UC Irvine

The innermost secrets of the mysterious neutrino are being revealed in underground detectors. Recent data on atmospheric neutrinos, primarily from the Super-Kamiokande experiment, confirm the neutrino flavor mixing and non-zero masses. The high precision measurement of angular distribution allows to determine the value of Delta m(2) between 0.0013 eV(2) and 0.0054 eV(2) at 90% c.l. Studies of up-down asymmetries in different event samples indicate that nu(mu) <->nu(tau) oscillations are more likely explanation of the data than nu(mu) <-> nu(s). The deficit of the observed solar neutrino Aux compared to the predictions of the standard solar model, often interpreted by neutrino oscillations, is further studied in the SuperKamiokande detector. The energy spectrum is measured above 5.5 MeV for the Sun's positions above and below the horizon. A day-night effect is observed at a statistical significance of 2 sigma. The K2K (KEK to Kamioka) is the first long-baseline neutrino-oscillation experiment. During runs in 1999 the first 3 events were observed in the SuperKamiokande detector caused by neutrinos produced at the KEK accelerator at a distance of 250 km. The predicted number of events without any oscillations is 12.3(-1.9)(+1.7).

* Presented at the Cracow Epiphany Conference on Neutrinos in Physics and Astrophysics, Cracow, Poland, January 6-9, 2000.