Neutrino physics with the Frejus underground detector

167

Nuclear Physics B (Proc . Suppl.) 14B (1990) 167-178 North-Holland

NEUTRINO PHYSICS WITH T E FREJUS UNDERGROUND DETECTOR Hans - Jürgen DAUM ( Aachen - Orsay - Palaiseau - Saclay - Wuppertal Collaboration ) Fachbereich Physik, Bergische Universität Gesamthochschule Wuppertal, Gaußstraße 20, D-5600 Wuppertal 1 Federal Republic of Germany The final data sample of neutrino interactions in the Fréjus detector recorded during 1245 live days is compared to the Monte Carlo prediction of atmospheric neutrinos . Good agreement is found between data and simulation . The exclusion regions for v. - vN, andy,, :vr oscillations can be extended down to AM 2 = 7* 10 -4 eV 2and 6* 10 -4 eV 2 respectively. The high energy neutrino interactions are used to set energy dependent neutrino flux on annihilation neutrinos from possible cold dark matter candidates trapped in the sun or in the earth. lations of atmospheric neutrinos based on recent flux INTRODUCTION

In recent years the study of atmospheric neutrinos has been a key point in large mass underground experiments

[1-7] dedicated to proton decay. Ultimately the sensitivity to nucleon instability will be limited by interactions of atmospheric neutrinos in the apparatus. Good understanding of atmospheric neutrino properties and interac-

tions is cntcial in order to improve neutrino background suppression in nucleon decay searches. Results on the

study of atmospheric neutrinos have been published by most of the experiments [8-11] .

On the otherhand deep underground experiments allow to attack some outstanding physical questions, not access-

ible by other means. The apparent solar neutrino problem [12,131 may be solved by neutrino oscillations [141 . Although the total statistics of neutrino interactions recorded in proton decay experiments is still very limited significant limits can be obtained from the data. The observation of missing matter in galaxies [15] may have particle

calculations including muon polarization effects; [1-8,191. The experimental ratio ve / v~L will then be used to define

exclusion regions for ve- vtL and vtL - vz oscillations . The high energy neutrino interactions are used to search for correlations with the sun and the center of the earth. Corrected integral flux limits for both sources will be given. Finally implications for possible dark matter candidates will be discussed.

THE EXPERIMENT The Fréjus nucleon decay detector is installed in an underground laboratory in the Fréjus highway tunnel

connecting France andItaly. The geographical coordinates of the laboratory are45 08'32"north and 6041' 21 "east . The rock overburden for cosmic ray muons reaching the detector averages to 1780 m. The detector has been described in detail in Ref. [201 . The fine granularity of this 900 ton tracking calorimeter is

physics aspects [16,17]. Possible dark matter candidates are weakly interacting massive particles which may be

achieved by a sandwich structure consisting of 912 vertical flashchamber (5mm x 5 mm cells) and iron (3mm) planes interspersed with 113 planes of Geiger tubes (15mm x

trappeddark matter is expected from the center of the sun or the earth. Limits on this flux may be inferred from the

event.

trapped by gravitational sources. Consequently a flux of high energy neutrinos produced by the annihilation of

measured neutrino spectrum .

In this paper the neutrino interactions observed in the Fréjus detector will be compared to Monte Carlo simu0920-5632/90/$03.50 © Elsevier Science Publishers B.V . (North-Holland)

15mm cells ) which provide the trigger. The detector cells are ~ :riented vertical and horizontal alternately, thus providing two independent orthogonal views for each

The trigger requires grouped hits in a small volume(,m3 ) typical for nucleon decay events corresponding to an energy thresholdof about 200 Me V forneutrino inter-

H.-J. Daum/The Fiéjus underground detector

168

actions. The average trigger rate is 45 hr -1 . Half of the

associated with the track . These tests yield an identifi-

events are due to cosmic ray muons whip the rest is induced

cation probability of 85% for showers ( e,y ) and 90%

produced by interactions of atmospheric neutrinos is about

95% above400 MeV for both particle types. No distinction

by local radioactivity and electronic noise. The event rate one event per week .

Continuous data taking started February 19, 1984 with a

mass of 240 tons . The size of the detector was gradually increased until the final mass of 900 tons was reached in

June 1985 . The experiment was stopped September 13, 1988.

All events areclassified during data taking by the physi-

cists on shift. Most of the neutrino interactions are found directly by this on-line scan . Subsequently the events are processed by an automatic selection program. The combined detection efficiency for neutrino events a 98%.

In oder to avoid additional measurement errors at low energies due to particles leaving the detector, a fiducial cut is applied to the events. A minimum distance of the neu-

trino vertex with respect to the su:ace of the detector is required dependingon the physics under consideration . For the studyof thegeneral features of neutrino interactions and the investigation of neutrino oscillations this cut is setto 50

cm. Since the neutrino direction is sufficiently well reconstructed at higher energies even if a fraction of energy

is leaking out the fiduci:i1cutcan be weakened to 25 cm for

the dark matter analysis thus increasing sensitivity. This reduces the fiducial mass to 554 tons and 700 tons leadingto an exposure of 1 .56 and 2.00 kiloton-years (kty) respectively .

Different from the analysis of ref [111 the results presented here are based on measurements using a pattern recognition program. Onl, the vertices of the events are determined visually on a graphic terminal while track finding, association of tracks in the two independent vi6ws, as

is made between electrons and photons. The visible energy of an event is determined by simply summing up the four

momenta of all reconstructed particles by assuming all tracks not interpreted as muons to be pions throughout this analysis .

Subsequently for each particle acontainmentis defined.

A track is considered to be contained if the extrapolation

ATMOSPHERIC NEUTRINOS

particle type identification

nonshowering particles ( p.,n ) at 200 MeV and more than

showers ( e,^l ' or

tracks(p,n,K,p) and momentum determination is done by program. The particle type of each track is determined

using a maximum likelihood test on the transverse profile and on the distribution of hits per plane for the hits

from the stop pointup to the surface of the detector crosses at least5 unlit planes . A shower is defined as containedif all its hits are at least 5 cm from the detector edges. The events

are contained if all particles are contained. In the analysis both contained and uncontained events are considered.

The events are classified as charged current ( CC R), v~L charged current ve ( CC e ) and neutral current ( NC ) interaction ( the symbol v denotes neutrino and antineutrino) according to a possible lepton at the vertex . An

event is defined as CC it if the event contains a noninteracting tracklinked to the neutrino vertex either with arange of more than 300 g cm-2, which corresponds to 3 interaction

lengths, or with a momentum, interpreted as muon, higher than 200 MeV andthe event contains neither a shower with

more energy nor an interacting track with a larger range before interaction. If thesecriteria are not satisfiedthe event is interpreted as CCe, ifit contains a shower produced at the neutrino vertex with an energy in excess of 200MeV and no

interacting track exists in the event with an energy loss larger than the shower energy before the interaction point. Events whichdo not fitoneof both categories are definedas NC .

Since the directionality of uncontained tracks cannot be

determined reliably, an additional cut is applied to the CCg events to avoid background induced by entering muons stopping in the detector . Uncontained CC g events are rejected either if they consist of a single track or if the total number of hits at the vertex not associated to an upward

pointing uncontained track is less than 14, which corresponds to twice the mean number of hits produced by

the decay positron of positively charged stopping muon.

H.-J. Daum/The Fréjus underground detector This cut reduces the total background to less than 3% . The probability for CC p.interactions to remain in the sample is

for CC p. and 84% for CC e events.

found to be 85% while the CC e and NC events are not affected. After all cuts 184 and 217 events are selected for 1 .56 and 2 .00 kty respectively.

This data sample is compared with predictions of a model of neutrino production by the decay of hadrons and muons produced in the cascade of primary cosmic ray interactions in the atmosphere . A simulation of neutrino andantineutrino interactions in the detector corresponding

169

Data

CCA CCe NC

total

vin,".s 108 62

14

184

CCe/ Clt .57± .09

" [î iair 74 54

10

138

.73± .14

Monte Carlo

I all events contained 113.9 69.7 69.9 13.4

196.9

.61± .08

60.4 11 .1

141.2 .87± .10

to an exposure of 15 .7 kty in the total detector volume has

been performed for neutrino energies between 200 MeV and 100 GeV. The energy dependence of the neutrino flux

at theFréjus latitude is taken from ref. [ 18,19] using a mean

solar activity . This flux has been extrapolated to 100 GeV including thecalculations of ref. [21] . The simulation of the

neutrino interactions in our apparatus is described in ref. [I 1] .The response of thedetector to electrons andpions has

TABLE 1 Neutrino event rates classified as charged and neutral current interactions . The numbers correspond to 1 .56 kty for data and Monte Carlo. The ratios CCe/CCpare given with statisticalerrors for thedata andestimated systematic errors for the simulation .

Table 1 shows theeventrates classified according to the

been calibrated [20] with a test detector of identical

lepton flavor and containment for the data in comparison

The trigger efficiency for charged current interactions

simulation are scaled to an exposure of 1.56 kty. Since the

structure at DESY and at the synchrotron at Bonn .

has a threshold at 200 MeV and reaches about 80% for neutrino energies of 1 GeV. The uncertainty on the trigger

with the Mote Carlo expectation . The numbers for the prediction of tire absolute rate has large uncertainties, it is

efficiency introduces a systematic error of 10% for CC e

better to compare the ratio of CC e / CC lt, which is less sensitive to the flux uncertainties. In table 1 the experi-

rises from a few percent at 1 GeV to 80% at 10 GeV.With

simulation . The error given for the Monte Carlo prediction includes a systematic error which is due to uncertainties in

events while for CC g interactions this error is negligible . The trigger efficiency for neutral current events is low. It respect to neutrino energy the triggerthresholdof the Fréjus

detector is very similar to those of the Cherenkov detectors [8] since at low energies most of the particles produced in

the neutrino interactions are unobservable with the Cherenkov technique. In addition the Fréjus detector is capable to observe and measure neutrino interactions nearly without limitation up to very high energies .

Ti :e simulated events were selected and measured in the

same way as thedata. Forasensitivity of 1 .56ktywe expect

mental ratio is compared to the values obtained from the

the neutrino flux calculations and the triggerefficiency for CC e events . The data agree well with the simulation. Especially no indication for adeficitofCC ttinteractions is observed as wasreported by the Kamiokande collaboration in ref. [9] .

The visibleenergy distributions forall events andseparated according to the reconstructed neutrino flavorsarepre-

sented in figure I for neutrino interactions occurring in the 554 ton fiducial volume. The observed zenith angle distri-

systematic error on the absolute rate is 20% resulting form

bution for CCp.and CCe events is shown figure 2. Within statistics the observations are in good agreement with the

chac,ge is expected due to variation of solar activity . From the simulation the lepton flavor identification probability

induced by neutrino oscillations or by dark matter

197 events while 184 events are observed in the data . The

the uncertainties in flux and simulation . No significant

using the pattern recognition program is estimated to 91 %

model predictions . Therefore the data allows to search for possible deviations from the expected neutrino spectrum annihilation .

H.-J. Daum / The R djus underground detector

170

30

c

a)

30

w > 20

20 10

10

a

W

0 -1,0

30 20

b) v

20

-0,5

0,0

0,5

1,0

h

b)

v e

10 5 15

-1,0

-0,5

0,0

1,0 0,5 cos ( O )

10

FIGURE 2 Zenith angle distribution for (a) CCp,and (b) CCe events. For aata and Monte Carlo prediction the same convention as in figure 1 is used. Upward directed neutrinos correspond to cos( O) values equal to 1 .

5 0

E [GeV] FIGURE 1 Visible energy distribution ofneutrino interactions for (a) -all, (b) CCN, (c) CCe events for a fiducial volume of 554 tons. The histogram shows the expectation of the simulation scaled to 1.56kty.The errors on the data are given by the variances from the Monte Carlo prediction. 25 Events are observed above 3 GeV while 27.8 are expected. NEUTRINO OSCILLATIONS If neutrinos are massive the Neutrino flavors (ve,vW v,t ) produced in weak interactions will be different from the neutrinomass eigenstates vl,v2, v3 as long as the neutrino masses are not degenerate . As a consequence the weak interaction eigenstates will evolve with time leading to neutrino oscillations . In the case of two family mixing the

probability for a neutrino created as viwith momentum pv [MeV/c] to be observedat a distance L [m' asyviis given in vacuum by P (vi ->vj ) = sin 2 20 sin 2 (1 .27 Orn2 L /E v)

(1)

where sin 20 and Grn2 [eV 2] denotes the neutrino mixing angle and thedifference ofthe mass squared ofthetwomass eigenstates. In largemass underground experiments neutrino oscillations can be studied usin g atmospheric neutrinos if the experiment provides neutrino flavor tagging. Neutrinos produced in the cascade of primary cosmic ray interactions in the atmosphere reach the detectors from all directions . The distance be w:cen neutrino creation and detection varies by

H.-J. Daum / The Fréjus underground detector three orders of magnitude from about 10 km for downward going neutrinos up to 13000 km for neutrinos passing the

earth. Despite of the flux uncertainties and the low event rate ( about 2 events per week in 1 kiloton ) these large

(NAis Avogardro's constant) since most of theupward going neutrinos only path through the mantle of the earth,

where the density varies slightly, without hitting the core .

distances together with the simultaneous observation of ve andvAevents allows to studytheregion of small Amtnotyet covered by other experiments .

For the studyof ve- v9oscillations the MSW effect [ 14]

v W

30 20

has to be included. Passing through matter, e.g . the earth, electron neutrinos behave differently from other neutrino

species since only for electron neutrinos both Z and W

10

exchange contribute to scattering off electrons . If the electron neutrino dominantly consists of the lightest mass

eigenstate vl the oscillation probability is enhanced for

neutrinos, even for small mixing angles, if the resonance condition

~8GF Ne Aw,2

=cps20

(2)

is met, where Ne stands for the electron density. For very

small Am2 the left-hand side of (2) is much larger than one andneutrino oscillations are suppressed. With the assumed

mass hierarchy oscillations of electron antineutrinos are always suppressed in matter . Az the left-hand side of (2) goes to zero the vacuum limit will be reached for both neutrinos and antineutrinos .

We have investigated v~ - v,r anti ve - e,A t;.cillat:ons

using the CC e / CC g ratio for all events interacting in the 554 ton fiducial volume. ve - v,, oscillations are not considered because of statistics. For each set of oscillation parameters we have calculated the expected neutrino

_

E [GeV]

FIGURE 3 Visible energy distribution of vg interactions with vp,-vj oscillations forAmt =10 -2 and sin220 = 0.7 . These parameters arefavoured by the I{arniokande observation [23] .

To illustrate the influence of neutrino oscillations on the measured neutrino spectrum, figure 3 shows the visible energy distribution for CC p. interactions in comparison with theexpectation forv9-- v,,oscillations with Amt =10-2 e,12 art; sin220 = 0.7, parameters which are suggested by

the Kamiokande data [23] .The discrepancy between the Fréjus data and the simulation including neutrino oscillations is obvious for this choice of oscillation parameters .

Using theratio CC e ./ CC p.tbissetofoscillation parameters is excluded by our data at 95 % confidence level.

spectrum and. the ratio of CC e / CC g derived from the reconstructed neutrino flavor. Each simulated event is weighted according to its oscillation probability calculated from the generated neutrino favor, direction and energy

taking into account the theori-tical ve / v1L ratio including muon polarization effects (19] .

For ve - v. oscillations we have taken hito account the MSW effect assuming the electron neutrino to be the

lightest neutrino . Although the radial density distribution of the earth exhibits acomplex structure 122] we used in our

analysis aconstant valueforthe electron densityof 2.5*N A

TAB' E 2 .02 Limits on neutrino oscillation parameters for Am 2 > 0 eV2 and for maximum mixing.

H.-J.

172

Daum /The F%jus underground detector

The experimental ratio CC e / CC It is compared to the Monte Carlo expectation without neutrino oscillations in

4 energy it is nearly impossible to be sensitive to Amt<10eV 2 using atmospheric neutrinos even with high statistics

table 1. Including the systematic error of 0.08 dueto uncertainties in the neutrino flux, cross section and CC e trigger

due to the suppression of oscillations in matter as discussed

0.73 and0.77at 90 %and95 %confidence level respectively. Theexclusion regions forve -v9and vlc vtoscillations

much lower neutrino energy thresholds would be needed.

efficiency the data exclude values of this ratio in excess of

as determined from the ratio CC e / CC It are shown in figure 4 together with the results obtained by the G6sgen

reactorexperiment [24] for ve - vAoscillations and by the

above. To circumvent this problem, present in currently running nucleon decay experiments, detectors having

Unfortunately the directionality in the neutrino nucleon scattering gets completely lost at low energies andfurther-

more CC g interactions will be suppressed due to the muon mass.

The IMB collaboration has excluded v.- v,toscillation

CDHS accelerator experiment [25] and the IMB proton decay experiment [8] for v,, - v t oscillations. Table 2

in the range 4.2*10-5 eV2 < Am2 <5 .4* 1Ô'4 eV 2 using the

periment.

events with muon decay signature [8]. They observed 15

summarizes the oscillation limits obtained by the Fréjus ex-

ratio of upward to downward directed contained one prong upward and 21 downward goingCC4events with Icos El I >

0.6. Although with this angular cut the statistics of CC4 events in the Fréjus experiment is comparable (17 upward

and 24 downward going events) it is not possible to reach such a low limit in Am 2 with our data. Common to all

experiments is the bad pointing accuracy due to the

kinematics of neutrino interactions below 1 GeV which

reduces the netobservable effect drastically . For ourdata

with a mean neutrino energy of 1.6 GeV we wouldexpect for maximum mixing 11 .6 upward and 19 .6 downward

going CC4 events at Amt=1.5*104 eV2 where the effect

due to v4-v,t oscillations should be maximal in the up to

down ratio. This has to be compared to the expectation of 18 .9 and 21 CC4events in the case of no oscillation . With

the limited statistics we consider it to be impossible to .J &-_ _ . _ . 10 -5 ' - . 10-s 0,0 0,2 0,4 0,6 0,8 1,0 0,0 0,2 0,4 0,6 0,8 1,0

sin 2 20

sin 2 20

FIGURE 4 Exclusion plot for the oscillation parameters sin220 and Am2forve-vgandv -v' oscillations . Theareaontheright of the curves (full ~ines)c are excluded at 90 % and 95 % C.L . respectively . The dashed lines indicate the results of other experiments . The sharp limitation of the exclusion area at lûw Am 2 for

vévA oscillations reflects the influenceof the MSWeffect.

The neutrino energy of the events observed in the Fréjus detector averages to 1 .6 GeV. For this mean neutrino

exclude v4 -V,t oscillations at such low Am 2 especially

with smaller mixing angles using the up to down ratio of muon neutrinos alone. DARK MATTER

It was already pointed out more than 50 years ago [26]

that a huge amount of dark matter is needed to explain the

redshifts observed in the Coma nebula . Measurements of the rotational curves of galaxies give the most striking

evidence that the major contribution to matter in the universe is invisible [15] . A recent r.viewon the problemof missing matter in the universe can be found in ref [271 .Astrophysics as well as particle physics may explain

H.-J. Daum /The Fréjus underground detector the origin of the dark matter problem. It was argued [17] that

only

nonbaryonic

matter

do

not contradict

observations . To be invisible this nonbaryonic matter should consistof particles having an interaction strength of

103

1 dN L dE

173

[ kty -1 GeV -1 1

at most weak interactions. Popular candidates for these particles are massive neutrinos [16] or the lightest super-

10

symmetric particle [28] whichis stable bec?,use of R-parity

conservation [29] . These weakly interacting massive particles (WIMP) are remnants of the Big Bang and thus

may even account for closure of a Friedmann - Lemaitre universe [28,30] .

Relict WIMPS with masses in the GeV range will have

velocities of theorder of 300 km sec-1 in ourgalaxy. Dueto

r 10-t

this small velocity they may be trappedby elastic scattering

in massive objects like the sun of the earth [31] . The

10 2

the solar neutrino problem also [32] . Trapped WIMMs annihilate with its antiparticles and will produce high 1( energy neutrinos[33-38] by direct pair production ( mono-

10 3 10-'

presence of dark matter in thecore of the sun may explain

lop

101

energetic ) or heavy quark decay. Since pions and Kaons interact before decaying they would not contribute to the neutrino signal. For a given WIMP mass the observable flux depends on the local density for particle and antiparticle ( which is either assumed to be equal or theparticle

is identical with its antiparticle), the capture rate, the evaporation rate and the annihilation cross section. We have analyzed ourneutrino interactions occurring in the enlarged fiducial volume, which corresponds to an

exposure of 2 kty, to set limits on a possible excess of neutrino interactions induced by an additional high energy neutrino flux from the sun or the center of the earth. We have used both contained and uncontained neutrino events

forthisanalysis . In the fiducial volume we observed in total 217 events while 239 events are expected from the simulation of atmospheric neutrinos in the energy range of 200 MeV < Ev < 100 GeV. The visible energy distribution

of the data is compared to the Monte Carlo prediction in figure 5. Good agreement is observed between data and

simulation especially no excess of events is found at higher

energies .

In order to search for a high energy neutrino signal form the sun or from the center of the earth cuts in the visible en-

E [GeV]

2

FIGURE 5 Distribution of the visible energy between 200 MeV and 100GeV forall neutrino interactions in the 700tonfiducial volume normalizedtothe luminosity . Theconvention used for data and Monte Carlo is the same as in figure 1. ergy and in the anglebetween the neutrino and the possible neutrino sourceshave been a )plied as afunction of the neu-

trino energy . The accuracy for pointing back to the direction of neutrino creation depends slightly on the neutrino

flavor due to differences in containment forelectrons and muons produced in charged current interactions. Theenergy dependence of theangularresoluron canbe approximated by

6(EV )=ß.+0.42E .l [rad] (3)

whereEv stands for the neutrino energy and ßo, is the high energy limitof the angular resolution (0.14radian forCCg and0.10radian forCCe events).To study WIMP annihila-

tion in the earth we have to account for the finite radius of the dark matter orbit [35] which gives larger values for ß. reaching the values quoted above for large dark matter masses.

IT-J. Daum /The Fréjus underground detector

174

factor of two higher than the flux of electron neutrinos the limits are nearly

independent of the neutrino flavor

indicating that the background of atmospheric neutrinos is still negligible for the exposure of the Fréjt,s experiment. The small difference in the flux limits for the two neutrino

flavors is

mainly due to different efficiencies for electron

and muon neutrinos. We estimate that two orders of magnitude could still be gained in sensitivity for E, > 10GeV before reaching the atmospheric neutrino level. These flux limits can now be used to set constraints on models predicting a high energy neutrino flux from the sun and the center of the earth produced by dark matter annihilation. Common to all calculations is the assumption

Evmin [ GeV ] FIGURE 6 Corrected upper limits on the integral neutrino flux at 90 C.L. (a) from the sun and (b) from the centerofthe earth for all neutrinos ( full line), CCg events ( dotted) and CCc events (dash dotted

The upper limits at 90 % confidence level on the integral neutrino flux from the sun and from the center of the earth are shown in figure 6 as a function of the minimal neutrino energy up to 100 GeV . The limits are given for all neutrinos and for electron neutrinos and muon neutrinos separaz ly . Our data sets neutrino flux limits for electron and muon neutrinos up to very high (even monoenergetic ) neutrino crriergies which is not the case irr,-~ " her experiment ; either :-ejection Aue to a low energy cutoff of 2GeV [39] or due to of muon neutrinos [40] because of containment cuts. The data are fully corrected and background subtracted . Although the atmospheric flux ofmuon neutrinos is about a

FIGURE 7 Corrected upper itimit on the integral event rate per kty (a) from the sun and ~"b) from the sun and the earth (full line ) compared to the expected rate for Dirac (dotted ) and Majorana neutrinos ( dashed ) from the sun and for Dirac neutrinos (dash dotted) from both tre sun and the earth [33,35] .

** r n .. id . .-J ". ..... .'The Fré j s

that a specific dark matter candidate contributes exclusi-

" ;1y to the galactic Ha'.o with a local density in the solar system of about0.3 GeV cm -3 andwith equal trapping rate for WIMPs and anti-WIMPs . If the cosmological constant vanishes, depending on the Hubble parameter,

a tau

neutrino mass of 50 -100 eV for instance couldaccount for

most of the mass necessary to close the universe thus the

abundance of other nonbaryonic big bang remnants would be reduced considerably.

In figure 7 thepredictions of the integral event rates per

ktydue to heavy neutrino annihilation in the sun andin both

175

underground detector

Scalar neutrinoscan be excluded entirely fromthe observed

neutrino spectrum in figure 5 since the expected eventrates

are tremendous [33] . The capability to distingu °sb between

electron neutrinos and muon neutrino allows to excluded both scalar neutrino flavors. The mass limits on generic

Higgsions areobtained from theintegmI flux limitshownin figure 6 in comparison with the expectation of ref. [34] including the corrections of ref. [35].

Particle

the sun and the earth are compared with the corrected and background subtracted upper limits at 90 % confidence

level. The predictions are based on the calculations of ref.[33,34] applying the corrections of ref[35] on the dark matter capture rate in ref. [31] which leads to an overall

correction factor of 1.5 and a WIMP mass dependent suppression due to mass mismatching in elastic scattering

VD

vM scalar ve scalar viL Higgsinos Neutralino

I

excl .Mass [ GeV/c2] Sun Sun + Earth <30 >75 <33 >51 <29 < 29 > 3 > 3 > 3 > 3 6-40 6-40 > 16 > 13

off nuclei. Since the sun consists mainly of hydrogen both

Dirac and Majorana neutrinos may be captured in the sun while in theearthheavy nuclei with zero spin dominate thus

only neutrinos with Dirac mass may be accumulated. Due to coherent scattering thecapturerate of Dirac neutrinos in theearth is enhanced significantly formasses which equals

roughly the masses of elements present in the earth in large quantities [35] . This effect may yield a neutrino signal from the center of the earth which is much stronger than the cor-

responding signal from the sun.

Table 3 summarizes the excluded mass regionsforheavy

neutrinostogether with the limits obtained forsome lightest

supersymmetric particles. Forall limits we have considered

only the flux of electron and muon neutrinos. A contri-

bution of tau neutrinos can notbe used fortheanalysis since interactions of tau neutrinos would not keep the direction-

ality due to suppression of charged current interactions at low energies and due to the missing energy in tau decay at higher energies . The combined event rate limits from the

sun and the earth gives slightly better mass limits forDirac neutrinos due to coherent scattering but it mainly increases the discrepancy between the experimental limit and the

expectation in the already excluded region. The evaporation rate sets a low mass limit of 3 GeV to all candidates .

TABLE 3 Mass limits on dark matter candidates derived from the neutrino flux limits from the sun and the earth. At presentit is notpossible to setmass limits on photinos because of the spin-dependent interaction with nuclei. In contrast if we consider the supersymmetric current eigen-

states, the Zino, the photinoand the higgsinos, to be different from the neutral supersymmetric mass eigenstates, the

neutralino, coherent scattering offspinless nuclei is possible yielding detectable neutrino signals [37,38] . In figure 8

the prediction of the combined integral event rate per kty produced by the annihilation of neutralinos trapped in the earth and the sun [37] is compared with the corrected and

background subtracted experimental upper limit at 90 % confidence level. The contributions of the annihilation signal from thesun andtheearthare also shown separately .

In the model alight higgs with amass of 10 GeV is assumed to be exchangedin the coherent neutralino scattering. Since the cross section I-. proportional to mH - 4 the expected

neutrir,o flux depends strongly on the higgs mass . Even without the uncertainties of unknown physics care has to be taken in interpreting the experimental results. Assuming a

H.-J.

M

x

Daum / The Fréjus underground detector

[GeV/c 2

the earth has improved the significance of the results. The interpretation depends on the assumptions made in the models, especially a finite neutrino mass could change the abundance of hypothetical dark matter candidates. The quest for the neutrino mass is one topic in particle physics. High mass underground experiments are a row °rful tool to attack this problem inregions not accessible toacceleratur orreactor experiment;. A 100 kilotonhigh resolution detector with properties better than those of the Fréjus detectorortheplanned Superkamiokande could gain several orders of magnitude in sensitivity in neutrino oscillations and in the nonbaryonic dark matter searches. A low energy threshold would be desirable for neutrino oscillation searches . The main goal of such an experiment should be the detection of the neutrino signal of a supernova explosion in our galaxy . Such a high statistics neutrino burst would allow to set limits onneutrino masses especially on the mass of the tau neutrino in the eV region.

FIGURE 8 Corrected upper limit on the integral event rateper kty from the sun and the earth ( full line ) compared to the expected rat°, for neutralinos from the sun ( dashed ), from the earth (dotted ) and from both the sun and the earth (dash dotted) [37]. The crosses indicate the event rate limits for the sun alone.

ACKNOWLEDGEMENT I would like to thank H. Meyer for many valuable discussions . This work !das been supported by the Bundesminister für Forschung und Technologie, Bonn, Federal Republic of Germany under contract number 55WT84P.

local dark matter density of 0.3GeVcm - 3 an overall uncertainty ofa factor of4 on the neutrino flux is estimated [36]. If this factor is accounted for in the theoretical expectation the combined integral event rate limit from the earth and the sun heavy Dirac neutrinos with 10GeV < My < 27 GeV and My > 80 GeV are still excluded .

REFERENCES 1 . M.L. Cerry et al., Phys. Rev. Lett. 47 (1986) 167 .

CONCLUSIONS

The r1e t,,trinn interactions

observed in the Fréjus detezfor are found to be in good agreement with the expectation of atmospheric neutrinos up to 100 GeV. From the data vN-v,, neutrino oscillations with An, 2 > 6*10 4 ev' 2 and ve - v9 oscillations with Otn 2 >7*10 -4 eV2 canbeexcluded atmaximum mixing. Using the corrected upper limits on the neutrino flux from the sun and the center of the earth we have set mass limits on various dark matter candidates within specific rriodcls .The combined flux limits from the sun and

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