Volume 232, number 3 PHYSICS LETTERS B 7 December 1989 ON THE INTERPRETATION OF THE KAMIOKANDE NEUTRINO EXPERIMENT E.V. BUGAEV Institutefor NuclearResearch of the Academy of Sciences of the USSR, 117312Moscow, USSR and V.A. NAUMOV Institute of Applied Physics, lrkutsk State University, 664003 Irkutsk, USSR Received 23 June 1989; revised manuscript received 22 September 1989 The comparison between the KAMIOKANDE data and theoretical estimations of the low-energy (E v< a few GeVs) atmos- pheric neutrino events is considered. Neutrino spectra calculated in our previous papers are used with the account of corrections due to the cosmic ray muon polarization. The discrepancy between observed and predicted ratios of electron-like and muon-like single-ring event numbers can be explained either by neutrino oscillations (of re '-~ %-type predominantly), or by misidentifying the particle species of single-ring events. 1. Introduction During the last few years about 103 events initiated by atmospheric neutrinos (AN) with mean energy ~ 1 GeV have been registered in a number of under- ground experiments, mainly with nucleon-decay de- tectors (see, e.g. the reviews [ 1,2 ] ). The experimen- tal counting rates and the energy distributions of neutrino events in IMB, Fr6jus, NUSEX and KGF agree (within experimental errors) with the expected ones. However, the more detailed measured charac- teristics such as the fraction of events accompanied by muon decay electrons (KAMIOKANDE, IMB), the numbers of fully contained events initiated by charged current (anti) neutrino interaction (Fr6jus), or ratio of numbers of muon-like (M-type) and electron-like (S-type) single-ring events (KAMIOKANDE) appear to be systematically smaller than the corresponding theoretical estimates, using as input the AN fluxes as calculated by Gaisser et al. [3,4]. The most sizeable discrepancies of such a kind are observed in the large water Cerenkov detector, KAMIOKANDE [5,1] ~t (this detector has rela- tively low thresholds for neutrino energy). For in- stance, the ratio of the measured value of NM/Ns to the calculated one (where NM(S) is the number of sin- gle-ring events of M (S)-type is equal to 0.60 + 0.09. The total observed number of neutrino events Ntot also appears to be less: (Ntot)obs//(Ntot )calc -- 0.82 + 0.05. (1) Such strong disagreements with theory cannot be explained by corrections, connected with the cosmic- ray muon polarization [6,7], which had not been taken into account in the calculations [3,4]. Some authors [ 7,8 ] tried to interpret the observed "muon deficit" as a manifestation of triple neutrino oscilla- tions with the dominating transition v~v~. At the same time, the results of other underground arrays (IMB, Fr6jus, NUSEX) on AN flavor ratio measurements do not require for their interpretation some kind of radical hypothesis. ~ The KAMIOKANDE results have not changed qualitatively after the 16%-increase of the statistics [ 2 ]. So we will discuss below only the results of the earlier detailed analysis [ 5 ] (2.87 kton yr of luminosity). 391