Sterile neutrinos in 4D and 5D in supernovae and the cosmo

From the introduction: [...] Although, of course, less under control than table-top or accelerator experiments, stars (core-collapse Supernovæ in particular) and the Early Universe (whose image is imprinted in the deep space that we observe today) are powerful laboratories for particle physics, and often come with features that make them complementary or even more useful than the traditional means of probing the microscopic world. The subject of this thesis fits in such a picture: focussing on the issue of the possible existence of sterile neutrinos, I discuss the constraints and the possible signals of their presence that come from the physics of Supernovæ and of the Early Universe. In the first Chapter I substantiate the link between the search for New Physics and the use of supernovæ and cosmological processes. This Chapter has the character of an introductory general overview, which consists of four parts: The Standard Model and why to go beyond, “Standard” neutrino physics and why to go beyond, The role of Supernovæ, The role of the Early Universe. Each of these parts can be essentially read in an independent way from the others (or even skipped by the learned reader). The specific work of this thesis builds up on this general background and knowledge, to which I will occasionally refer in the subsequent Chapters. In the second Chapter, based on ref.[1], I focus on the case of a single (4D) sterile neutrino. After the presentation of the adopted four neutrino formalism, I describe the derivation of bounds and signals on its mixing parameters with the active neutrinos, considering BBN, LSS, CMB and supernovæ. Including the consolidated mixing of active neutrinos among themselves in all these contexts can lead to outputs quite different from those in the literature and to new features. As a result, I present plots of the combined excluded regions in the space of the mixing parameters. In the third Chapter, based on refs.[2, 3], I move to the case of an infinite tower of sterile neutrinos motivated by Extra Dimensions. After the description of the 5D setup, I discuss the model of modified evolution developed for the SN inner core, first qualitatively and then in detail. I show that a careful study of the SN physics can allow a relaxation of the previous bounds in the literature by several orders of magnitude, giving also rise to an interesting modified phenomenology. For the three cases of electron, muon and tau neutrinos mixed with the extra dimensional states, I present the possible signatures of extra dimensional states in the neutrino signal from the next SN.