The efficient discrimination of electron and pi-zero events in a water Cherenkov detector and the application to neutrino oscillation experiments

Super-Kamiokande, a large water Cherenkov detector, observed atmospheric neutrinos produced by interactions of cosmic rays in the atmosphere. By observing zenith angle and energy of the particles produced by the neutrinos in single ring events in Super-Kamiokande, it became apparent that the muon neutrino (numu) undergoes oscillations. During the oscillations, nu mu changes into another kind of neutrino, which can be either the tau neutrino (nutau) or a sterile neutrino (nu s). In addition, a small component of nue is possible but not considered here.; With the standard single ring analysis, using two-flavor oscillations without matter effects, it was hard to discriminate between these two possibilities because, while nus doesn't interact at all and nu tau does, interactions of nutau don't produce a significant single ring signal. Due to the large mass of the taon (tau), charged current interactions of nutau are suppressed in the observed energy region. However, it is possible to study neutral current interactions in order to discriminate between the two oscillation possibilities.; This work describes a technique for extracting the neutral current interactions from Super-Kamiokande data by means of selecting pi0 → gammagamma two ring events. Additionally, two ring events are used to independently confirm the neutrino oscillation observation.; A crucial step in the selection of neutral pion events is the reconstruction of the two Cherenkov rings produced in pi0 decays. Described here is a new reconstruction algorithm that uses a maximum likelihood method to fit the expected pattern of light to the observed pattern of light. A fast method is used for generation of the light pattern (including scattered light) produced by a hypothetical particle track configuration. Also described here are software components created in order to develop the fitter, including a custom data visualization program, and inter-language interface.; A new generation of long baseline neutrino oscillation experiments is coming up. An important goal of these experiments is search for electron neutrino (nue) appearance. Neutral pions are an important background. This algorithm is a valuable tool for discrimination between electrons produced by nue and background neutral pions in experiments such as K2K and T2K.