distributions using the knowledge ofthe beam plus resolutions in the apparatus.

306

B.C. Barish, Experimental aspects of high energy neutrino physics

‘4-~!t

IO38~20.6

0.2

I

I

I

I

I

I

I

i~i

NEUTRINOS

+

ANTINEUTRINOS

I I I I I I I00 E~(GeV)

II

I

I

I

I

I

I

200

Fig. 3.13. Test ofcharge symmetry at y = 0 from the Caltech—Fermilab experiment. The inner error bars are statistical and outer error bars include systematic errors. The best two-parameter fit and an energy independent value (see text) are shown.

with E in GeV. For exact charge symmetry a,, = av and b,, = b~.In addition, linear dependence of ~ on E implies b~= b~= 0. The best fits yield (in units of 10- 38 cm2/GeV) Neutrino a = 0.771 ±0.061 b = —(0.66 ±0.65) x i0~ Antineutrino: a = 0.745 ±0.060 b = —(0.24 ±0.50) x i0~ Overall : a = 0.75 ±0.04 b = —(0.37 ±0.39) x i0~. These results are consistent with ~

=

or 0 and with no statistically significant deviation from the

expected linear behavior with energy. The best fit to the combined data for the y = 0 cross section gives 2/GeV o0/E = (0.719 ±0.035) x 10 38 cm at an average incident neutrino energy of 100 GeY. The wide angle data from the Caltech—Fermilab experiment have also been analyzed and include charged current events for O,~ 360 mrad. and E~ 2.5 GeV. A small extrapolation has been made for events beyond this cut (7 % in the worst case), and total cross sections have been obtained. The result [3.16] is shown in fig. 3.14 for neutrinos. Assuming that the dominant mechanism for deviations from simple scaling arises from the effects of a vector boson propogator, these data exclude charged bosons of mass M~ < 30 GeV. A comparison of cross sections and mean y values for neutrino and antineutrinos has also been presented for these Caltech—Fermilab data [3.17]. Measurements of neutrino cross sections and average y in the energy region between 45 and 200 GeV yield (0.60 ±0.06) x 10_38 cm2/GeV ~=0.474 ±0.015 =

with no visible dependence on energy. For antineutrinos (0.29 ±0.03) x r= 0.322 ±0.010. =

10_38

cm2/GeV

B.C. Barish, Experimental aspects of high energy neutrino physics

50

-

2) (I0~cm 00

-

0

307

Mw~

I 100

I

E~(GeV)

I 200

Fig. 3.14. Neutrino total cross-sections as a function of the incident neutrino energy from the Caltech—Fermilab experiment. The Wboson curve is drawn assuming = 0.24E.

The observed energy dependence of both the antineutrino cross section and average y appears small (‘s- 20 ±10 %) over this energy region. These results are in conflict with the striking energy dependence reported for the HPWF experiment both for and the behavior of the neutrino and antineutrino cross sections. The ratio of the average slopes of the antineutrino and neutrino cross sections is 0.48 ±0.05, where the smaller error results from partial cancellation of the scale errors. An analysis [3.18] has been performed on these data similar to the analysis of Barnett [3.4] on the HPWF results. In this case, the fit is to absolute cross sections for neutrino and antineutrino (instead of the ratio) as well as <~> for both neutrino and antineutrino. The phenomenological model incorporates both charm production and scalebreaking. Fits to the data are shown in fig. 3.15 with and without inclusion of a b-quark. In contrast to the HPWF results, these data are quite consistent with the standard GIM 4-quark model [3.19] and imply that a right handed b-quark with usual coupling strength (if it exists) has Mb > 8 GeV. The data can be fit satisfactorily with or without including scalebreaking terms, but fit better with scalebreaking. The indications for scalebreaking come from <~>~which is somewhat smaller than expected with scaling, and the value of o~/Ewhich is significantly lower than the values (o~/E -~ 0.74) reported at CERN—Gargamelle energies. Both these characteristics would have a natural explanation with scalebreaking terms. It is interesting to note that the fits to the data both with and without scalebreaking are in reasonable agreement on the amount of charm production. Charm production accounts for roughly 10~of the antineutrino cross section and 8 % of the neutrino cross-section at 175 GeV. As is discussed in section 5.4.1, combining these data with 2j~tdata from the Caltech—Fermilab experiment, we can estimate that the average branching ratio for charm decay to muons is about 100/

308

B.C. Barish, Experimental aspects of high energy neutrino physics

.8 -

0 x

.6

-

.4-

-

.2

-

ANTI-NEUTRINO NEUTRINO

VV -~-~-

‘b—Quark’ Mass O

.8

~ot

I I

I

-

.6

(IO38Cm2/GeV)

~ ‘b—Quark’ Mass 0 I

0

100

E (Ge V)

200

Fig. 3.15. and a, e(j)

—

01/E versus E data from Caltech—Fermilab experiment are shown. For comparison curves for model including p type scalebreaking are shown with and without inclusion of a b-quark.

3.4. Experimental tests of sum rules 3.4.1. Gluons The fraction of momentum carried by gluons can be determined by combining eN and vN results. In terms of the quark structure functions, the electron structure functions are written as, JF~(x)dx =Jx(~[u(x) + ü(x)] + ~[d(x) + d(x)] + ~[s(x) + ~(x)])dx

(3.23)

JF~(x)dx = Jx(~[u(x) + ~(x)] + ~[d(x) + d(x)] + ~[s(x) + ~(x)])dx

(3.24)

and

B.C. Barish, Experimental aspects of high energy neutrino physics

309

where u(x) are the number of up quarks in the proton, etc. The ~ are due to the fact that electromagnetism varies like the squares of the charges. Averaging over these two expressions, the electron structure function becomes, JF~(x)dx = j~xdx[~(u(x)+ ~(x) + d(x) + a(x)) + ~(s(x) + ~(x))].

(3.25)

Similarly, for neutrinos the scattering in terms of quark structure functions is given by the expression: JF~(x)dx = Jxdx(u(x) + ~(x) + d(x) + d(x))

(3.26)

neglecting the coupling to strange quarks. The total fraction of momentum carried by quarks is

f= 9JF~(x)dx - ~fFdx.

(3.27)

Experimentally, the integral over the electromagnetic structure functions from SLAC [3.20] and from muon scattering at Fermilab yields, JF~NdX= 0.15 ±0.01. The integral over the neutrino structure function has been reported from the CERN—Gargamelle experiment obtained from the expression 1

JF~NdX

=

3ir 4G2ME(°5+

-

0V)

(3.28)

which assumes particular relations between the structure functions (e.g., Callan—Gross relation). They obtain

J

F~dx = 0.48 ±0.05

CERN—Gargamelle.

An evaluation of this integral is also obtained from the small-y Caltech—Fermi.lab data using eq. (3.21). It should be noted that this measurement is insensitive to assumptions about the Callan— Gross relation, etc., and agrees quite well with the CERN—Gargamelle result, JF~dx = 0.45 ±0.02

Caltech—Fermilab.

310

B.C. Barish, Experimental aspects of high energy neutrino physics

From these numbers, we obtain the fraction of momentum carried by interacting quarks is about 50 %. The rest is presumably carried by the “gluons” that bind the quarks. 3.4.2. Mean square charge of constituents The sum of the slopes of the rising cross section in neutrino and antineutrino scattering from nucleons is related to the magnitude of the electromagnetic scattering from nucleons through the mean square charge of the constituents. This can be seen since summing neutrino and antineutrino cross sections yields 3~

—

~ + ~

=

0v~0r

4G~M E,,

=

J F~(x)dx.

(3.29)

From electron scattering, we have (Q +

~)

=

JF~dx

(3.30)

where is the mean square charge of the constituents in units of electric charge. Taking the ratio, we obtain = JF~dx/JF~dx ~ This ratio can be evaluated either using the total cross section values at CERN—Gargamelle or the evaluation of ~ F~dx from the Caltech—Fermilab small-y data at high energies. The Caltech—Fermilab data can be compared with the SLAC data in approximately the same range. The result of taking this ratio yields a result quite compatible with models with fractional charge constituents (including the GIM 4-quark model) which predict ~ ~ from eqs. (3.25) and (3.26). Integral charge quark models (Han—Nambu) also agree, depending on whether the energy is below or above color threshold. 3.4.3. Adler sum rule A test of the Adler sum rule [3.21] has been reported for the CERN—Gargamelle data [3.22] for E,, < 10 GeV. Perkins [3.23] has discussed this test, including the unpublished HPWF data [3.10], which indicates a violation. The sum rule states that the 1’\ lirn1t~j~—~ —-~-~)= A. it (do” do A is a small number that depends on the Cabbibo angle in SU 3 schemes. The right-hand side is identically zero for the GIM 4-quark model. 2/2ME, so that v 0 means the It is convenient plotshows the data against ofthe variable, = q product xy —÷ 0. Fig.to 3.16 the results this test for vthree bins in v-energy from the Gargamelle experiment. The data indicate a small value of A at v = 0 consistent with either SU 3 or SU4 predictions. Unpublished HPWF data discussed by Perkins [3.23] and Cline [3.10] are shown —+

B.C. Barish, Experimental aspects of high energy neutrino physics I

I

I

I

311

I

I

1.6 .4 I.2

-

CERN GGM HPWF 1A4-6 •2-4 oI5 A35~(E)GeV EGeV.

.

-

iTT[j~

j

T

t.6-I2

I 0.04 I I I O 0.02 0.06 0.08 0.1

064J

EMPIRKAL HTTO

I O.I5 vq2/2ME

I 0.2

-

-

0.25

Fig. 3.16. Test ofAdler sum rule for CERN—Gargamelle data. The data are expected to converge to a very small value as v —r 0. HPWF data, which have been discussed by Cline and Perkins, are shown also. These data indicate a possible breakdown at high energies.

5. 4 -

CONVERGENCE OF G-LS SUM RULE

32\ I

-

OO

5-

0.1

I 0.2

I 0.3

0.4

0.5

x,

GLS SUM RULE

00 E (GeV) Fig. 3.17. Test of Gross—Llewellyn-Smith sum rule from Gargamelle data. The experiment yields that the number of valence quarks is 2.8 ±0.8.

312

B.C. Barish, Experimental aspects of high energy neutrino physics

for comparison. A possible violation at high energies is indicated from this result. No other test at high energies on this sum rule has been reported. 3.4.4. Gross—Llewellyn—Smith sum rule This sum rule, again based on current algebra relations, yields the number of valence quarks in a constituent model, j’[u(x) + d(x)

—

~(x)

—

~(x)]dx

=

Nq

—

3

—

Nq~1

it 1~X[d~~N d0rN 2G2MEJ x[dx dx 0

=3. This is a very difficult sum rule to evaluate, due to the convergence problem at small x, e.g.,

1 do x dx

~

—-—-—*

asx —*0.

The convergence of the CERN—Gargamelle data is shown in fig. 3.17 along with the result which is consistent with three valence quarks. That is, Nq N4 = 2.8 ±0.8 for an overall fit. Clearly, at low energies this sum rule appears valid and indicates that there are 3 valence quarks in the nucleon. No result for this sum rule has been reported, thus far, at high energies. —

4. Neutral current results 4.1. Existence of neutral currents Theoretical motivation from predictions of various gauge theory models, in particular the Weinberg—Salam model [4.1], stimulated vigorous searches for neutral current events in neutrino reactions. Prior to that time, almost all information on neutral currents came from studies of the decays of strange particles. Strangeness changing neutral currents (AS = ±1) have been demonstrated to be extremely small. Precise upper limits or very low rates have been measured for a variety of processes that would involve AS = ±1 neutral currents. For example, K ltVV ~ 1.2 x i0-~, ~ K —+it°e Ve

pete —+ne V

<

Even the decays that have been observed are at a level where they are explainable without invoking strangeness changing neutral currents. For example, K—+ L = (10.2 ±2.3) x i0~ K1 all —*

B.C. Barish, Experimental aspects of high energy neutrino physics

313

is consistent with the lower bound from unitarity, CPT, and quantum electrodynamics combined with the measured rate for KL y~’. The other strangeness changing neutral current type reaction that has been observed is —+

K~~it~e~e 0 + =(5±2)x 106. K + —*lre Ve However, this result is quite compatible with calculations of second order electromagnetic processes. Tests for non-strangeness changing neutral currents cannot be made with comparable precision. Rather than studies of decays, the experiments involve searching for neutrino induced processes having a neutrino rather than producing a charged lepton in the final state (e.g., v + N v + X). Since the limits for strangeness changing neutral currents were so small and comparable sensitivities were not possible in neutrino experiments, searches for non-strangeness changing neutral currents were not vigorously pursued until the theoretical motivations (gauge theory models) were put forth that not only predicted neutral weak currents, but also incorporated a mechanism, or new quantum number, for suppressing strangeness changing neutral currents (i.e., CHARM). The first observations of neutral current reactions were reported by the Gargamelle collaboration [4.2] in 1973 in the inclusive reaction v,~+ N VH + X. They found neutrino-like interactions without final state muons with a rate of about five times that expected from neutron background. The same group also reported the observation of events of the pure leptonic reaction ~ +e ~ + e, which could only occur via a neutral current interaction. The existence of neutral currents was soon verified by the HPWF group [4.3] and the Caltech— Fermilab group [4.4] for the inelastic channel v + N “v” + N at high energies. It was difficult to make an absolute convincing case for neutral currents from any single experiment because of the experimental problem that for neutral current reactions both the initial and final state neutrinos are non-observables. This means that the reaction is under-constrained and alternate mechanisms for the observations are possible. Rather than actually “seeing” neutral current events, other explanations for apparent events of this topology must be explained away. For this reason, it was vital to look for neutral currents in a variety of experiments and reactions with different background problems. At low energies (Gargamelle), the main experimental problem was from neutron interactions and uncertainties in the magnitude of this background, while at high energies (Fermilab) for both the HPWF and Caltech—Fermilab experiments, the prime difficulty came from not identifying the muon in the ordinary charged current inclusive reaction v + N p + hadrons. I will now review the evidence for existence at high energies from the Caltech—Fermilab experiment. The technique and results from the HPWF experiment are similar, except for details of muon identification and the fact that the experiment was performed in a broad-band neutrino beam. The process searched for was v~+ N v~+ X. Neutral current candidates are events where no muon has been identified in the final state. Figure 4.1 shows one such event in the Caltech—Fermilab detector. An interaction occurs well inside the Fe target and no identifiable muon is observed. This particular event has a measured hadron energy, Ehad = 18.4 GeV, while the most penetrating particle in the event only traverses 70 cm of Fe from the interaction point. In order for a muon to be present, it must have E~< 1 GeV or ~ 400 mrad. —+

—+

—~

—*

—~

—+

-

3I 4

B.C. Barish, Experimental aspects of high energy neutrino physics Neutral Current Interaction v

+

N

-~

v

+

hadrons NEUTRAL CURRENT EVENT

ENERGY DEPOSITED IN CALORIMETER 30 25 III20

I’

0

~

/ /FIRED

fi

‘~f l

~

STEEL TARGET (5’

v

IS

SCINTILLATION COUNTERS

BEAM

X

51

~

area,

501

long)

SPARK CHAMBERS

~

~

MAGNET

Edron

energy I8.41 Fig. 4.1. Neutral current candidate from Caltech--Fermilab experiment.

Unfortunately, this single event cannot unambiguously be identified as a neutral current process. All that has been observed is a neutrino-like interaction with no identifiable muon in the final state. Various backgrounds also satisfy these conditions (e.g., neutron interactions, charged current interactions with a very wide angle or slow muon, etc.). For this reason, one must compare the number and characteristics of such observed events with expectations from the various background sources. For performing the neutral current search, a short spill (‘-..~ 1 msec) was used in order to minimize cosmic ray backgrounds. The trigger did not require the presence of a muon, only that there was a significant local energy deposition in the calorimeter target. For the original search, a minimum energy of Ehad ~ 3 GeV was required and full efficiency was reached for Ehad ~ 6 GeV. (This biases the data against low inelasticity and the consequences of this will be discussed later.) Ignoring 2p and 3p events, the observed topologies fall into four types: Type 1: Charged current events with the muon through the magnet (e.g., v,, + N —s p + X). -

haclrons

Muon spectrometer

B.C. Barish, Experimental aspects of high energy neutrino physics

315

For this case O!L~ P,~ and Ehad are measured and from this the physics variables x, y and E~ are determined. Type 2: Charged current events with the muon identified by penetration (L ~ 1.4 meters of Fe), but missing the magnet.

L ~L

H

H~

hadrons

Muon spectrometer

For this case the p is identified, and O,~and Ehad are measured. Relying on information about the incident neutrino spectrum, these data are also used to study the physics variables and cross sections for charged currents. Type 3: Charged current events with the muon either of too low energy or too wide angle to be identified.

H

L-.j /P

_

Muon spectrometer

For this case, L 1.4 meters and only Ehad is measured for the event. It is uncertain whether a muon is present, therefore these events are “neutral current fakes” and must be subtracted from the neutral current signal. Type 4: Neutral current events.

~ L

hadrons

For thise case, L

Muon spectrometer

1.0 meters (the length of the hadron cascade) and only Eh5d is measured for the event. The typical length of the hadron cascade (the depth of the most penetrating particle) is shown in fig. 4.2. This length is only a slow function of the energy, Ehad. 2 and v The acceptance is illustrated below.for the various type charged current events in terms of the variables Q

316

B.C. Barish, Experimental aspects of high energy neutrino physics

~ /

Type 2 (~harged current events

Type 3

—Neutriil current fakes Q

0 I

Miss magnet

Type I

Thru ~Prnax~Ev

As is discussed in the section on charged currents, at least approximately, the inelasticity distributions (y = Ehad/E~)for neutrino and antineutrino can be described by the expressions dN/dy dN/dy

‘-~

const (1

—

(for neutrinos) 2 (for antineutrinos). y)

(4.1) (4.2)

This means that there are less events at large y (or large O,~,or large v) for antineutrinos and therefore less charged current fakes to subtract from the neutral current signal. Figure 4.3 shows the measured O~distribution for type 1 and type 2 events where the muon is identified. The fit to the neutrino On-distribution for flat or (1 y)2 distributions are also shown. Estimating the charged current fakes essentially means extrapolating this fit to larger O~,where the muon is not identified. In order to display all events (types 1—4) on the same graph, length distributions, instead of —

MEASURED (n) vs DEPTH 00

~

50GeV PROTONS

TYPICAL HADRON

010

PENETRATION-

0

0.5

1.0

DEPTH-Fe (meters) Fig. 4.2. Typical length of hadron cascade in a Fe calorimeter.

B.C. Barish, Experimental aspects of high energy neutrino physics

317

DISTRIBUTION IN 9

100 ,—RELATIVE EFFICIENCY C,,

I

~ / ~5O~

00

2

fly) (I— y)

~

\~,—f(y)’CONST.

00

200

—

ê(mrad) Fig. 4.3. 8,, distribution for neutrino initiated charged current events from the Caltech—Fermilab experiment.

0-distribution, have been presented. Figure 4.4 shows data where the distribution of the material traversed by the most penetrating particle for both neutrinos and antineutrinos are shown. Note from the scale at the top of the figure that, if a muon is present in the final state, the penetration is inversely related to the muon angle. There is not, however, a one-to-one correspondence because of the finite transverse size of the target and neutrino beam. The top scale represents the mean muon angle corresponding to a given penetration before escape out of the sides. The most striking feature of both graphs is the large peaks for P -~ 10 collision lengths 1 meter of Fe). These peaks for both v and 13 have been interpreted as a neutral current signal. The peak is typical of the distribution of the most penetrating particle expected if only hadrons exist in the final state. The best estimate of the number of charged current events (v(15) + N —s ~ + hadrons) under this peak, determined by extrapolating the charged current data, is also shown in fig. 4.4. This background represents scattering at such large angles that the muon does not penetrate much matter before escaping out the sides of the apparatus. The background curves are drawn using a simple fit to the charged current distributions P > 13 CL (collision lengths: 1 CL = 10 cm of Fe) and extrapolating to large angles (or short penetrations). The signal far exceeds this estimated charged current background both for neutrinos and antineutrinos. Since a narrow-band beam was used for the experiment, it is possible to perform another test of whether an anomalous (or mis-estimated) number of wide angle muons from the charged current reaction could account for the results. The mean energy of neutrinos from it-decay striking the apparatus was —~ 40 GeV. Kinematically, as the muon from charged current interactions goes to larger angles more and more of the neutrino energy must be carried away by the hadrons. Conservation of energy and momentum requires the hadrons to carry most of the energy. (—j

0 M xy sin2-~= 1 ~

—

(y is large).

(4.3)

318

B.C. Barish, Experimental aspects of high energy neutrino physics 9/.L)

<8~>(MILLIRAD) 300 200

100 I

I

(

300 200

75 I

160 —

(MILLIRAD)

I

40

-

160 -

-

120

-

PENETRATION 998 v EVENTS

-

(b)

-

V)

I

—

z

2

2

o

(/~Ioo-

~

75

I

(a) U,

00

I

140

-

20

-

PENETRATION 646 ~ EVENTS

-

IT~%3::::~:~ -

-

~I0O-

-

00020304050

PENETRATION, P (COLLISION LENGTHS OF STEEL)

PENETRATION, P

(COLLISION LENGTHS OF STEEL)

Fig. 4.4. Penetration curves for the most penetrating particle in neutrino and antineutrino collisions.

Therefore, from kinematics O~~_Li~g~ (Eg~~smalI)

Figure 4.5a shows the distribution in hadron energy for all charged current events with an identified muon (P> 14 CL). Note the distribution has a significant number of events with little hadron energy (low inelasticity). In contrast, fig. 4.5b shows the hadron energy distribution for charged current events with 18 CL P 26 CL characteristic of <0~,,> -~ 200 mrad. For these data, the hadrons tend to carry away a larger fraction of the neutrino energy. The solid line is the predicted shape. For charged current events giving P < 14 CL, is so large that Ehad -~ E,, as reflected in the solid curve in fig. 4.5c. The normalization is the calculated level of charged current background using the fits described above. Thus, for the charged current backgrounds, the hadrons carry

B.C. Barish, Experimental aspects of high energy neutrino physics

Ia)

1~fll

so

319

Pa

~

4 Ci

40

> ,~

~IO ~ Iz >

—r (b) I8EP26CL

71 ~

CHARGEDCARLO MONTE CURRENT

C’

Ic) 40-

PeI3C~L CHARGED I

20

-

C

0

~~__+MONTE

20

40

CURRENT

CARLO

60

ICC)

80

HADRON ENERGY, E,. 1GeV)

Fig. 4.5. Hadron energy distributions for (a) all charged current events, (b) wide angle or short penetration charged current events, and (c) events with no penetrating particle.

most of the energy of the incoming neutrino and the distribution of hadronic energies must reflect the beam energy ( 40 GeV). Figure 4.5c shows that for the neutral current candidates, the energy of the hadronic shower is often rather small and similar to charged current hadron energy distribution, indicating that the hadrons often carry only a fraction of the energy of the incoming neutrino (i.e., Eh = E~m E~ out). This, of course, is not consistent with events coming from a larger charged current background. In fact, this result should be viewed as evidence, at least statistically, for missing energy in the final state (presumably carried off by a neutrino). Other backgrounds, such as cosmic rays, Ve events, etc., have been checked in detail and cannot explain the signal. The experimentally observed rates for neutral currents in this experiment were approximately —.‘

—

R~(~) -~ 0.22,

R~(~)

-~

0.33

for Eh> 6 GeV

where NC neutral current and CC charged current. It should be emphasized that these represent only partial cross-sections since a minimum observable energy in hadrons (minimum y) is needed to even observe neutral currents. To obtain the ratio of total cross-sections ONC/occ, the neutral current hadron energy distribution must be extrapolated to zero hadron energy. This extrapolation and a quantitative discussion of~neutral current parameters are discussed in sections 4.2 and 4.3.

320

B.C. Barish, Experimental aspects of high energy neutrino physics

4.2. Pure leptonic neutral current reactions Once the existence of neutral currents was established, the main task experimentally has been to obtain information on the space-time structure, isospin structure, etc. of the neutral current interaction. This is a long difficult program but some progress has already been made. The pure leptonic reactions are the most ideal to study because of the ease of interpretation, but, unfortunately, they are the most difficult to measure experimentally because of the small crosssections. The semi-leptonic reactions are more model-dependent to interpret, due to the presence of hadrons but more accessible experimentally. For the pure leptonic interactions, the general form for the Lagrangian can be written as [(~~(1 + ys)vIL)(~yp(Cv+ CA y5)e)]

=

(4.4)

then 2m ~ 2Git

do” dy

[

A + B(1

dy

y)2 + Cm Cyl

L

(4.5)

E~j

2G~m~ Er[B + A(1 it L

=

—

—

y)2 + Cmey

E 6

where A=(CA~CV),

if

Eva

me 2mE[ 2G

C=(~4~)

(4.7)

>~

2G2m E-[

e v [B

=

4

B=(Cv2CA),

B1

+

Al

-~-j

~ o~/a~ ~ 3.

(4.8) (4.9) (4.10)

Within the gauge theories these cross-sections have contributions from the ordinary V—A charged current interaction and/or the neutral current interaction. In the Weinberg—Salam model [4.1], the neutral current J~ is a mixture of the third isospin component of the V—A current J~ and the electromagnetic current: =

J~ 2sin2OWJ~M. —

The only free parameter in this model is the mixing angle sin20~.This one parameter, then, determines the magnitudes and distributions for all four possible pure leptonic reactions. The following table gives the expected value of C~and CA in the Weinberg—Salam model and for the standard V—A charged currents alone.

B.C. Barish, Experimental aspects of high energy neutrino physics

321

Table 4.1 Cv and CA for pure leptonic neutral current channels Reaction

v—A model

W.S. model CA

(1) ~‘e+C—’v~+C (2) V~+ ~ —eV,, + C (3) v,, + e v,, + e (4) V,, + C —“6,, + C

1/2 1/2 — 1/2 —1/2

2O~ +2sin + 2sin28~ + 2sin28~ + 2sin28~

1/2 —1/2 — 1/2 1/2

C~ 1 1 0 0

CA +1 —1 0 0

Reactions (1) and (2) have both charged current and neutral current contributions. The mere existence of these reactions does not imply neutral currents, however the rates are dependent on the coupling. Reaction (1) has not been observed. However, a major project to study this reaction in the energy range 15—50 MeV is planned for the Los Alamos Meson Facility. The observation of reaction (2) has recently been reported [4.5] from an experiment conducted near th~Savannah River reactor. The first objective of this difficult experiment was to “see” a reactor-associated signal, and second, to determine that they have actually observed reaction (2). The table below shows the severe background problems and the difficulty in observing a signal from the reactor. Yet they appear to have a definite positive signal. Energy 1MeV)

Reactor ON

Reactor OFF

Reactor associated

Standard deviations from zero

1.5—3.0 3.0—4.5

45.1 ± 1.0 2.4 ±0.19

39.2 ±0.9 1.2 ±0.14

5.9 ±1.4 1.2 ±0.25

4.1 4.8

The main backgrounds from the reactor that could simulate reaction (2) are: 1. Ve + ~ .- n + e~, 2. neutrons from reactor, 3. gammas from reactor (ye

—s

ye).

These backgrounds have been studies in some detail and the authors conclude that they indeed are observing reaction (2). The results in two different energy bands are: o experimental = (0.87 ±0.25) ~ (1.5
=

322

B.C. Barish, Experimental aspects of high energy neutrino physics -CA

2.5 SAVANNAH

R)VER

2.0-

.5 V-A

REACTOR

.5-3MeV

.0

-1.0

-0.5

0.5

28~ x~sin 1.0

~Oee 0.5

0 /,/

.0

.5

2.0

2.5

C

V

Fig. 4.6. CA versus C~for 6~e scattering results.

Reactions (3) and (4) are not allowed via the ordinary V—A charged current reaction. Among the early evidence for neutral currents was the observation of an event apparently due to reaction (4) in Gargamelle [4.6]. That experiment has now been completed and a spark chamber experiment by the Aachen—Padova group has also observed these reactions. For the Gargamelle experiment they have reported VIL+e—SVP+e VIL + e —5 V~+

3events

e

0 events

for the CERN neutrino (antineutrino) beam with — 1.5 GeV. To unambiguously select good events several cuts have been made to the data. (1) Only a negative electron is observed in the event. (2) The electron is at a small angle, characteristic of v—c scattering (0e < 5°). (3) 0.3
=

(i.o ±1.3) x

1042

cm2

B.C. Barish. Experimental aspects of high energy neutrino physics

323

GARGAMELLE

c

2

sin

1.0-

.

-

fC~,,+e—ø.~+5

w~+e—...e~,,+e ~

0.35

2

—I

-

0.I
Fig. 4.7. Allowed contours for CA versus Cv for Gargamelle v,, e results and ~s,, e results. These are determined using the 900/ confidence limit values for both v,, e and 6,, e.

and 0cc

~ 2.6 x 1042 cm2 E~ (90% confidence limit).

The measurement for 17~—eand the limit for V~—ecan be used to determine the parameters Cv and CA. Using the 90 °/~confidence limits for these values, the resulting contours for C~and CA are shown in fig. 4.7 along with a comparison with the Weinberg—Salam model. The Aachen—Padova experiment [4.7] was constructed to make statistically better studies of the same reactions at the CERN P.S. The experiment was a large spark chamber experiment with 1 cm Al plates with a total of 30 tons of detector. The spark chambers were triggered every machine pulse, so no special trigger requirements were used. The background problems are similar to Gargamelle with the exception that this detector had no magnetic field and therefore the discrimination between e4 and e was not possible. An additional criterion on selecting events in this experiment was that the kinematical condition 0~<2me/Ee be satisfied, making allowance for measurement errors. Altogether, they collected 19 17~eevents (estimated background, 2.9 events) and 25 v~eevents (estimated background, 11.8 events). After background subtractions have been performed cross-sections are obtained for both v~and 13~ reactions: aRe =

Ove

=

(5.4 ±1.7) x 10

42

cm2 E

6 2E (2.4 ±1.2) x 1042 cm 6

(0.2
There is somewhat of a discrepancy between the Aachen—Padova and Gargamelle 1~eresults. The difference is about 2o and might be statistical or might be due to either an underestimate of background in the Aachen—Padova experiment or too tight criteria (losing efficiency) in the Gargamelle experiment. The Aachen—Padova results20~.The are alsoresulting consistent with the modelin yielding contours for Weinberg—Salam Cv and CA are shown fig. 4.8. a somewhat larger value for sin

B.C. Barish, Experimental aspects of high energy neutrino physics

324

2 G~ 1.0 -

sin

C,~

AACHEN-PADOVA

-1.5

P 1~-*-e—..5,,+e

26~<0.7 0.4< sIn

Fig. 4.8. CA versus C~for Aachen—Padova v,~e andy,, e results.

One interesting result of this experiment is that the ratio of cross sections, Rye

=

Ov

e/0ve

=

0.44 ±0.26.

This is the first measurement where an 1~cross-section is found to be larger than the v cross-section. This may, at first, seem surprising but, in fact, is what is expected in the Weinberg—Salam model for a typical sin20~-~ 0.3—0.4. For that model, as can be seen by using the values of Cv and CA from table 4.1 in equations (4.8) and (4.9), C~= 0 if sin2O~= 0.25 and 0y = o~.For sin2O~> 0.25, 0~ > 0~. (In contrast, for semileptonic reactions, we will see that cr~= °~for sin 2O~= 0.5. So, for typical measured values like sin2O~ 0.3—0.4 one expects o~>o~for pure leptonic reactions and ov < 0~ for the semileptonic reactions.) Overall, the data on pure leptonic channels is not yet quantitative enough to make strong quantitative statements. However, the data is consistent with a (V,A) theory, indicates parity violation (since o~ op), and gives an average value [4.7] (although there is great scatter in the data) for = 0.38 ±0.06 which agrees quite well with results from semi-leptonic reactions. 4.3. Semi-leptonic neutral current reactions

—

Exclusive channels

4.3.1. The elastic scattering reactions + p V~+ p

—s

V~+ ~ 13~, + p

have now been observed at BNL by two experiments [4.8, 4.9] working in the same beam. The Brookhaven beam is a broad-band horn focused beam. The peak of the spectrum is -~ 1 GeV. The Harvard—Pennsylvania—Wisconsin (HPW) detector consists of a totally active (liquid scintillator) segmented target which enables good background separation. The total weight is 30 tons and drift chambers are used for position detection. Events with one charged track originating within the detector are identified. The angle and energy of the proton are measured, thus giving good event selection. Also, an active shield is used around the apparatus. The results of this experiment, along with comparisons of the charged current quasi-elastic

325

B.C. Barish, Experimental aspects of high energy neutrino physics

channels are shown in figs. 4.9 and 4.10. In total they have observed: Reaction

Events

Estimated background

v,,+p—ev,,+p

30 22

7 8

V,,+p—eV,,+p

The various backgrounds that could simulate these neutral current channels include neutron induced events, single pion production with one hadron of very low energy, etc. Another potential problem for the antineutrino measurement comes from neutrino contamination in the beam. Since the 17 (or v) is a non-observable, one cannot distinguish in the apparatus whether an event originates from a neutrino or antineutrino. A 30 % contamination of v’s in the 13 beam would account for the entire 13-signal. The authors have concluded that this background is not that large and place an upper limit of 10 % on the v-contamination. This was done by looking at the charged current events (e.g. i~p-.-. p~n).Unfortunately there is no magnetic field on the detector to identify 2 distribution they infer a limit + ‘s originating from 13’s, however, from the sharpness of the q on the v contamination. Finally, they conclude that they have established the existence of the reaction vp —s vp with a 7 a confidence level and 17p —s 13p with 5 a. The ratios o(vp

vp)/o(vp -÷ pp)

—s

I

I

0.17 ±0.05,

=

—÷

I I

p~n)= 0.2 ±0.1

__________________________________

300

0

o(13p —s13p)/o(13p

.

0.2

0.4 0.6 0.8 q2 (GeV/c)2

Fig. 4.9. vp elastic scattering from Brookhaven AGS.

HP\V

1.0

experiment

0

at

the

0.2

0.4 0.6 q2 (GeV/c)2

0.8

1.0

Fig. 4.10. ap elastic scattering from HPw experiment at the Brookhaven AGS.

326

B.C. Barish, Experimental aspects of high energy neutrino physics

for 0.3 < q2 <0.9 GeV/c2. These two measurements yield a ratio o(13p 17p)/o(vp —s vp) = 0.3 ±0.2 (statistical error only). —*

For a pure vector interaction, this last ratio should be unity. The HPW result indicates V,A interference (or parity violation) in neutral current interactions involving hadrons. This experiment does not favor vector-like theories [4.10] of the neutral weak interaction. Within the context of the Weinberg—Saiam model of weak neutral currents the antineutrino— neutrino ratio determines the V,A combination or Weinberg angle. From this data one concludes sin2 O~= 0.3 +0.05 —01 The elastic scattering channel has also been investigated in the Columbia—Illinois—Rockefeller (CIR) experiment at the Brookhaven AGS. The apparatus consists of large Al spark chambers with scintillation counters interspersed. Time-of-flight was used to reduce backgrounds due to neutrons in the beam. Events with proton momentum larger than 550 MeV (Q2 > 0.3 GeV2) and O~,> 25°were selected. They found 77 candidates for the quasi-elastic charged current reaction v~n—s ~p and 38 candidates for the neutral current reaction v~p—s v~p.In this kinematic region nuclear effects, rescattering, and np charge exchange are not serious. Other backgrounds have been estimated and after subtraction from the data, they obtain: =

o(v~p—s v~p)/o(v~n —s ip)

=

0.23 ±0.09

for 0.3 < < 1 GeV2. This result is in reasonable agreement with the neutrino elastic scattering result of the HPW experiment. The CIR group have not reported a measurement of the antineutrino reaction. 4.3.2. Single pion production channels The single pion production channel affords the opportunity to study the isospin structure of the neutral current. So far, the data are not good enough to make significant tests. Positive evidence for the reactions (1) (2)

v+n—+v+A° v+pv+A~

would be evidence for an isovector piece of neutral current. The experimental way to observe these reactions is to look for a peak in the (it-nucleon) mass plot of the final state. Detailed calculation by Adler [4.11] show that, in the case of pure isovector currents ~(1) + a(2) o(3) + o(4)

—

R

=

025

—

where (3) and (4) are the following reactions: (3) (4)

v+p-s+A~ v+n-+A~

and that: R’ °

=

o(vN 2o(vN

-+

—s

vN’it°) j[N’it°)

=

2R

=

05

B.C. Barish, Experimental aspects of high energy neutrino physics

327

These single pion production channels have been investigated by several groups. In order to investigate the isospin properties of the neutral current the CERN—Gargamelle group [4.12] has compared ratio of it4/it/it° for the single pion production channel. If only Al = 0 were allowed this ratio would be 1/1/1 for a target with equal number of protons and neutrons [4.13]. The experimental results are and

it°/it -

=

it°/it

=

1.4 ±0.2 2.1 ±0.4

for v-induced single pion production for 13-induced single pion production.

Comparing these results to expectations (it°/it = 0.9) for FREON (which is not isoscalar), the data are inconsistent with a purely isoscalar neutral current. In the CIR experiment [4.14], they have searched directly for the existence or non-existence of A-production in the neutral current interaction. Results from Argonne [4.15] for the charged current reaction are consistent with a pure isovector transition, Al = 1. They find that the it°N final state is dominated by A(1236) production. The reactions vN —÷ ~N’it° CC reaction and vN —s vN’it° NC reaction have been studied in the CIR experiment. Figure 4.11 compares the n°pinvariant mass plots for the NC and CC data. The data has been divided into small angle (0 < 20°)and large angle (0 > 20°)samples. It should be noted that the CC background in the NC sample is expected to be largest at small angles. Unfortunately, the data is not yet good enough to draw quantitative conclusions from these mass distributions, although the results are not inconsistent with A-production in the neutral current channel [4.16]. Detailed predictions on the single pion production channels have been made by Adler [4.11] for various coupling hypothesis which can be compared with the data. CC (a)

0

NC (b)

Io~650M~11~ 20

(c)

-

6

>20°

(d)

P~>65OMeV/c

~o 20-

-

~

I ~ I I

H,

1.2

: 8

Ct) &~~Op>2O

Pp>500MeV/c

1.4

.6

1.2

ir°pInvariant Mass

1.4 1.6 (GeV)

Fig. 4.11. Single it production invariant mass plots (Columbia—Illinois—Rockefeller), for the reaction v,, + p -. v,, + p + it° (b, d, 1) and a comparison with the charged current reaction v,, + n —o ~r + p + it° (a, c, e).

32S

B.C. Barish, Experimental aspects ofhigh energy neutrino physics

Measured values of R°

—

o(v~p

—*

2a(v~n—s j~pit°)

—

d ~

v~pit°) + o(v~n—s v~nit°)

o(13~p—s 13~pit°) + o(1Y~n—s 13~nit°) an 2o(13~p-÷ jf~nit°) have been reported. Interpretation of these results are difficult since a model must be used to calculate nuclear rescattering effects. The CERN—Gargamelle experiment has reported values of —

—

0.1
..

(68 % probability limits)

for Freon. The CIR experiment has reported values of R° = 0.17 ±0.04, R° = 0.39 ±0.18 in good agreement with the CERN—GGM results. Using the predictions, corrected for rescattering by Adler, these results correspond to sin2O~< 0.35 in the Weinberg—Salam model. Finally, the Aachen—Padova group has also reported a measurement of the ratio of antineutrino to neutrino induced single pion production in the neutral current channels, o(I3N

—+

13Nit°)/o(vN

—*

vNn°) =

0.51 ± 0.12.

This result indicates parity violation in the neutral current. The same group reports somewhat larger values of R°and R°, R° = 0.40 ±0.06,

R° = 0.61 ±0.10.

The ratio of these two values, however, is consistent with a value of sin20~= 0.4 ±0.1. 4.4. Semi-leptonic neutral current reactions

—

Inclusive channels

The initial experiments in the inclusive channel were convincing in demonstrating the existence of the neutral current channel, but for a variety of reasons were not sufficient to make detailed studies of the nature of the interaction. Improvements have been made and new data have recently been reported in this channel. In all cases only partial cross-sections are measured, due to a minimum hadron energy requirement. Also, different beam spectra are used, backgrounds are different, etc. For these reasons, it is not always easy to compare data from the different experiments. In the following sections, I will review the status of these experiments separately and compare results where appropriate. 4.4.1. HPWF neutral current results [4.18] This experiment has been run using a variety of broad band incident v and 13 spectra and a small fiducial volume. The experimental set-up and beams are shown in fig. 4.12. The two most serious problems hadron punch through (misidentifying neutral current as charged current events) and muon detection inefficiency (misidentifying charged current as neutral current events) —

B.C. Barish, Experimental aspects of high energy neutrino physics

329

HPWF NEUTRAL CURRENT EXPERIMENT MUON DETECTORS LIQ. SCINT. MODULES #1 2 6789IOII...~I5r •I2345

(a)

~ I~r~1r~

1LJ’ L~0N1 Ii ILl

4~ ~~SPARKA

HPWF NEUTRAL CURRENT DATA 1.0 - (a) SDETECTOR I -

.~ADRON

CHAMBERS

ABSORBER

20.8

__________________________ i

a. ~ z

~

I

I

I

I

—v,HORN,400GeV

IO~~(b)

~ QUAD 380GeV

w

I— .

T V

0 010

—+——

0.6 0.4

-

0.2

-

0DETECTOR 2

-

__________ +

I

0c1 ~(b)

-

S

II

I

I

I -

2IO~ ~ 0.6 LU

c’J

> IO~ -

~

0

......---.. I 40

I 80

i

-

‘S Is..i 120

LU

0.4

0 c.~

I 160

-

-

_____

~ _______ I I I

ENERGY (GeV)

5

6

s-

, 7

I

I

I

I

8

9

10

II

12

MODULE NUMBER

Fig. 4.12. (a) Sketch of the target-detecting apparatus showing the small size of the fiducial volume used in the neutral current experiments. (b) Neutrino and antineutrino spectra used in the neutral current experiments,

Fig. 4.13. (a) Raw ratio of the number of muonless events to the number with detected muons versus module, number, as shown in fig. 3a. (b) The raw ratio corrected for muon detection inefficiency and hadron punch-through versus module number.

is much better understood than for the original NC studies by this group. The magnitude of the correction from raw NC/CC ratios to corrected ratios is shown in fig. 4.13. The summary of the results for different beam conditions is shown in table 4.2. In order to interpret the results it is necessary to extrapolate the data to obtain the “missing signal” for Ehad <4 GeV. The extrapolated cross-section ratios for antineutrino/neutrino induced Table 4.2 HPWF measurements ofpartial NC/CC ratios for different beam conditions Beam type

R

Single horn

0.31 ±0.06

Pure v-beam 53 GeV

1974 Quadrupole triplet

0.24 ±0.06

Mixed v, V beam 78 GeV

1975 Quadrupole triplet

0.29 ±0.04

Mixed beam 85 GeV

Double horn with plug

R~

0.39 ±0.10

Comment

Enriched V-beam — 41 GeV

330

B.C. Barish, Experimental aspects of high energy neutrino physic.s

interactions had been obtained by assuming various forms for the interaction. In addition, a correction for an antiquark component has been applied, assuming a 5 ~ contribution. Various hypothesis are tested and the results are tabulated in table 4.3. Table 4.3 Measured values of uNC/a~C after extrapolation to Eh~d= 0 under different model assumptions. A correction for an assumed 5 antiquark contribution is also included

Interaction assumed

Measured value

Predicted value

V—A V or A V +A

0.61 ±0.25 0.40 ±0.17 0.37 ±0.16

0.38 1.00 2.65

The results are clearly inconsistent with a V + A interaction, and are 3 standard deviations from pure V or pure A. They conclude that these results require significant parity violating component in the weak neutral current, and are consistent with V—A. The best fit to the data is of the form (V—0.8 A). The total rates are consistent with the Weinberg—Salam model. 4.4.2. Caltech—Fermilab determination of V,A coupling parameters In the Caltech Fermilab experiment, using the dichromatic beam, hadron energy distributions for v and 13 induced neutral current interactions have been measured [4.19]. A minimum energy for the hadrons of Eh 12 GeV was required in the analysis for a beam setting with v-peaks at approximately 45 and 130 GeV. From this data, partial ratios of neutral to charged currents have been obtained. = 0.28 ±0.03 and E~=0.35±0.1l for Eh > 12GeV. In order to interpret these results in terms of V,A coupling parameters a simple quark-parton form of the structure functions has been assumed for both charged and neutral currents. For charged currents, under the usual scaling assumptions and assuming a V—A form, the differential cross-sections are written as ~(CC)

where ~

=

=

E[(1

=

E[~

—

+ ~(1

~)

+ (1

—

~)(1

y)

2]

—

(4.11)

—

y)2]

(4.12)

(G2ME/ir) ~ F

2(x)dx. The shapes of the y-distributions and the relative magnitudes of the total cross-section are determined by the parameter, c~,usually interpreted as the “antiquark”2component in for the13nucleon. The observed, dominantly flat distributions for v events, and distributions events are a consequence of V—A coupling and the dominant negative (1 y) helicities of the interacting nucleon constituents (e.g. quarks). —

B.C. Barish, Experimental aspects of high energy neutrino physics

331

Predictions of gauge theories, as well as the analogy to charged and electromagnetic current couplings, suggest that the neutral current also couples through a combination of V and A. Assuming V,A type coupling, the neutral current distributions are similar in form to the charged current distributions: 2] (4.13) do”(NC) = Eg0[(1 P) + P(1 y) —

do(NC)

=

—

Eg

.

2]. 0[P + (1

—

P)(1

y)

—

(4.14)

P, analogous to c~in equations (4.11) and (4.12), is a “positive-helicity” parameter. In this case, however, it receives contributions from both a) V—A coupling to the antiquark component in the nucleon and b) V + A coupling to the quark component. The structure of the neutral current coupling affects only P, while the strength of the coupling determines g 0 (measured relative to the usual charged current coupling). In the event that neutral currents and charged currents scatter from the same nucleon components, we can make some direct predictions. If the neutral current coupling is pure V—A (like the charged current), P = and if the coupling is pure V or pure A, P = ~.(The last statement is independent of the nucleon constituent.) The measured Eh distributions reflect the y-distributions, since Eh = E0y. Both the neutral and charged current distributions have been fitted [4.20] to the form of the above equations, including relative normalization, required by assuming charge symmetry at y = 0. Figure 4.14 shows the results of a two parameter fit for g0 and P. The 1, 2 and 3 standard deviation contours for this fit are shown in the figure The best values for the parameters are g0 = 0.31 ±0.02 and P = 0.36 ± 0.09. This value for P is about three standard deviations from pure negative helicity scattering and about 1.5 standard deviations from pure V or pure A. Using these fitted parameters, it is possible to extrapolate the hadron distributions to Eh = 0 in order to obtain the total cross-section ratios. .

0.5

~ ~ I I I I I NEUTRAL CURRENT I

COUPLING CONSTANTS 0.4 o

-

-

-

g500.I99±O.O23 g oO.lIO±O.037

0~’

V or A

—

p

a.

-

—— °‘

0.2

-

0.I

-

_~~4—p°O.36 -

=

0~ 0

—, ——

~\__.)

I

—

I

O.I

I

I

I

2

I

02

STANDARD DEVIATIONS

3

I

I

I

0.3

I

I

I

I I 0.4 I

I

0.5

g~c(I—P)g0 Fig. 4.14. Determination of positive and negative helicity strengths for the inclusive neutral current channel for the Caltech—Fermilab data. The contours represent the result of a two parameter fit.

332

B.C. Barish, Experimental aspects of high energy neutrino physics

This extrapolation yields, o”(NC)/a~(CC)= 0.27 ±0.02 aV(NC)/oV(CC)

=

and ov(NC)/ov(NC)

0.40 ±0.08

0.75 ±0.15. ti(NC) and a0(NC) are expected to be equal in some vector-like theories The cross-sections o [4.21]. This result, while not inconsistent, does not favor this possibility. Extracting the amount of V—A and V + A coupling for the neutral currents is by necessity more model dependent. In addition to the neutral current parameters g 0 and P determined above, the antiquark component of the nucleon must be known. This is not very well determined for the charged currents and for neutral currents it is assumed that the value of ~xdetermined from the charged current data is appropriate for neutral currents. The relations between these three parameters then, are =

g~=(1 —P)g0=(1 —~)g +~g~ and g~= Pg0

=

(1

—

ix)g + + cg

-

where g and g~are the absolute magnitudes of the neutral current V—A and V + A coupling strengths, respectively. The Weinberg—Salam model [4.1] can be directly compared with results presented in this form since the positive helicity contributions from the antiquark component of the struck nucleon has been removed. In the Weinberg—Salam model, these couplings can be 20~as follows: expressed in terms of a single parameter sin = — sin20,~+ ~ sin~0,~ -

and g~=

~

sin40~,

neglecting small affects of the Cabibbo angle, etc. Figure 4.15 shows this curve in the g versus g~plane along with the results for the neutral current parameters from the Caltech—Fermilab -

.5 I

I

I

I

I

I

I

I I

I

I

I

I

I

.9

.4

WEINBERG—SALAM

.

8

-

MODEL (AS FUNCTION

OF SIN2 8~) .3-

PUREV

7

.2’

.6

.

.1

PURE V-A

+

2 I L1

.0

I

I

.1

I

I

I

I

I

.2

I

.1

Ii

.3

0

.4

.5

q Fig. 4.15. V — A and v + A coupling parameters determined in the Caltech—Fermilab experiment. The result is consistent with the weinberg—Salam model and yields a value. sin2O~ = 0.33 ±0.07.

B.C. Barish, Experimental aspects of high energy neutrino physics

333

experiment, using ~ = 0.17. The data agree with the Weinberg—Salam model in magnitude and yield a best fit sin20~,= 0.33 ±0.07. More generally, scalar, pseudo-scalar, and tensor couplings could, in principle, contribute to the neutral current signal. In an extreme case, pure scalar or pseudo-scalar coupling would produce a distribution do/dy c~y2 for both v and 13. This is inconsistent with both the shapes and the relative magnitude of the measured hadron energy distributions, and is ruled out at the level of 5 standard deviations. 4.4.3. CERN—Gargamelle inclusive neutral current results A complete analysis of the inclusive neutral current data from the CERN—Gargamelle experiment has been presented [4.22]. The total data sample consists of 175 neutrino and 151 antineutrino induced neutral current candidates. After subtracting neutron contamination (16 neutrino and 11 antineutrino), a neutral current signal has been obtained. At this point a raw comparison of rates with the charged current reaction can be made. However, a severe bias to the data results from a minimum energy requirement of 1 GeV for the hadrons in the reaction. This requirement is imposed on the data due to the large neutron background present at lower energies. The mean neutrino energy at the CERN—PS is -~ 2 GeV, which means the Eh > 1 GeV cut makes the experiment primarily sensitive to neutral current interactions with large inelasticities (large y). The observed ratios of neutral to charged currents, in this region, are R~= 0.25 ±0.04 and R~= 0.56 ±0.08 for Eh> 1 GeV,

‘-~

2 GeV.

In order to correct for the y0~ condition and to compare with theoretical predictions an analysis similar to that used for the Caltech—Fermilab data, has been performed. Equations (4.1)—(4.4) have been assumed to determine the V,A coupling parameters. In the notation of section 4.4.2 they obtain = =

IX

=

(1 — P)g0 = 0.232 ±0.048 g0P = 0.068 ±0.022 0.05 ±0.02 from the ratio of v and 13 slopes for charged current total cross-sections.

Using these values, they obtain the extrapolated total cross-section ratios, 0(NC)/a0(CC) = 0.26 ±0.04, OV(NC)/oC(CC) = 0.39 ±0.06. o These results are shown in fig. 4.16 in comparison with the Weinberg—Salam model and models with pure V or pure A coupling. This experiment differs from the predictions of pure vector (or axial-vector) coupling by about 3o. The experiment is quite consistent with the Weinberg—Salam model and yield a best value of sin20~= 0.32 ±0.05. In comparison with the Caltech—Fermilab experiment at higher energies, the final results are

334

B.C. Barish, Experimental aspects of high energy neutrino physics

I

I

¶

060

0.10

I

I

I

I

(a)

WEINBERG-SALAM 2 SIfl

~(VA)

1~~2OI

-

0.I0

0.40 0.50 0.60

0.20 0.30

gN~(I—P)go

I

—

I

9 .8

I

I

I

I (b)

~.5

-

WEI NBERG-SALAM sin2 —

-

I IaI~ 6I~ b~ Q~

I

.4

-

.64-

-

I

5

0

.~

-

.2.1 ~

~

~:

.1

.2—

-

.1

.2

p

I .3

.4

.5

I .6

.7

ov(NC)

R~ OR o~’(CC) Fig. 4.16. The top figure shows the negative and positive helicity coupling parameters determined from the CERN—Gargamelle neutral current data. Using a value of a = 0.05 for antiquark component, the resulting theoretical predictions are shown for comparison. In the bottom figure, the neutral to charged current ratios are shown. For the solid curve, the observed data are plotted and the theory has been corrected for the Eh > 1 GeV cut. For the dotted curve, the data have been corrected for the E 5 > I GeV cut as in text.

B.C. Barish, Experimental aspects of high energy neutrino physics

335

very similar and yield almost the same values for cross-section ratios and Weinberg angle. However, it should be noted that the Caltech—Fermilab data has a somewhat larger positive helicity contribution both for the charged and neutral current channels. —

5. Direct production of new particles 5.1. Search for direct production of W-bosons by neutrinos In addition to the possibility of “seeing” the W-boson through propagator effects (section 3.3.2), direct production of W-bosons by neutrinos should occur if the center of mass energy exceeds their rest mass values. This implies that a COM energy = > M,,,, is required. The direct production process involves coherent electromagnetic scattering off of nuclei followed by decay of the W-boson:

+g_P

~

+

The leptonic decay rate can be calculated and for large masses this sets minimum decay rate F > 1018 sec Therefore, the process is “prompt” from an experimental point of view and only the final products can be observed. The production cross sections for W-bosons have been calculated [5.1] however the leptonic branching fraction of the subsequent decay is less certain ~.

B

=

F(W~—s p’~v)/F(W~ all) =? -+

For that reason the experimental results are often presented for an assumed (estimated) value of B or as a function of B. For incident vp, the cleanest experimental signature of W~production would be through this channel which would produce events with the following characteristics: (a) p~i final state; (b) minimal hadron excitation as a consequence of the coherent nature of the scattering; (c) E~+>> E,~as a result of kinematics surrounding heavy W production. A detailed search for events of this topology has been carried out by the Caltech—Fermilab neutrino group [5.2] and in a sample of 1522 high energy ~C events no events satisfying the above criteria were found. From this experiment, the mass limit as a function of B is shown in fig. 5.1. The conclusions are: ~ 8 GeV 90% confidence if

B

=

~

0.25.

A somewhat higher mass limit can be set from the larger samples of data now available. The sensitivity to direct W-boson production should be increased to 10—12 GeV. However, for much

336

B.C. Barish, Experimental aspects of high energy neutrino physics IC

I

I

I

-x~x

6

/

W-BOSON

X

-

/

-

MASS LIMIT

2-

0 0

-

I

I

I

I

0.I

0.2

0.3

0.4

0.5

W—’~~~.Ll, Fig. 5.1. w-boson mass limit (Caltech—Fermilab experiment), from direct search for w-boson production in high energy v-interactions.

larger masses, the W-boson will only be observable by indirect means from neutrinos. In particular, effects of the W-boson propagator on the ordinary charged current reaction are observable up to much larger masses (section 3.3.2) and potentially, with the “Energy Doubler” at Fermilab, propagator effects up to approximately 100 GeV should be observable. 5.2. Search for production of gauge-theory type heavy leptons In the gauge theories, the existence of a positively charged heavy lepton with muon number and/or a neutral vector boson is required to cancel the divergent process, v~+V~W~+W.

The cancellation takes place with the diagrams

and/or

implies neutral currents v0 + N

—0 V0 +

X

implies new heavy lepton (v 0, g, Y~) extended family

B.C. Barish, Experimental aspects of high energy neutrino physics

337

If there is such a heavy lepton, the production by neutrinos, v,0 + N —~Y~+ hadrons, obeys the same physics as single muon production, v,~+ N —s j~ + hadrons, except (1) the coupling

w

~

need not be the same as

w~

(2) the heavier mass of the Y~relative to the ~i leads to more severe phase space restrictions for Y + production; 2, v) and (3) order terms will not, inenter general, be negligible,expressions. so terms involving the W4(Q W 2,m~/E~ v) structure functions the cross-section Using5(Q quark-parton structure functions the cross-section formula becomes, d2a dxdy

=

G~s[7 ~~[I\1

/

m2\

— —)q(x) + (1

—

m2\

y)~l

—

y—)q(x)

where x

Q2/2Mv,

y

v/E 5,

G~= g~G/g and q(x), ~(x)

are the momentum distributions of quarks and antiquarks. The allowed phase space region is obtained by noting 2/s.the condition for positivity of the antiquark contribution to the cross-section, i.e., x(1 — y) m y

allowed region m2 /s

The cleanest experimental signature for Y~production is the occurrence of “wrong sign” muons resulting from the decay y~ ~ v,~+ ~ + v, 5.

beam~~~~p

~

“wrong sign” final state muon

I N

signature

hadrons

However there are other decay modes, Y~ v,~+ e~+ v~ -+

—*

hadrons (in the quark model this has a weight of 1 below charm threshold and 3

if quarks are colored).

338

B.C. Barish, Experimental aspects of high energy neutrino physics

Current algebra argument yield the estimate F(Y~—s v~1.pv~)/F(Y~all) —+

0.2—0.3.

So again, as in W-boson production, the decay branching ratios are model dependent and somewhat uncertain. However, in this case a search has also been made for the hadronic decay mode and the final results are independent of the branching ratio. A set of experiments [5.3] have been carried out in the Caltech—Fermilab apparatus which were specially designed for this search. For example, great effort was made to minimize the antineutrino background in the neutrino beam. In the search for wrong sign muons, a v beam uncontaminated by V’s is required. A nearly pure beam is obtained by using bending magnets to send only positive hadrons toward the decay pipe. The experimental results for observation of charged current events with a ~[ versus ~ for 170 GeV positive and negative hadron beam settings are given below.

v t~

Right-sign muon

Wrong-sign muon

Expected background from non-pure beam

1522 60

8 6

10.5 6.1

From the expected production rate for the incident neutrino spectra, even if all 8 wrong sign events for incident neutrinos really result from Y + decay, M~~7 GeV. Furthermore, these wrong sign events are entirely consistent with being due to a small 13 contamination in the beam. Also, the results of the 13 run show a much larger relative number of wrong sign events; strong evidence for v contamination rather than Y production and decay. Finally, the measured energy and measured y distributions of the wrong sign events agree well with the background expectations, and very poorly with the heavy lepton model curves. Figure 5.2 shows the final result of the search for wrong sign muons. No evidence for a gauge type charged heavy lepton has been observed. If G~.= G~,then M~~ 7 GeV, where the equality hold for the improbable case that all 8 observed wrong sign events, consistent with background estimates, are actually Y~decay. Using fig. 5.2 the limit can also be stated in terms of the relative coupling strength. For example, if M~= 1 GeV then from the lack of observed events we conclude G~<0.02 G~. It is worth pointing out, that there have been no significant searches, at this time, for other types of heavy leptons that might couple to the muon neutrino. For example, a heavy neutral lepton or a sequential heavy lepton of the same sign and lepton number as the muon. In fact, some interpretations of the HPWF trimuon events (section 5.5) hypothesize production of new heavy leptons. 5.3. Charm baryon candidate A very interesting candidate for a charmed baryon event has been observed [5.4] in the 7-foot bubble chamber at BNL. The a cryogenic 3. Itdetector can be isfilled with H bubble chamber of 2m diameter and 3m high. The visible volume is 7 m 2 or D2.

B.C. Barish, Experimental aspects of high energy neutrino physics 10

I

I

I I

339

/

90% CONFIDENCE LEVEL

1.0

-

-

THEORETICAL B

O.OII

0 Fig. 5.2. Heavy lepton mass limit (K

= Y

2

I

4

I

6

8 M~(GeV/c2)

-. jz~vv/Y—o all) from search

10

12

for reaction

v 0+ N

.+

~

+ X followed by decay

p~+ v0 + v0 in the Caltech—Fermilab experiment.

In order to separate the j~’sfrom the ir’s, there are 4 steel plates immersed in the liquid. The size of each plate is 1.5 m x 1.5 m x 5 cm. Charmed baryon disintegration A possible way to look for charmed baryons is to look at strange particle production events. In most theories the charm quark is coupled with the “Cabibbo strange quark” S~,= s cos O~ d sin O~•So the branching ratio of “charmed particle —s strange particle” must be large. A study of events containing strange particles has been carried out by the BNL—Columbia group. The result of their study is as follows: —

Number of pictures

Charged current events

Strange particle events

H2

204000

400

6

D2

278000

686

4

The strange particle events are mostly events with a V°(A or K°).These are the only strange particles with good detection probability. For an invariant energy of the hadronic system greater than 1.5 GeV, the number of V°events and of charged current events are, after corrections for background, etc.: V°= 9.14 events CC = 309 events. This yields a ratio of V°/charged currents system W between 1.5 and 3.5 GeV.

=

0.03 ±0.01 for invariant mass in the hadron

340

B.C. Barish, Experimental aspects of high energy neutrino physics

The repartition of strange particle events is as follows: AS= —AQ 1 event vp —s j~K°n~p AS = +AQ 7 events with associated production ~

1 event ambiguous vp

K0

—s

(

ir~j[

or

I will concentrate here only on the event with a single A°identified, which is shown schematically below:

7T+

(3k’ (4) p (6)

e 1r+ (2)

—

e’

/ /A° p //

iT

+

It,

•

Q

II

A~ ~

~+

,~+

o—

+1.+2~lQ-+l~ 0 •

1 ~ AS —

1

J

~

As

—

—SQ

L_...appgrent

violation of the rule AS • SQ

The parameters of this event are E,, q2 x

= = =

13.5 GeV 2.2 GeV2 0.31.

The mass of the hadronic system is M (A°ir~ir~ir’~ir) = 2425 ±12 MeV. The mass of the A, 2ir it system is, depending on which it + is left ~

-

2260 ±12 MeV or 2090 ±12 MeV. The reconstructed mass of the incoming particle is almost zero (within a few MeV) so one can assume that no neutral particle is unseen.

B.C. Barish, Experimental aspects of high energy neutrino physics

341

A detailed study of possible backgrounds has been presented [5.4] with a total probability of P ~ i0’~that this event is due to any of these sources: (1) associated production of K~, (2) associated production of K°,with an undetected K°decay, (3) interaction not produced by a v-interaction, (4) missing K°. This event can be explained as a charm baryon event as follows:

w+ {d

~: J” = ~: J~=

~ ~

I I

= =

1 1

charge charge

= =

2 2.

No events of this type have been observed in the FNAL 15 ft. bubble chamber [5.5]. The relative sensitivities are difficult to estimate, due to the different beam spectra, uncertainties about the production mechanism, etc. The BNL group has estimated that 0.1 events would have been expected in the FNAL data reported in ref. [5.5]. Other estimates of the sensitivity of the FNAL sample are somewhat higher. More events have now been analyzed from the 15 ft. chamber with no events of the type discussed here reported. However, at this time the uncertainties in the relative sensitivities, etc., make comparisons unreliable. It should also be noted that a search for non-leptonic states has been carried out by the Gargamelle group [5.6]. Looking for V°’s,they have observed 69 A°,40 K°,34 ambiguous (A°/K°)and 13 with two or more V°’s.Mass plots of Ait, A,rit, Airitir, Kp, Kpir, Kit, Kirir have been presented, but there are no striking features, possibly because of high background from associated production. 5.4. Di-lepton events One of the most interesting developments in neutrino physics during the past couple of years has been the discovery of neutrino induced events with 2 it’s in the final state. The first reported observation of such events was reported for the HPWF experiment [5.6] and it was soon confirmed in the Caltech—Fermilab experiment [5.7]. Considerable data has now been obtained, including complementary data on events with a 4ue final state. It is now becoming possible to study some of the characteristics in some detail in order to uncover the physics sources of these events. 5.4.1. 2p-results Events with 2~z’sin final state have been’observed for -~ 1 % of all v-interactions. For comparison, events with no muons v + N v + X occur for 20—40 %, of v-interactions. Fig. 5.3 shows an example of a 2~event in the Caltech—Fermilab apparatus. The two muons are clearly identifiable —+

342

B.C. Barish. Experimental aspects of high energy neutrino physics

ENERGY DEPOSITED IN CALORIMETER I

I

6’0

I

I

50

I

I

40

I

I

IhdIi1

30

20

BEAM

-

I

SCINTILLATION THAT

0

~/

~

v

I

[1 r

~

ELEVATION VIEW

SPARK

_

VBEAM

PLAN VIEW Fig. 5.3. Example of 2~sevent

MAGNET in

Caltech—Fermilab apparatus.

by their penetration through Fe. This particular event has opposite sign final state au’s and a total observed energy of well over 100 GeV. Observation of these events could be the signal for production of new leptons, new hadrons, or possibly even W-bosons. However, before considering these possibilities it first must be established that the events do not result from a background source. The main potential source of background is due to charged current events with a second muon coming from it or K-decay, v + N —s ~u + hadrons

{ir~

Non-prompt background sources The fundamental difference between events originating from it or K-decay and sources such as new hadrons or leptons is the lifetime of the parent particles. The lifetimes of the new particles is so short r 10_b sec that, for all practical purposes, both muons are “prompt”. That is, they decay so rapidly that both muons appear to originate at the neutrino interaction point. Non-prompt background sources are usually measured by varying the density of absorber for it’s and K’s and extrapolating to infinite density — (e.g., prompt sources). For v-induced 2p events in the Caltech—Fermilab experiment, no such empirical test has been performed since the target is all one density. In the HPWF experiment some events originate in liquid scintillation and some in Fe. Although the geometric acceptances are different and the two absorbers are not really two ‘~

B.C. Barish, Experimental aspects of high energy neutrino physics

343

discrete well defined densities, the HPWF group have compared the rate from the two different targets. Fig. 5.4 shows the result of this test. At the lo level only 30 % contamination from it and K-decay is allowed and the probability that the entire signal is non-prompt is ruled out at the 4o level. In the absence of good quantitative measurements of the level of it and K-decay background calculations have been made in the following way. (1) Assume that the hadron distributions in v-collisions is identical to that observed in hadron collisions. (2) Calculate the hadron cascade including the possibility of it and K-decays. This gives the probability to get a JL from it and K-decay and the expected energy spectrum. (3) Fold this calculated probability against the measured hadron energy distribution from single ‘u-events. Fig. 5.5 shows the experimental data for the energy distribution and the expectations from it and K-decay. A clear excess of events appear, especially for higher energy second muons. However, the contamination from it or K-decay might be significant and the effects of this contamination must be considered in interpreting the results. The main features of 2ji events The following rates have been determined for 2j~tevents in the HPWF experiment: cr(v

—+

j[ji~)/o(v

o(13

—s

p~j~)/o(v

~j~)

~ 0.8 ±0.6

~tj~)/o(v

~

~

o(v

-÷

-+

,~)

-~

10~

0.1.

Thus production is observed both by neutrinos and antineutrinos with cross sections that are statistically equal. The general features of events made with v’s versus V’s (with limited statistics) seem to be quite similar. Muons are dominately of opposite sign (same sign ~‘s occur at 10 % the rate of opposite sign), the “right sign” muon (~ifor incident v’s, ~ for incident 13’s) tends to 10~

Fraction(7r.KIDecay All Dimuons

-29 -30

~

~.

-33

E

~ 0.1

(b) ~ a

‘~.05—

~-

(a) I

I

I

I

I

I

I

I

I

I

I

50 100 Absorption Length (cm)

I

I

ISO

Fig. 5.4. Emperical test of prompt versus non-prompt 2/1 signal from the HPWF data. Rates from two targets of different absorption length are compared. The extrapolation to zero absorption length yields the prompt 2g signal.

344

B.C. Barish, Experimental aspects of high energy neutrino physics 00

I

0

I

I

I

2SC~IIIIIIIIIIiiiIIIIT I

DATA

-

200

A ~

~Iso~,:

W

I

BACKGROUND-~’

-

01

00I -

A

IO0~

-

DATA

A

Pws

50

0

I

I

3

I

I

6

I 9 I

I

I 12

I

0

15

ENERGY-MUON (GeV) Fig. 5.5. Calculated non-prompt 2/1 signal versus energy requirement on second muon to data for Caltech—Fermilab experiment.

0

~

25

.5.5. }~A~5.’~~.Y4,52

50

75

-

4.5’

00

~WR0NG SIGN Fig. 5.6. Camparison of Pft versus P,~for Caltech--Fermilab data. P~,corresponds to p with same lepton number as incident v (e.g. ~i for va).

be more energetic, etc. A plot of ~rs versus P~ from the Caltech—Fermilab experiment is shown in fig. 5.6. It should be noted that experimental biases cut-off the distributions for muons less than E~6GeV. An important question in interpretation is whether there is a threshold in production of 2ii’s. Previously a negative search had been reported at Serpukhov for E 5 15 GeV and no events had been observed at Fermilab for E5 ~ 30 GeV. This was interpreted [5.8] as possibly indicating a threshold for 2~iproduction. However, it has now become apparent that the second muon tends to be very slow and the requirement that 21L’s be produced with ~ 2 GeV for 15 GeV neutrinos represe~1tsa serious bias. A new experiment has been performed at Serpukhov [5.9] and they now report a positive signal for E5 > 10 GeV. Due to the bias against low energy muons, the results for E5 < 10 GeV are inconclusive. The Serpukhov results are shown in fig. 5.7. Physics possibilities for sources of 2j.i events 1. W-boson production v + Z —s j~ + W~ + Z ~ R~+ —

V

B.C. Barish, Experimental aspects of high energy neutrino physics

345

SERPUKHOV FLUX RUN I

—

~E ~ ~

IN. 5

10

15

20

25

30

E~GeV

SERPUKHOV DATA 0

• 0 —

RUNI RUN2 FROM ir&K DECAY

(N

0

I

2

2)

E[GeV]

Fig. 5.7. Evidence for prompt 2p’s from Serpukhov data.

This hypothesis is ruled out for two reasons: (1) the observed energy in the hadron system is large while for W-boson production it should be very small; and (2) the observed energy of the j1 is large and ~ small, while for W-production the opposite is expected. 2. Heavy lepton production v,1 + N —s L°+ hadrons. —

~ ~

+ V~+

VL

zo ~hadrons

Pais and Treiman [5.9] have shown that for production of a heavy lepton with generalized

346

B.C. Barish, Experimental aspects of high energy neutrino physics

coupling V,A or S,P,T the ratio of the mean value of the R 0.5

j[

and ~

momenta is bounded,

/lies between R

2.1 where

= mean energy of u =

mean energy of u~.

The data from the HPWF group yields Rmeas = 3.7 ±0.7, which seems to rule out the hypothesis of a neutral heavy lepton. However, several corrections should be applied to the data before a definitive conclusion can be made. For example, any antineutrino contamination in the neutrino beam tends to reduce the measured ratio, the cut-off on low energy muons also reduces the measured ratio, the cut-off on low energy muons also reduces the measured ratio, however any it or K-decay muons could greatly increase the observed ratio. This comment is just cautionary. The data indicates that the Pais—Treiman bound for heavy lepton production is violated which favors the interpretation of production of new particles at the hadron vertex. 3. Heavy hadron production v~+ N —s ~u + X(m,K,C~,...) [~ ~+ + v + hadrons. —

hadrons

This is an example of charmed hadron production. If we consider only valence quarks, the production of a charmed object can occur only for neutrino interactions, but it is suppressed by sin2O,

sin2 0~

cos2 0~

Hv

COS

Slfl

B.C. Barish, Experimental aspects of high energy neutrino physics

347

The x-distributions from events produced off these valence quarks should have a wide range of x-values. If we also consider the possibility of production off the “sea”, we have two additional diagrams:

2 O~

cos

cos2 O~

In this case, the x-values should be confined to small x. Thus, by comparing the absolute rate for dimuons by neutrinos and antineutrinos and the x-distributions in the two cases the validity of this general picture can be checked. At present the data are consistent with charm production being the prime source, but more data on distributions, backgrounds, etc., are needed, to determine whether charm accounts for all the prompt dimuon signal or whether other sources are also present. For example, some events in the HPWF data may indicate other sources (see section 5.5). Also, whether there is a like-sign dimuon signal above it and K-decay background levels remains to be proven. The relative energy dependence of dimuon production by neutrinos and antineutrinos can indicate new sources of dimuon events. For example, if a b-quark existed, above threshold, more 2~.tevents would be observed by antineutrinos but not by neutrinos. The diagram below has

no analog for neutrinos. A test of this has been performed with the Caltech—Fermilab 2~u data [5.10] as shown in fig. 5.8. The test is not very precise due to the limited statistics at high energies and uncertainties in the level of it and K-decay background. However, the data appear completely consistent with the GIM four quark model and inconsistent with a b-quark of Mb ~ 5 GeV. This same conclusion was reached from the recent charged current data from the Caltech—Fermilab experiment (section 3.3). Since the data are consistent with being dominantly due to charm production, this same data can be used to estimate the charm semileptonic branching ratio to muons. From the data of section 3.3 on the charged current reaction, it was estimated that the contribution of charm to the total cross section was about 8 % for v and 10 % for 13. Using the 175 GeV two muon data point, and subtracting it and K-decay background, the charm contribution to the 2p signal is obtained. Comparing, 2p and 1j~icharm production we obtain a semileptonic branching ratio of ~ 10% for charm decay to muons. (There is a small additional contribution for muons less than 2.5 GeV, which are not observed.)

348

B.C. Barish, Experimental aspects of high energy neutrino physics

ANTI-NEUTRINO DIMUON RATE .03

,/4~MbZ5GeV

-

MbO7GeV

.02

-

GIM .01

+

background

-

-

_~~t~~ackground

NEUTRINO DIMUON RATE cr2~

-

.0I-

0

-

Tx ~~stimated

GIM ÷background

background

200

100

Et,(GeV) Fig. 5.8. Dimuon rates for v and P versus E~from the Caltech—Fermilab experiment. The estimated level of background from it- and K-decay (which has not been subtracted from the data) is shown. The curves through the data are from Barnett and are normalized to the neutrino data point at 175 Gev. The expected added contribution for a b-quark (assuming branching fraction equal to charm) is also shown.

5.4.2.

/~e events

Events have been observed [5.11] in the 15’ Fermilab bubble chamber of the type v~Z + ... . Similar events have also been reported in Gargamelle and at Argonne at lower energies. —~

Fermilab e~.tevents The results of the Fermilab experiment are 5000 events single muon 15 candidates ite final state. The most startling feature of these first bubble chamber events is that 11 out of 15 have “Vees” — —

B.C. Barish, Experimental aspects of high energy neutrino physics

349

representing observed strange particles in association with the events. This number of strange particles is even larger than expected in simple models of charm production. A larger data sample now being analyzed appears to have somewhat smaller strange particle production [5.12]. The main expected backgrounds in these bubble chamber experiments are far different than in the 2~counter experiments making the experiments very complementary. Briefly, the primary background sources are 1. Asymmetrical Dalitz pairs vt,Z -s jiit° + X L+ ye~e N

slow estimate: < 0.7 events. 2. vt,Z—+irK~K°+X I ) e+itove estimate: < 0.3 events. 3. ‘~eZ—s e~ + X (mis-identify ~) estimate: <0.02 events. 4. Ke~itv estimate: <0.02 events. Therefore, the total background estimate < 1 event. The detection efficiency for e~has been estimated to be approximately uniform for E > 1 with a value of E = 0.48 ±0.07 and falls off below 1 GeV. The observed detection rate, corrected for efficiency is R 102

=

~ie~/ji

=

0.55

±0.2 for

{~ >

1~8GeV

The main correction necessary to obtain the real rate involves estimating the number of positrons below 0.8 GeV. This correction is obviously model dependent, but could be quite large. For example assuming charm decay D~ e~K°v0as the origin of the events the true rate calculates to be -+

Rcorr

=

,ue~/~(~ 1.2%.

This rate is quite consistent with the observed Fermilab 2~rate. The mean neutrino energy here is -~ 25 GeV. Again, there is a fast “right-sign” muon and a slow “wrong-sign” lepton. Qualitatively, strange particle production is prolific as expected in charm production, though the first results are even larger than expectations of charm models. Figs. 5.9 and 5.10 are examples of e~zevents in the Fermilab bubble chamber. These data again violate the Pais—Treiman bound. / ~ 6.6.

The large number of K~’sdetected indicates that a process such as v+Z-s1u+C+nK’s+X I )e~+K’s may well be responsible for these events. Larger samples of j~eevents are now being analyzed and more detailed information on the characteristics can be expected shortly.

350

B.C. Barish, Experimental aspects of high energy neutrino physics

EXAMPLE

EXAMPLES (~.ieEVENTS)

e° K°

Fig. 5.9. Example of event of the type v

0N .-+ pe + X from the

Fermilab 15 ft. bubble chamber.

Fig. 5.10. Example of event of the type v5N the Fermilab 15 ft. bubble chamber.

—o

pe + X from

5.5. Trimuon events The first observation of trimuon production by neutrinos was reported [5.13] in the CaltechFermilab experiment. Two events were observed out of a sample of 12 000 v and 6000 13 interactions. Both events contain an energetic jC and two additional muons with low kinetic energy in the hadron rest frame. These events have been interpreted, by the authors, as being consistent with production at the hadron vertex. Two mechanisms which may contribute to this signal are (1) low mass muon pairs from virtual photons and/or decay of vector mesons, and (2) associated production of new hadrons which decay leptonically. (I)

(2)

~~drons

The HPWF group have recently reported [5.14] the observation of six trimuon events in the initial data taken with their new detector “NEULAND”. Two of the more interesting events, superposed on the detector are shown in fig. 5.11. The new detector has several key features important for studying multimuon events. A new 8-meter diameter iron toroid has been installed, directly following the neutrino target, that measures the charge and momentum of muons out to -~ 600 mrad and with minimum momentum of 3 GeV. This large toroid is followed by the original 4-meter toroids to provide ±15% momentum resolution for muons up to 3about GeV. The mass200 = 250 tons); v-target now consists of three different densities: (1) Fe-target (-.-~8 g/cm

B.C. Barish, Experimental aspects of high energy neutrino physics

80

EVENT

60 40

-~~flhflifl ,~—~--

MUON HODOSCOPE

[]Li LI

VETO COUNTER

~ [~(-)IIGeV SCALE J0.5ni

FI (D....20

Jo

EVENT 119-017911 EHZI3GeV

~~Llf

3_r,l[li~ñiiiUON HODOSCOPE 1-132GeV

___ COUNTER

46-040090

83GeV

EH: -

VETO

351

~] JJ

~

LI

U U Li

SCALE J0.5m

H.FeT -“+.— Li q C— ~FeC*~~ SPECTROMETER~ Fig. 5.11. Two trimuon events observed in the (HPWF) NEULAND detector at Fermilab. These events, having a pair of energetic muons with a small opening angle are difficult to explain as originating at the hadron vertex.

3, mass = 45 tons); and (3) Fe-calorimeter (-..~3 g/cm~,mass = (2) Liquid scintillator (-~.-0.8 g/cm 90 tons). Several of the observed events are characteristically different from the events reported in the Caltech—Fermilab experiment. Both events shown in fig. 5.11 have two rather energetic opposite sign muons with a small opening angle between them. The authors have calculated that the probability that these events all have their ori&in at the hadron vertex is extremely unlikely. This leads to the intriguing prospect that trimuon production might have source at the lepton vertex. This hypothesis has been proposed by the authors and some characteristics of both these trimuon and some of their dimuon events have been interpreted as supporting this conjecture.

352

B.C. Barish. Experimental aspects’ of hIgh energ *‘ neutrino physics

~lr~

Fig. 5.12 shows P~_versus P0~for a sample of opposite sign dimuon events from the HPWF experiment. They note that although most dimuon events fall along a band with large P0_ and small P0+ consistent with charm production, ten events are observed within the bounds P,~= 2P0+ and P0~,= ~ ~ and having combined momenta greater than 40 GeV. They note that the calculation of Barger et a!. [5.15] yields a probability < iO~ per 2~[’ event of finding an event in that region from charm production. They conclude that a feature common to all models that seek to explain the properties of trimuon events and symmetric dimuon events will be the inclusion of a neutral heavy lepton. I 220

I

I

F

I

-

-

20

60 P

100 +

140

180

(GeV/c)

Fig. 5.12. P5- versus P~.for 2p events in the original HPWF experiment. The probability of finding an event due to charm outside the charm boundary is P < i0~ per observed 2p event. The authors hypothesize that the symmetric events beyond this boundary may have a leptonic origin.

B.C. Barish, Experimental aspects of high energy neutrino physics

353

6. Future prospects 6.1. CERN—SPS facilities The neutrino facilities at the CERN—SPS have recently started to come into operation. A review of the experimental facilities, beams, etc., installed at the SPS has been made by Steinberger [6.1]. A horn beam (broad band) and a high quality dichromatic beam have been built. The narrow band beam represents an improvement over the present Fermilab beam in several respects: (1) improved optics for the hadron beam, yielding better energy resolution for the knowledge of neutrino energy (the resolution AE,, is typically 10% for this beam); and (2) the contamination of wrong sign neutrinos (e.g., neutrinos in the antineutrino beam) has been reduced, which should help neutral current experiments, in particular. In contrast to the Fermilab scheme of interchanging beams on “train-loads”, the two SPS beams (broad band and narrow band) exist side by side and use the same decay region, etc. This means that changing from the narrow band to broad band beam should require minimal downtime. The region occupied by the narrow band beam elements is 120 m long, the decay region 300 m, and the shield 350 m. The shield is more homogenous than Fermilab allowing the possibility of using monitors inside the shield to measure the muon flux at different depths, in addition to monitoring schemes used at Fermilab. Measuring muons was the basic scheme for normalization of v data for the Gargamelle experiments at the CERN PS. The shield at the SPS is made mostly of Fe, which makes the shield substantially shorter than Fermilab, increasing the solid angle acceptance of the detectors. This shorter shield, however, creates a larger parallax problem (uncertainty in the incoming neutrino angle due to the uncertainty in decay origin), which contributes substantially to the uncertainty in neutrino energy in the narrow band beam. There are four large detectors at the SPS. 6.1.1. Gargamelle This very successful heavy liquid bubble chamber has been moved from the PS to the SPS. The chamber is filled with freon or propane and is in the shape of a cylinder, aligned along the beam direction, 4.9 m long and 1.9 m diameter. The fiducial volume is -~ 3 m3, or 5 tons of freon, and the magnetic field is 20 kg. At the SPS, an external muon identifier made from Fe plates and covering 30 m2 will be used. Wire chambers will identify the positions of muons penetrating the absorber which will then be matched to tracks exiting the chamber. ‘—j

6.1.2. BEBC A big European bubble chamber (BEBC) has been constructed for use at the SPS. It is shaped in a vertical cylinder 3.5 m diameter and 4 m high with -~ 10 m3 fiducial area. The magnetic field is -.~ 30 kg from a large superconducting magnet having a return path in Fe at 12 m diameter. Outside the Fe is an external muon identifier of 150 m2 similar to the Fermilab 15 ft. bubble chamber EMI. The chamber can be filled with H 2, D2 and Ne. 6.1.3. Electronic detector (CDHS) A collaboration from CERN—Dortmund—Heidelberg—Saclay (CDHS) have installed a very large electronic detector at the SPS. Fig. 6.1 shows a side view of this detector. The apparatus is

354

B.C. Barish, Experimental aspects of high energy neutrino physics

~

a/—beam

~5meters~ FINE GRAIN TARGET

COARSE GRAIN TARGET~

-—

CERN -S PS 400 TONS SCINTILLATOR-DRIFT CHAMBER

III

~

Fig. 6.1. CERN—5P5 electronic facility for CDHS experiments. It consists of 1400 tons of magnetized target and is on a 2.5’ slope following the direction of the beam coming from the SPS.

somewhat different conceptually from the original Caltech—Fermilab and HPWF detectors in that it is a combined function device. That is, the target-calorimeter and Fe toroids for muon spectroscopy are not separate elements. In this case, the magnet toroids are segmented, thereby serving as target and magnet simultaneously. The total detector consists of 19 magnetic toroids (3.75 m diameter), containing 75 cm of Fe (65 tons). Between toroids are drift chambers for tracking muons. The first seven toroids are finegrain (segmented into 5 cm Fe plates) and the downstream twelve toroids coarse-grain (15 cm Fe plates). Between plates, scintillation counters are placed to do hadron calorimetry. Basically, this device, in addition to being extremely massive, has very good angular_acceptance for muons, due to the combined function, and good calorimetry (t~Eh 0.8 ~/E(GeV)) for the fine grain portion. The resolution on the muon momentum is 10%. 6.1.4. Electronic detector (CHMR) A collaboration from CERN—Hamburg—Moscow—Rome is building a detector which will occupy the region just downstream of the CDHS detector in the same experimental hall. This apparatus should be particularly useful for neutral current studies since it is designed to measure the direction of the hadron shower as well as the energy. The direction and angle of the hadrons along with knowledge of the neutrino energy from the beam define the kinematics and allow determination of the usual deep inelastic variables. This apparatus, under construction, is being made with marble plates having scintillators and proportional chambers inserted in the gaps. The marble slabs are 8 cm thick x 3 m x 3 m. Altogether, there are 80 plates and they will be surrounded with a magnet to provide a magnetic field. In addition to the other applications, this device will also be used to measure the polarization of muons coming from neutrino interactions in the apparatus upstream.

B.C. Barish, Experimental aspects of high energy neutrino physics

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6.2. CERN—SPS initial results At the time of writing this review, initial results are just starting to appear from the SPS program. It is not my goal to try to review these results, as they will almost certainly be supplemented with many interesting results over the next year and should rightfully be the subject of another review. However, several of the initial results are particularly relevant to some of the Fermilab results reviewed in this report and I will briefly give only these results. The CDHS experiment has presented results on opposite sign dimuon events [6.2] and charged current events [6.3] from a narrow band run carried out during the first half of 1977. This run consisted of 53000 v and 1500013 induced charged current events. From the same sample, they have found 257 v and 58 13 induced dimuon events. The experiment detected a minimum energy of 4.5 GeV for muons, so these represent the partial rates above that cut. They conclude, as the Fermilab experiments have, that the dimuon signal cannot be accounted for from non-prompt sources. They estimate, from~calculation, a 13 ±4 % background from it and K-decay in their neutrino dimuon data. Fig. 6.2 shows the observed ratio of 2~i/1j.ievents versus energy from this experiment. Also shown on the right-hand scale is the detection efficiency (mostly the effect of the muon energy cut-off) assuming the production mechanism is charm. The shapes and relative v and V signals appear consistent with this source and they conclude that the data shows no sign of b-quark production. This conclusion is quite consistent with that of the recent Caltech—Fermilab I

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356

B.C. Barish, Experimental aspects of high energy neutrino physics

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data (see section 5.4, fig. 5.8). The raw rates (above it and K-decay background) appear somewhat smaller in the CDHS data; however, the Caltech data have a lower energy requirement on the second muon (E~> 2.5 GeV) and a direct comparison is difficult. The charged current analysis, thus far, has been directed toward the existence/non-existence of the y-anomaly reported by the HPWF group (section 3.3.1). Fig. 6.3 shows the antineutrino/neutrino cross section ratio reported by the CDHS group and compared with the sharply rising cross section ratio data presented for the HPWF data. Overall systematic errors in normalization (10%) and between E < 100 GeV and E> 100 GeV (10%) are not reflected in the CDHS data points, but it is quite clear that the trend reported for the HPWF data is not confirmed. This was also true for the recent Caltech—Fermilab data (section 3.3.2, fig. 3.15) which also showed no dramatic energy effects the for HPWF o~and data cr~versus E5. 5 and 5. Againdependent for comparison, showing Fig. 6.4rising showsv the CDHS data onenergy are shown. This CDHS result, like the Caltech—Fermilab a sharply at increasing result (section 3.3.2, fig. 3.15) does not confirm this trend. The line in the figure is for B = 0.8 (c~= ~-(1 B)) including the acceptance of the CDHS experiment. This experiment is consistent with B = 0.8 (10% antiquark), independent of energy for the antineutrino data. This result is somewhat in conflict with the Caltech—Fermilab result 19%) at high energies. However, for the CDHS data, only the antineutrino <~> has been used in determining B and, for example, the neutrino data shown in the same figure do not agree well with B = 0.8 and would fit better to a lower value of B (larger antiquark component). —

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Overall, the general conclusions of the Caltech—Fermilab and CDHS experiment agree in so far as there are no strong energy dependent effects observed for either the dimuon or charged current data. This is in sharp contrast to conclusions from the HPWF data. The first results from BEBC with a Ne—H 2 mixture exposed to the SPS narrow band beam have also been presented. In this case, absolute cross sections have been presented (fig. 6.5) which yield o/E for neutrinos indicating a slow energy dependent decrease, and a/E for antineutrinos with very little change with energy. The o-~/o~ ratio grows with energy, but no sharp energy dependent effects, as reported in the HPWF experiment (section 4.3.1), are observed. These results are very similar to the Caltech—Fermilab (sectionresults. 3.3.2). A slow decrease in the 2>/E from BEBC cross along section with theresults Gargamelle Fig. 6.6 shows
358

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6.3. Fermilab facilities At Fermilab the future program is more one of evolution than instituting a new program. A new high quality dichromatic beam is under construction that will give improved v or 13 purity and AE,, —~ 10 %. The electronic detectors are both being upgraded considerably. Initial data with an improved detector in lab-C were already reported here for the observation of trimuon events (section 5.5). The apparatus consists of three density targets and a 24 foot diameter muon spectrometer. The original Caltech—Fermilab detector has been removed from the Wonder Building and a new detector is being installed in lab-E. This new detector consists of a rear portion (450 tons) of magnetized detector (20 cm granularity) of 11.5 foot diameter. The front portion has -

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B.C. Barish, Experimental aspects of high energy neutrino physics

359

a two-density target for studying non-prompt sources of dimuons and part of what will ultimately become a large fine grain target-calorimeter (670 tons). In the Wonder Building, an experiment is being tested to investigate v,~—escattering. The EMI for the 15’ bubble chamber is being extended for additional coverage and being made into a double plane. Finally, all new developments have in mind the ultimate uses of neutrinos up to 750—800 GeV when the energy doubler is completed at Fermilab. We can look forward to a bright future of investigating high energy neutrino interactions at both CERN and Fermilab.

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[4.4] B.C. Barish et al., Phys. Rev. Lett. 34 (t975) 538. [4.5] H.S. Gurr, F. Reines and H.W. Sobel, Proc. Neutrino Conf., Balatonfüred (1972) Vol. 1, p. 147; F. Reines, H.W. Sobel and H.S. Gurr, Proc. Neutrino Conf., BalatonfUred (1975) Vol. I; H.S. Gurr, F. Reines and H.W. Sobel, Phys. Rev. Letf. 34(1976) 315. [4.6] F.J. Hasert etal., Phys. Lett. 46B (1973) t2l. [4.7] H. Faissner et aI., Proc. Neutrino Corf., Aachen (1976); H. Faissner, Proc. XVIII Intern. Conf. on High Energy Physics. Tbilisi (1976). [4.8] D. Cline et al., Phys. Rev. Lett. 37 (1976) 252; 37 (1976) 648. [4.9] W. Lee et al. Phys. Rev. Lett. 37 (1976) 186. [4.10] A. De Rujula et a!.. Phys. Rev. D12 (1975) 3589; R.L. Kingsley et a!.. Phys. Rev. D12 (1975) 2768; H. Fritzsch et al., Phys. Lett. 59B (1975) 256; S. Pakvasa et a!., Phys. Rev. Left. 35 (1975) 703. [4.11] S. Adler, Phys. Rev. D9 (1974) 2125 and Proc. Sixth Hawaii Topical Conf. in Particle Physics (1975). [4.12] F.J. Hasert et al., Phys. Lett. 59B (1975) 485. [4.13] V. Brisson, Proc. Eleventh Rencontre de Moroind, Vol. 11(1976) p. 253. [4.14] K. Goulianos, Proc. Eleventh Rencontre de Moriond, Vol. 11(1976) p. 283; W. Lee et a!., v-75 Conf., Aachen, Germany. [4.15] 5.J. Barish et a!., Phys. Rev. Lett. 36(1976)179. [4.16] It should be noted that Goulianos has concluded in ref. [4.14] p. 289, that both the charged and neutral current process show an enhancement in the ~\(1236)region. [4.17] Aachen—Padova Report, v-76 Conference. Aachen, and XVIII Intern. Conf. on High Energy Physics, Tbilisi, USSR (1976). see S.S. Gershtein, p. B141. [4.18] P. Wanderer et al., HPWF/I preprint (1977) submitted to Phys. Rev. D. [4.19] F. Merritt el a!., Caltech Preprint CALT 68-600 (1977) unpublished. [4.20] F. Merritt et a!., Calfech Preprint CALT 68-601 (1977) unpublished. [4.21] A. De Rujula et a!., Phys. Rev. D12 (1975) 2768; H. Frifzsch et a!., Phys. Lett. 59B (1975) 256; S. Pakvasa et a!., Phys. Rev. Left. 35 (1975) 703. [4.22] J. B!ieteshau ef a!., Nuc!. Phys. B! 18(1977)218. [5.!] R.W. Brown and J. Smith, Phys. Rev. D3 (1971) 207. [5.2] B. Barish et a!., Phys. Rev. Left. 31(1973)180. [5.3] B. Barish, Proc. Sixth Hawaii Topical Conf. in Particle Physics (1975) p. 245. [5.4] B. Barish et a!., Phys. Rev. Lett. 32(1974)1387. [5.5] E.G. Cazzoli et al., Phys. Rev. Left. 34(1975)1125; B.P. Roe, Proc. 1975 Lepton-Photon Symp., Stanford University (1975). [5.6] C. Rubbia, XVII Intern. Conf. on High Energy Physics, London (1974); A. Benvenuti et a!.. Phys. Rev. Lett. 34(1975)419. [5.7] B. Barish, Proc. La Physique du Neutrino a Haute Energie, Ecole Polytechnique, Paris, France (1975) p. 131. [5.8] B. Barish, Proc. Intern. Conf. on Lepton—Hadron Interactions, Stanford(1975) p. 511. [5.9] A. Pais and SB. Treiman, Phys. Rev. Letf. 35(1975) 206. [5.10] B. Barish eta!., Caltech Preprint CALl 68 :603 (1977) unpublished. [5.11] J. von Krogh et a!., Phys. Rev. Lett. 36(1976) 7)0. [5.12] C. Baltay, Aachen Neutrino Conf. (1976). [5.13] B. Barish eta!.. Phys. Rev. Lett. 38 (1977) 577. [5.14] A. Benvenuti et a!., Phys. Rev. Lett. 38(1977)1110. [5.15] A. Benvenuti eta!.. Phys. Rev. Lett. 38(1977)1179; C. Albright eta!. Phys. Rev. Let). 38(1977)1183; V. Barger et a!.. Phys. Rev. Let). 38(1977)1187. [6.1] J. Steinberger, CERN School of Physics, Wépion, Belgium (1976), CERN Report (unpublished). [6.2] M. Holder et a!., CERN Report (1977), submitted to Phys. Left. [6.3] M. Holder et a!., CERN Report (1977), Phys. Rev. Lett. 39(1977)433. [6.4] P.C. Bosetti et a!., CERN/EP/Phys. 77-37 (unpublished) submitted to Phys. Lett. B.