Matter Effects on Conversion of Neutrinos from Supernovae and Cosmological Sources

This thesis collects in a unitary presentation the results of a series of papers [25, 54, 55, 56] in which we studied the effects of matter on oscillations of cosmic neutrinos from various sources. After a first introductory chapter, in chapter 2 we consider the conditions for significant matter effect; in particular we show that a minimum width of medium is required, independently of the neutrino oscillation parameters and energy, and of the matter density profile. In chapter 3 this condition is applied to high-energy neutrinos propagating across the universe: -vve find that substantial effect is produced by the relic neutrino background provided that it has a large CF-asymmetry and the neutrino source is at cosmological distance. The effect and its observability are discussed in detail. The chapters 4-6 are devoted to supernova neutrinos and their regeneration in the matter of the Earth. A detailed description of the Earth matter effect is presented in chapter 4, and in chapter 5 we give an application to the neutrino signal from SN1987 A. vVe find that the Earth effect could reconcile the difference in the energy spectra of the Kamiokande and IMB data. In chapter 6 we consider future supernova neutrino detections: we show that the Earth matter effect is observable and has important implications on the reconstruction of the neutrino mixing scheme and mass spectrum. The discussion is completed by three appendices.