Theoretical Calculations for Electron Spectra and Neutrino-Nucleus Scattering in Context of Reactor Antineutrino and Gallium Anomalies

Abstract

This thesis consists of fourteen publications and an introductory part on rare beta decays and neutrino-nucleus scattering. The overarching theme of this work is to provide state-of-the-art theoretical calculations for statistical inferences related to the existence of eV-scale sterile neutrinos. The first body of research relates to forbidden beta spectra and the reactor antineutrino anomaly. As a first application, the theoretical formalism is applied to several forbidden beta decays in order to study how adopting quenched values of the weak coupling constants gA and gV affects the shape of the electron spectrum. Next, the formalism is applied to the first-forbidden unique decays of argon isotopes 39,42Ar in order to produce high-precision spectra which could be utilized for modeling background radiation in rareevent searches. Following this, the mesonic enhancement of the ƴ5 matrix element, also known as the axial-charge matrix element, for decays between neutron-rich nuclei is studied in the A ≈95 and A ≈ 135 mass regions. These include transitions which play a role in reactor antineutrino experiments. The theoretical spectra are then compared with the experimental results of COBRA for 113Cd and EXO-200 for 137Xe in order to assess the quality of the theoretical predictions, and excellent agreement is found. The formalism is then applied to the 36 most important first-forbidden contributors to the cumulative β-spectrum from nuclear fission in order to reassess the nuclear structure uncertainties related to the reactor antineutrino anomaly. The nuclear structure uncertainties are found to be significantly larger than has been previously assumed. The second body of research focuses on charged-current scattering reactions for neutrinos and nuclei, with special emphasis on the gallium anomaly. Firstly, new estimates for solar and supernova neutrino cross sections for scattering off 40Ar are derived. Next, a new estimate for the solar neutrino cross section of 205Tl, an excellent candidate for radiochemical experiments, is evaluated. The formalism is then applied to the case of 71Ga in order to re-evaluate the theoretical cross section and to investigate possible shortcomings of chargeexchange reactions as a means of evaluating cross sections in the context of the gallium anomaly. The statistical significance of the gallium anomaly is found to be reduced by the new calculations.