Anomalies and sterile neutrinos – Implications of new theoretical results

Abstract

. The reactor antineutrino and the gallium anomalies have been long unexplained. Possible explanations for both of these anomalies include new physics, such as the existence of one or more eV-scale sterile neutrino. However, the previous theoretical calculations, which do not replicate the experimental results, rely on many simplifying approximations. We have performed shell model calculations in order to gain insights into these issues. In the reactor-antineutrino analysis the beta decays contributing to the cumulative electron spectrum are usually assumed to have allowed spectral shapes. However, many of these decays are actually first-forbidden. Moreover, these decays dominate the experimentally observable region. Based on the recent results, the use of this allowed approximation can at least partially explain the so called reactor antineurtino anomaly. Our new large-scale shell model calculations regarding the neutrino-nucleus scattering cross section off 71 Ga decreases the gap between theory and the experimental results of GALLEX and SAGE experiments. Conflict between charge-exchange BGTs and the neutrino-nucleus cross sections can to some extent be explained by destructive interference between Gamow-Teller and tensor contributions.