Neutrino Flavor Conversions in High-Density Astrophysical andCosmological Environments

Abstract

The topic of this thesis is the study of the neutrino flavor conversions in high-density environments: the supernovae and the the Early Universe. Remarkably, these represent the only two cases in which neutrinos themselves contribute to the \"background medium\" for their propagation, making their oscillations a non-linear phenomenon. In particular, in the dense supernova core, the neutrino-neutrino interactions can lead in some situations to surprising and counterintuitive collective phenomena, when the entire neutrino system oscillates coherently as a single collective mode. In this context, we have shown that during the early SN accretion phase (post-bounce times $<$ 0.5 s) the matter density is so high to dominate over the neutrino density, suppressing the collective flavor conversions. We have characterized this suppression numerically solving the neutrino propagation equations. We have also supported our finding with a stability analysis of the neutrino equations of motion. Neutrino flavor conversions in the Early Universe are another fascinating problem involving collisional damping, refractive effects from charged leptons and neutrino self-interactions. In this thesis, we have studied the flavor conversions of active-sterile system of neutrinos in the Early Universe. This study has been motivated by hints for low-mass sterile neutrinos, coming from precision cosmological measurements and laboratory oscillation experiments. We performed an accurate solution of the kinetic equations for the evolution of the active-sterile ensemble in order to determine the relic abundance of sterile neutrinos. We took into account a possible primordial neutrino asymmetry $(L > 10^{-3})$ in order to suppress the sterile neutrino production and to find a better agreement between the cosmological and laboratory hints. Finally, we discuss the implications of our results on Big-Bang Nucleosynthesis and on the Cosmic Microwave Background from data measured by the Planck experiment.