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STUDY OF NEUTRINO OSCILLATIONS IN THE FREJUS EXPERIMENT

Claude LONGUEMARE'Fréjus CollaborationbLaboratoire de l'Accélérateur linéaire, 91405 Orsay cedex FRANCE

Nuclear Physics 13 (Proc . Suppl.) 16 (1990) 490-492North-Holland

The Fréjus nucleon-decay detector has been operated between 19-02-1984 and 13-09-1988 . The sample of atmo-spheric neutrino interactions recorded is compared with the predictions of a Monte Carlo simulation . A searchfor neutrino oscillations is performed . No evidence is found, and new limits on oscillations varameters are givenat 95% CL.

1 . INTRODUCTIONThe search for nucleon decay, motivated by GUT

theories predictions, has stimulated both theoreticaland experimental work on atmospheric neutrinos to im-prove confidence in the background estimations of suchstudies . Atmospheric neutrino signals have also beenused to set limits on fluxes of high energy neutrinoscoming from dark matter annihilation in the sunlll, andon fluxes of cosmic photinos121 . Such studies have beendone by the main nucleon decay experiments . I.M.B.,Kamo-kande Fréjus and NUSEX13 ~4 A have publishedor reported their observations. The most striking re-sult was published one year ago by the KamiokandeII collaborationlal . A deficit of muon neutrino inter-actions in the low energy bound of the spectrum wasreported . This discrepancy has been afterwards inter-preted as an evidence of possible neutrino oscillations .Here, we present the neutrino event sample recordedin the fiducial volume of the apparatus . This sample iscompared with the results of a Monte Carlo simulationof atmospheric neutrino interactions . This comparisonis used to search for possible neutrino oscillations .

The apparatus was located in the Modane under-ground laboratory, situated in the middle of the Fréjushighway tunnel connecting France and Italy, under theAlpes . The rock cover averages 1780m . The Fréjusdetector has been described elsewhereH . It is a finegrained tracking calorimeter, consisting of sandwichesof Geiger and flash chamber planes . The trigger is pro-vided by the Geiger tubes. It requires a coincidenceof at least 5 hits in 5 adjacent Geiger planes, with atmost 3 hits in the same plane . The average trigger rateis 45 per hour, 20 due to cosmic muon and 25 to localradioactivity.

0920-5632/90/$3 .50 ® Elsevier Science Publishers D.V .North-Holland

2 . DATA AND MONTE CARLO ANALYSISEvents are selected and classified on line by the shift

physicist . A rescanning based on a pattern recognitionprogram is then carried out . The cumulated efficiencyof these two independent scannings is about 98% . Themain sources of background has been removed from theevent, sample : events with a visible vertex located out-side a fiducial volume (50 cm from the detector's edges,554 tons fiducial mass), events associated with a cross-ing muon track, events with only an outgoing chargedtrack leaving the detector and thus possibly missiden-tified with a stopping cosmic-ray induced muon. Se-lected events are measured on a graphic terminal . Anevent is defined as contained if all prongs are contained .We however do not discriminate contained and uncon-tained events in this analysis . All events are classifiedas muon or electron charged current, or neutral currentinteractions (respectively CCe, CCp and NC) accord-ing to possible leptonic prong linked to the vertex asexplained in reference 4 . Visible energy and momen-tum are computed for each event : the leptonic trackbeing identified as mentioned, all other electromagneticshowers are treated as 7 and all charged hadrons as pi-ons .

Atmospheric neutrinos are generated by decays ofparticles such as pions, kaons and muons, producedin cosmic ray showers. In the simulation program weuse a neutrino flux which takes into account the muonpolarisation effectshl ; The neutrino interaction simula-tion program is described in details in reference 8 ; it isbased on both experimental data and phenomenologyof weak interactions in the GeV energy range . The de-tector simulation introduces systematic uncertaintieswhich have been estimated by comparing the output

a present address : L .P.C., University of Caen, F-14032 Caen cedex, Franceb Physikaliiches Instinct der RWTH Aachen, L .A .L . Orsay, LPNHE Ecole Polytechnique, DPhPE-Saclay,

Univer3itiat-Gesamthochschule Wuppertal

C. Longuemare, Fréjus Collaboration/Neutrino oscillations in the Fiéjus experiment

of different programs. The analysis uses a simulatedsample corresponding to a 10 kt.y sensitivity, it hasbeen scanned and measured in the same way as realdata . The efficiency of our recognition procedure hasbeen checked . The probabilities for a v,, CC interaction(resp. ve) to be recognized is 95% (resp . 86%).

The 188 selected events are classified using the cri-teria described above . From the Monte Carlo, we ex-pect 212 events at the same sensivity (1.56 kt.y) . Theagreement is good within the 20% systematic error onthe absolute normalisation of the flux['] . The data andMonte Carlo samples have been compared in reference4 . The e/p ratios for the data and the Monte Carlo areshown below with estimated errors .

Data

: 0.53 f 0.09(statistical)e/p Ratios

MonteCarlo : 0.56 f 0.08(systematic) (1)

The value quoted for the Monte Carlo includes allsystematics (flux and simulation) . Visible energy andangular distributions have presented elsewhere [4] andit was shown that agreement between simulation anddata is satisfactory within statistic .

3 . ANALYSIS OF NEUTRINO OSCILLATIONSThe study of neutrino oscillations is one way to im-

prove our knowledge of neutrino physics . Oscillationshave been searched for in various experiments, studyingreactor induced v e fluxesP] or high energy acceleratorsv,/9, beams11° i . Neutrino oscillations, if they exist,can modify the atmospheric neutrino fluxes . This mod-ification can have three aspects . Oscillations clearlywill change the flux flavor composition . If the oscilla-tion length are greater than the atmosphere height, butless than the Earth's diameter, only neutrinos comingfrom the opposite side of the Earth have a significantoscillation probability. Therefore the angular distribu-tion will be distorted . If, for some subsample of thestatistics, the oscillation length is of the order of theflight distance, the visible energy distribution will beaffected by a E,,/D modulation . Under some hypoth-esis, tau neutrinos can be generated by oscillations .This neutrino can interact in our apparatus throughNC and CC interactions with a threshold of about 3.5GeV, therefore the tau neutrino can be, as a first orderapproximation, considered as "sterile" in our experi-ment . The Fréjus detector provides good identificationefficiencies of the CC neutrino interactions [section 2]but energy and angular resolutions, together with thelow statistics, make impossible to test for oscillationsusing energy and angular informationt81 . Thus the onlyreliable measurement that can be used to test neutrino

oscillations in our experiment is the flux flavor com-position. This information is represented by the e/pratio, defined as the ratio between the number of CCeand CCp events . To determine the excluded values for®m2 and sin 220, the e/p ratio have been be exper-imentally determined, and predicted by Monte Carlosimulation without oscillations . Combining statisticalerror and systematic uncertainties, the experimentalresult excludes at 90% confidence level values outsidethe interval

Taking oscillations into account a modified atmosphericneutrino flux is computed for each values of sin220 andAm2 from the flux given in reference [7] . Events of the10 kt-y simulated sample are then weighted by the ra-tio of the fluxes with and without oscillations . In eachhypothesis, the predicted value of the e/p ratio is com-puted . The interval given by (2) is then used to obtainconservative limits on the oscillations parameters .

In the channels invoving ve matter effects are takeninto account using of a realistic electronic density forthe Earth . However, it should be noted that for anyexcluded value of Am2 matter effects are roughly neg-ligible for this analysis.

0.36 <_ e/p < 0.76

(2)

sin$(20)

491

MCL

M CL

Fig. 1 : (Om2 ,sin 2 20) diagram for ve(ie ) +-+ VI(v0

oscillations . The two e/p = constant curves (plainline) correspond respectively to 90 and 95 °% confidencelevels . Area (A) [(C)f is excluded by accelerator [reac-tor] experiments .

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REFERENCES

C. Longuemare, 1Wjus Collaboration/Neutrino oscillations in the I+lréjus experiment

simma(20)

CL

Fig. 2 : (Am2,sin 220) diagram for v~`VN,) +-+ v,(-P,)oscillations . The two e/la = constant curves (plainline) correspond respectively to 90 and 95% confidencelevels . Area (A) is excluded by accelerator experiments.

Figures 1 and 2 show e/Ft constant curves in theusual (®m2, sin 220) diagram . The excluded areas areindicated and compared with other results from accel-erator or reactor experiments .

CONCLUSIONThe study of atmospheric neutrinos properties is

a unique opportunity to search for oscillations at low®m2 . This paper presents an analysis of the datataken by the Fr6jus detector, corresponding to the final1 .56 kt-y sensivity of the experiment . No evidence forneutrino oscillations is found, and new limits on theparameters are set . Existing limits are improved, byabout one order of magnitude in the v,,( -P,) H v,,('v,.)

channel, and by about two orders of magnitude in thev,,(°v.) H v,(v,) channel . However, these results arelimited to relatively large mixing angles .

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