Measurements of the Atmospheric Neutrino Flux at Super-Kamiokande

Abstract

Three direct measurements of the atmospheric ν e +ν e and ν µ +ν µ neutrino fluxes were performed using the Super-Kamiokande water Cherenkov detector: the directionally-integrated energy spectra, the azimuthal spectra, and the modulation of the fluxes with time over the 11-year solar cycle.In particular, the energy spectra in the sub-GeV to TeV range was measured and compared to the predictions of various published flux models. While none of the models were strongly inconsistent with the data, some preference was seen for the HKKM11 model as the most realistic model.The azimuthal analysis measured the east-to-west asymmetry in the neutrino flux, caused by the geomagnetic field, for both flavours at > 5 σ . Measurements were made of the strength of the effect as functions of energy and zenith angle. There was also an indication that the alignment of the asymmetry was dependent on zenith angle, seen at the 2.2 σ level.A search for a long-term correlation between the atmospheric neutrino flux and the solar magnetic activity cycle was performed, however the expected effect based on the HKKM model was calculated to be relatively small. An indication of a correlation was seen at the 1.1 σ level. During particularly strong solar activity events, known as Forbrush decreases, no theoretical prediction is available, but a deviation from the expected neutrino event rate is seen at the 2.4 σ level.