Atmospheric Neutrinos in Super{Kamiokande

Since the initial announcement of evidence for oscillations of atmospheric neutrinos [1], Super{ Kamiokande has increased its atmospheric neutrino sample by 40% to 45.3 kiloton-years of fullycontained interactions and 42.2 kiloton-years of partially-contained interactions. The data exhibit signi cantly low values of R ( ( =e)DATA=( =e)MC) in both the sub-GeV sample (R = 0:67 0:02 0:05) and multi-GeV sample (R = 0:66 0:04 0:08) due to a zenith-angle dependent de cit of muon neutrino induced events. The observed de cits cannot be explained by any combination of known systematic uncertainties but are consistent with neutrino oscillations of $ with sin 2 > 0:86 and 1 10 3 < m < 8 10 3 eV. The data are also well t by oscillations of $ sterile with sin 2 2 > 0:87 and 2 10 3 < m < 7 10 3 eV. With the current sample, these two oscillation modes cannot be distinguished.