A study on the eμ identification capability of a water Čerenkov detector and the atmospheric neutrino problem

We carried out an experimental test of various characteristics of a water Čerenkov detector, especially the particle identification capability, using the 12-GeV proton synchrotron at KEK, Japan. The detector characteristics so far studied are well reproduced by the Monte Carlo simulation. Especially it is able to identify electrons and muons with mis-identification probabilities less than a few percent in the momentum range of 250–1000 MeV/c except the particle positions in the extreme edge of the detector volume. Based on the present results, the deficiency of μ-neutrinos in the atmospheric neutrinos is not an artifact caused by some peculiar detector characteristics.