A search for oscillations of muon neutrinos in an experiment with L/E ≅0.7 km/GeV

Abstract A low-energy muon neutrino beam was used to illuminate two similar fine-grained neutrino detectors placed at 123 m and 903 m from the beginning of the neutrino source. The fiducial masses of the “close” and the “far” detector were about 36 tons and 120 tons respectively. The average energy of the selected neutrino events was 1.5 GeV. Data were recorded for an integrated flux of about 10 19 protons. Quasielastic charged current events initiated by ν e 's have been searched for and an upper limit of 2.7%, at 90% confidence level, is obtained for the fraction of ν μ 's transformed into ν e 's. For complete mixing this correspnds to a limit Δm 2 ⩽ 0.20 eV 2 for the transition ν μ → ν e and to a minimum value of the mixing parameter sin 2 2 θ = 0.04. In the same data sample we compared the rates in the two detectors of ν μ -initiated charged current events, mainly of quasielastic type: we conclude that oscillations of ν μ 's to ν τ 's, and possibly heavier neutrinos, do not appear. For complete mixing the limit in this case is Δm 2 ⩽ 0.29 eV 2 .