Experimental study of the reaction νμe−→μ−νe

Volume 122B, number 5,6

EXPERIMENTAL

PHYSICS LI'TT'I'ERS

17 March 1983

vlae- ~ li-Ve

STUDY OF THE REACTION

CHARM Collaboration

F. BERGSMA, J. DORENBOSCH, M. JONKER, C. NIEUWENtlUIS, F. U i ) O NIKItt'JF, Amsterdam, l'hc Netherlands J . V . A L L A B Y , U. A M A L D I , G . B A R B I E L L I N I M. M E T C A I , F , J. P A N M A N , K . W I N T E R

1, L. B A R O N E

2, A . C A P O N E

2,W. FLEGEL,

CERN. Geneva. Switzerland I. A B T , J. A S P I A Z U , F . W . B ( ) S S E R , t1. D A U M A N N , K . H . R A N I T Z S C I I , P. S T J k H E L I N

P . D . G A L L , E. M E T Z , F. N I E B E R G A L L ,

11. lnstitut fi~r Experimentalphysik 3, Unirer~itiit llamburg, Hamburg, Germany

P. GORBUNOV, E. GRIGORIEV, V. KAFTANOV, V. KttOVANSKY, A. R O S A N O V Institute for Theoretical and Experimental Physics, Moscow. USSR and A. BARONCELLI4,

B. B O R G I A S , C .

F. d e N O T A R I S T E F A N I

BOSIO4,F.

FERRONIS

E. L O N G O S , P .

5, p. P 1 S T I L L I 5. C. S A N T O N I 4, L. T O R T O R A

MONACELLIS,

4 a n d V. V A L E N T E

~'

Istituto Na:ionale di t.'isica Nucleare, Rome, Italy Received 14 December 1982

We present data of the r e a c t i o n v j s e - ~ t a v e, the inverse of m u o n decay, collected in a long exposure of the CHARM neutrino detector to the CERN SPS hornfocussed wide band neutrino beam. After corrections for efficiencies, a sample ot" 594 * 561stat.) + 22(syst.) events has been found. The ratio of the measured and expected cross sections is 0.98 t 0.12 assuming pure V - A interaction. In an SU(2)I. X SU(2) R X U(1) model containing two charged intermediate vector bosons, one can relate this ratio to the ratio of the masses of the two vector bosons and to their mixing angle w. I r o m the result of this experiment, the limitsMlWe)/M(Wl) ~ 1.9 and w ~< 15 ° at 90'), cl can bc derived. A search Ior the r e a c t i o n / ; u c -, u be, allowed by multiplicativc lepton n u m b e r conservation, gave a limit of o(b-~e -* u Ve)/O(v~e " ~ ta ~'e) < 0.05, at 90";• confidence level.

I On leave of absence from INI'N, LN Frascati, I. rascati, Italy. 2 On leave of absence from lstituto di Fisica, Universit~t di Roma, Rome, Italy, and INt.N Sezione di Roma, Rome, Italy. 3 Supported by Bundesministerium ffir l-orschung und Tcchnologie. Bonn, G e r m a n Federal Republic. 0 031-9163/83/0000-0000/$

03.00 © 1983 North-Holland

'~ INI:N Sezione Sanitzl and Istituto Superiore di Sanit~t, Rome, Italy. s lstituto di I.'isica, Universit'2t di Roma, Rome, and INr:N Sezione di Roma, R o m e , Italy. 6 Laboratori Nazionali INI:N I.rascati, I.rascati, Italy. 465

Volume 122B, number 5,6

PIIYSICS LETTERS

A study of the reaction r u e - -* la-Ve

(1)

usually called inverse ninon decay, provides information about the chiral structure of the leptonic charged current interaction [1] and the two-component nature of neutrinos [2]. Assuming completely left-handed charged leptons and making no assumptions on the helicity of the neutral leptons, the differential cross section for reaction (1) can be written as [3] do/d(cos 0") = [G2(s - m2)2/16ns] X [(1 +P)(1 - X)(I - cos 0*)(a -- b cos 0 ' ) + 4(1 - P)(1 + X)l,

(2a)

where 0* is the angle in the cm system between the incoming v u and the outgoing/a-, a = 1 + m 2 / s , b = 1 - m 2 / s , X = - 2 ( g v g A ) / ( I g v I 2 + IgAI2), and P = [N(VR) - N(VL)]/[N(VR) + N(vi_)] is the neutrino polarization. In the limit s >> m 2 the cross section (2a) can be expressed in terms of the inelasticity y = (1 - Eu/Ev) do/dy = G2(s/4n)

× [(1 +/')(1 - x ) ) , 2 + (1-/')(1 + X)l.

(2b)

The v 2 term in (2b) describes the scattering of possible right-handed vu by left-handed e - , coupled by S, P terms in the effective lagrangian. A pure V - A structure of the interaction implies ?, = 1, and left-handed two-component neutrinos implyP= -1. In a different approach reaction (1) can be described in terms of a l e f t - r i g h t symmetric electroweak model based on the gauge group SU(2)L × SU(2)R × U(1) [4]. In this model two charged gauge bosons exist, W L and WR, whose exchange in reaction (1) produces respectively the constant part and the y2 part in cross section (2b). A symmetry-breaking mechanism causes the W L and WR to mix with an angle co forming two mass eigenstates W 1 and W 2. When co = 0 and M(W2) = 0% Wl =WL is the ordinary W-boson of the standard model. In this model the rate of reaction (1), normalized to the integrated V - A cross section, is a function of the mass ratio r = M(W 1)/M(W2) and of the mixing 466

17 March 1983

angle co [5]. A measurement of the rate of reaction (1) then tests directly the V - A coupling dominance of all the leptons involved, and in the l e f t - r i g h t symmetric model it limits the mass ratio of the two charged bosons and their mixing angle. Theories of multiplicative conservation of the lepton number [6] allow the final state of reaction (1) to be induced also by "wrong" neutrinos: ~ e - ~ / ~ - Ve •

(3)

Up to now only few results have been presented on reaction (1), because of the high energy threshold and the small cross section [7,8]. We have studied reaction (1) and searched for reaction (3) using the CHARM neutrino detector, exposed to horn-focussed wide band neutrino and antineutrino beams produced by 400 GeV protons from the CERN SPS, with a mean energy of 31 GeV for the neutrino and 24 GeV for the antineutrino beam. The data presented here have been taken from 1979 to 1981; the previously published data [8] concern only 1979 wide band runs and are part of the sample we present here. A detailed description of the CItARM detector may be found elsewhere [9]. Briefly, it consists of a segmented ionization calorimeter surrounded by a magnetized iron frame, and a muon spectrometer. The calorimeter is made of 78 modules, each one consisting of a marble plate of 3 × 3 m 2 surface area and 8 cm thickness followed by a plane of 128 proportional drift tubes and a plane o f 20 plastic scintillators oriented at 90 ° with respect to the tubes. The muon spectrometer is composed of four toroidal iron magnets providing information for the identification and the momentum analysis of forward going muons. Each magnet is segmented in iron disks, 5 cm thick in the first module (the "end calorimeter"), and 15 cm thick in the next three modules (the "end magnet"), for a total iron length of 3 m, with an external diameter of 3.7 m, and a central hole of 16.7 cm diameter. Eighteen planes o f proportional drift tubes, interspersed between the disks, are used to measure the muon momentum with a resolution of 16% on average. Six planes o f scintillation counters have been inserted in the gaps of the last three magnets to detect and to measure muon "bremsstrahlung" energy loss. The detector was triggered requiring that at least four scintillator planes were hit, that the detected

Volume 122B, number 5,6

PIIYSICS LETTERS

ionization loss was larger than 50 MeV, and that a track had penetrated through at least four planes o f the muon spectrometer end magnets, equivalent to 1.35 m of iron. In later runs it was further requested that at least nine consecutive planes in the calorimeter were hit. The two samples have been compared in all the relevant variables, and found compatible with each other. The particular characteristics of reaction ( l ) are the energy threshold £'v > 10.8 GeV, the small muon emission angle 0u < 10 mrad, and the absence of a visible recoil. Consequently, the analysis aims at selecting recoil-less single-muon events above threshold. A fiducial volume was defined to ensure good track measurement. Tracks starting between plane 3 and 69 (59 for later data), with ly, z I < 150 cm, were accepted. Tracks were required to have a distance from the beam axis of 25 < R ~< 180 cm, to ensure full detection efficiency in the muon spectrometer. Events were selected which satisfied the following conditions: (1) Less than 92 MeV deposited energy in the first six planes of scintillators following the vertex, less than 18 MeV in each of the first three scintillator planes, and the sum o f the deposited energy in the first two planes less than 20 MeV (the peak value of the Landau distribution of the energy deposited by a minimum ionizing particle in one plane of the calorimeter is 6 MeV). In addition, less than 10 MeV in each of the first three scintillator planes was required if more than one proportional drift tube was hit in one of the first three planes. (2) Negative (positive) muon charge for neutrino (antineutrino) runs. (3) The time-of-flight measured between the scintillators in the last plane of the calorimeter and the scintillation counters in the last magnet of the muon spectrometer was compatible with a track following the direction of the neutrino beam. Out of 43478 neutrino and 99596 antineutrino quasi-elastic candidates, the final sample selected in this way contains 16843 neutrino and 44703 antineutrine events. Above E v = 10 GeV the q2 dependence of quasielastic scattering

run ~ ,u -p

(4a)

17 March 1983

and of one-pion production vuN ~ / I - n N

(4b)

is expected to be energy independent and equal for v and for ~ on an isoscalar target. On the other hand the reaction (1) is expected to occur at very low values of q2. Our selection criteria imposed a limit o f q 2 < 0.1 GeV 2 on the detection of quasi-elastic v and P scatterin g, because of the rejection o f proton recoils at larger q ~ a n d excluded at the same time events belonging to reaction (4b). This fact allowed us to separate the background from the signal in the v sample, in the region 0 < q 2 < 0.02 GeV 2, by subtracting the q2 distribution of the P sample normalized to the equivalent v sample in the range 0.02 ~< q2 < 0.1 GeV 2. We approximate q2 by p~, the muon transverse momentum squared, because 0u is very small. For q2 < 0.02 GeV 2 we observe an excess of/a events (fig. 1),

Nv(la- ) - ~Nv (/a+) = 937

(0.407 X 1355)

= 386 -+ 3 6 ,

(5)

where e = Nv(t.t ; 0.02 ~< q2 < 0.1 GeV2)/Nv(la+; 0.02 ~< q2 < 0.1 GeV2), which we attribute to reaction (1). The binning in fig. 1 is of the same order of our q2 resolution. '

'

'

'

I

i

600

,

,

m

,

H. ÷

o

z

400

b.J

13

z

20C

i

o

I

I

1

I

1

o.05 qZ ( GeV )2

i

I

I

o

l:ig. 1. q 2 distribution of single recoil-less/a and U+ events, normalized to equal numbers for 0.02 ~
~

/a

v e.

467

Volume 122B, number 5,6

PItYSICS LETTI-RS

l ' h e efficiency of the selection criteria has been determined experinaentally by applying them to later parts o f the same tracks, starting at least 18 planes after the vertex, with the result: esc I = 0.882 + 0.003 .

(6)

T w o other corrections must be applied to the n u m b e r of events obtained: (i) for the efficiency ecut of the cuts (q2 < 0.02 GeV 2 and p , > 10 GeV/c), which is estimated using the observed 1:',, spectrum and the simulated d e t e c t o r resolution Ccut = 0.789 + 0.030 ,

(7)

(ii) for the m u o n track fitting efficiency Etit 6fi t = 0.934 -+ 0.002 .

(8)

The q u o t e d errors of the efficiencies are mainly o f systematic origin. Correcting the result for the efficiencies (6).---(8) we find ; i

500-

I

N(uue-

17 March 1983 ~ At re) = 594 + 56(star.) + 22(syst.)

(9)

for a total n u m b e r of acceptance-corrected inclusive o n e - m u o n events with Evis > 10 GeV o f ( 1 . 0 4 6 -+ 0.063) × 106, or a relative rate o f R = N ( u u e - ~ At- U e ) / N ( u u N -+ At X ) l E v i s > 10 (;eV = [5.68 + 0.53(stat.) + 0.40(syst.)] X 10 4 Integrating eq. (2) over the observed l:'t, spectrum, we obtain the predicted n u m b e r of inverse m u o n decays

N=.~N0[(I+P)(1

X)C+8(1

-- P)(I + X ) ] ,

(10)

where C = 2.98 is given by the integrated neutrino spectrum and N 0 is the expected number of events for pure V - A interaction. Assuming respectively left-handed or right-handed neutrinos, we predict the following rates for reaction (1): - - - ' r - -

: 1

- T

!

T

" -

---~

-'T

-

-

b

a --

v-A

15C

200-

--

v-A

I

%

10C

3

~D

_o

O

ioo

-

z

z 5©. LaJ

! o-

j---

. . . . .

J

°I

I

-40. I I

o

i

i

i

i

]

i

;

0.05 qZ (GeV)z

i

i

0. I

o

50

IOO

py- ( GeV/c )

Fig. 2. (a) Observed q 2 distribution of the difference N u ( u - ) - e:V~u+), normalized to equal rate for 0.02 ~ q2 < 0.l GeV 2, compared to the prediction for inverse muon decay' assuming V --A coupling and left-handed neutrinos (solid line). (b) Observed u - momentum distribution of the differenceNvOa-;q 2 < 0.02 GeV 2) - Nu(ta-;q 2 > 0.02 GeV 2) normalized to 386 events, and V - A prediction for inverse muon decay. 468

N 1 =607-+35,

N2 = 2 2 6 ± 1 3 .

Comparing these predictions with the observed signal (9) we obtain: RI =0.98-+ 0 . 1 2 ,

17 March 1983

PIIYSICS LI.ITTt-RS

Volume 122B, number 5,6

R2 = 2.63 -+ 0 . 3 3 .

R 1 expresses the ratio between the observed rate and the rate expected from V A coupling among all the leptons in reaction (1), while R 2 excludes at the level of 6 standard deviations the d o m i n a n c e of S, P couplings in the interaction. Fig. 2a shows the q2 distribution of the inverse m u o n decay candidates and fig. 2b shows their P m o m e n t u m distribution. We note that fig. 2b does show the expected threshold behaviour of reaction (1). In fig. 3 we compare our result in terms o f V - A interaction and left-handed neutrinos to the previous experiments [7,8], and show the statistical significance and correlation with which the parameters X and P of eq. 12) are determined. Evaluating now R 1 ill the SU(2)L X SU(2) R X U(1 ) model [5], as a function of the ratio of the masses r = M(WI)/M(W2) and of tire mixing angle co we lind the 90¢7~ confidence limits co ~< 15 ° and M(W2) ~> 1.9 × MlWl). We conclude that the V A structure of the le D tonic charged current interaction and the two compo-

n e n t theory with left-handed neutrinos is confirmed again, on the basis of four times larger statistics than in the previous result [8]. Candidates of reaction (3) have been searched for in the a n t i n e u t r i n o exposure. We observed 9406 recoil-less single-/a events in a n t i n e u t r i n o runs, and found an excess of 120 ± 20 # events with p u > 10 GeV/c and q2 < 0.02 GeV 2. We expect a c o n t r i b u t i o n from reaction (1), due to n e u t r i n o c o n t a m i n a t i o n of the a n t i n e u t r i n o beam. t h i s can be c o m p u t e d by scaling the observed excess in n e u t r i n o runs, taking into account the fitting efficiencies for focussed and defocussed m u o n s : 386 × 9 4 0 6 / 4 3 4 7 8 = 84 e v e n t s . A n o t h e r c o n t r i b u t i o n to the p - sample in antineutrino runs may be due to the reaction ue e

-+p

/)u

(11)

induced by the Ue c o n t a m i n a t i o n of the a n t i n e u t r i n o beam. This can be c o m p u t e d from the knowledge b f the beam composition to be: 84 X -_'V(ue)× <_E(#e_)! x _o(~£e) = 7 e v e n t s , (l:'(v,)>

,'V(vu) o(v,e) with N(Ve)/N(vu)= 0.11 and (E(Fe))/(E(vu)) = 1.89 derived froln the calculated n e u t r i n o flux spectra [10] ~', and o(vee)/o(vue)= 3/8 for V - A interaction [3]. Tire candidates of reaction (3) are thus 120 - 84 - 7 = 29 -+ 2 2 .

,--.

Ji=

90 % C.L.

OO~/'" /t CHARM

,,,/ C~c",~2~ ' 0

o(Pue-'la__Pc! = A'. . .(3)/A . . . . . . . ~.-(./a o(uue-+ta Ue) N(l)/,'Vv(~ •

I

-!L -I

If we assume a multiplicative lepton n u m b e r conservation [6] which allows reaction (3) to occur, we can evaluate the ratio between cross sections of reaction (1) and (3), with the result

I

X : -2% % I%12. .gAle I"ig. 3. 905; confidence contour of the V A parameter X and left-handed neutrino polarization P deduced from the observed rate of the inverse muon decay reaction. Previous limits from the (;argameUe Collaboration [71 and from the CHARM Collaboration [8] are shown as well.

I

+

) <0.05 )

(12)

at the 90% confidence level, where N,-(,u+) and A,',(/J ) are respectively the n u m b e r of quasi-elastic antineutrino and neutrino events. Our result seems to exclude multiplicative lepton n u m b e r conservation ,z I The fluxes have been checked using the neutrino beam Monte Carlo program by Visser II 1 I. J-2 Willis et al. {121 give a 90:.; confidence upper limit ~d" R < 0.098. 469

Volume 122B, number 5,6

PHYSICS LETTERS

We w o u l d like t o express o u r g r a t i t u d e and apprecia t i o n to o u r n u m e r o u s t e c h n i c a l c o l l a b o r a t o r s a n d to J. A u d i e r , C. Busi a n d G. DeBilio for t h e i r c o m p e t e n t help w i t h the d a t a h a n d l i n g . We wish to express o u r g r a t i t u d e to Dr. A.N. D i d d e n s a n d Dr. A.M. Wetherell for their c o n t r i b u t i o n s to this e x p e r i m e n t . We t h a n k G. Altarelli and J. M a a l a m p i for e n l i g h t e n i n g discussions.

R eferen ces

[ 1 ] E.('.G. Sudarshan and R.E. Marshak, Phys. Rev. 109 (1958) 1860; R.P. Feynman and M. Gell-Mann, Phys. Rev. 109 (1958) 193. [2] H. Weyl, Z. Phys. 56 (1929) 330; A. Salam, Nuovo Cimento 5 (1957) 299; L. Landau, Nucl. Phys. 3 (1957) 127; T.D. Lee and C.N. Yang, Phys. Rev. 105 (1957) 1671. [3] C. Jarlskog, Lett. Nuovo Cimento 4 (1970) 377; M.K. Gaillard, SPSC Workshop on Neutrino physics. CERN/SPSC 78-153 (1978) p. 71.

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[41 J.C. Pati and A. Salam, Phys. Rev. D10 (1974) 275; H. t.ritzsch and P. Minkowski, Nucl. Phys. B103 (1976) 61; M.A.B. Beg, R.V. Budny, R. Mohapatra and A. Sirlin, Phys. Rev. Lett. 38 (1977) 1252. [5] J. Maalampi, K. Mursula and M. Roos, University of Ilelsinki preprint ItU:FI.T-82-4 (1982). [6J N. Cabibbo and R. Gatto, Phys. Rev. Lett. 5 (1960) 114; G. Feinberg and S. Weinberg, Phys. Rev. Lett. 6 (1961) 381. [71 Gargamelle Collab., N. Armenise et al., Phys. Lett. 84B (1979) 137. [8] CIIARM Collab., M. Jonker et al., Phys. Lett. 93B (1980) 203. [91 CItARM Collab., A.N. Diddens et al., Nucl. Instrum. Methods 178 (1980) 27. [ 10] ABCMO Collab., P.C. Bosetti et al., Proc. Intern. Conf. Neutrino 78 (Purdue University) p. 939. [ 11 ] C. Visser, Neutrino beam Monte Carlo program ( 1979). 112] S.E. Willis et al., Phys. Rev. Lett. 44 (1980) 522; 45 (1980) 1370 (E).