The Zenith Angle Distribution of Fully Contained Events in SuperKamiokande and the Impact of Quasi Elastic Scattering on their Direction

Abstract

Quasi Elastic Scattering (QEL) is the dominant mechanism for producing both Fully Contained Events and Partially Contained Events in the SuperKamiokande (SK) detector for atmospheric neutrinos in the range 0.1GeV to 10GeV. In the analysis of SK events, it is assumed that the zenith angle of the incident neutrino is the same as that of the detected charged lepton. In the present paper, we derive the distribution function for the scattering angle of charged leptons and derive the average scattering angles as well as their standard deviation due to QEL. Thus, it is shown that the SK assumption for the scattering angle of the charged lepton in QEL is not valid. Further, for a real analysis of the experimental data, we should examine the correlations between the zenith angle of the charged leptons and those of their incident neutrinos, taking into account the effect of the azimuthal angle of the charged particle over its zenith angle. We conclude that it is not possible to reliably assign the zenith angle of the incident neutrino to that of the charged lepton, and so the zenith angle distribution of charged leptons does not reflect that of the incident neutrinos under the SK assumption. This result has clear implication for attempts to detect neutrino oscillations from an analysis of Fully Contained Events and Partially Contained Events in SuperKamiokande.