The atmospheric neutrino background for proton decay via p -> e(+)pi(0) in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, 3.14x10(5) neutrino events corresponding to about 10 megaton-years of atmospheric neutrino interactions were collected by a 1000 ton water Cherenkov detector (KT). The KT charged-current single pi(0) production data are well reproduced by simulation programs of neutrino and secondary hadronic interactions used in the Super-Kamiokande (SK) proton decay search. The obtained p -> e(+)pi(0) background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is 1.63(-0.33)(+0.42)(stat)(-0.51)(+0.45)(syst)(megaton-year)(-1). This result is also relevant to possible future, megaton-scale water Cherenkov detectors.

Single charged pion production in charged-current muon neutrino interactions with carbon is studied using data collected in the K2K long-baseline neutrino experiment. The mean energy of the incident muon neutrinos is 1.3 GeV. The data used in this analysis are mainly from a fully active scintillator detector, SciBar. The cross section for single pi(+) production in the resonance region (W < 2 GeV/c(2)) relative to the charged-current quasielastic cross section is found to be 0.734(-0.153)(+0.140). The energy-dependent cross section ratio is also measured. The results are consistent with a previous experiment and the prediction of our model.

We performed an improved search for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}$ oscillation with the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment, using the full data sample of $9.2\ifmmode\times\else\texttimes\fi{}{10}^{19}$ protons on target. No evidence for a ${\ensuremath{\nu}}_{e}$ appearance signal was found, and we set bounds on the ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}$ oscillation parameters. At $\ensuremath{\Delta}{m}^{2}=2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{eV}}^{2}$, the best-fit value of the K2K ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ disappearance analysis, we set an upper limit of ${sin}^{2}2{\ensuremath{\theta}}_{\ensuremath{\mu}e}<0.13$ at a 90% confidence level.