Disaccoppiamento cosmologico del neutrino in presenza di interazioni Non Standard

Neutrinos are important and fascinating particles that could represent the defining key for a new theory that can describe the fundamental interactions. In fact, experimental evidence today confirms unequivocally that neutrinos can change their flavor through the oscillation property. This leads to the consequent evidence that the neutrinos have masses: this possibility is not allowed by the Standard Model, therefore it is necessary to define a Beyond Standard Model (BSM) theory. Moreover, we know that the non-zero neutrino masses usually come with Non Standard Interactions (NSI) that might violate leptonic flavor and break the weak universality. The objective of this thesis will be to study how the NSI neutrinos have with the couples e± in the radiation dominated era alter the picture of the neutrino decoupling from the cosmic plasma, and what is the consequence they have on the Neff parameter, the effective number of neutrinos. This parameter is related to the cosmological density of radiation ρR, quantifies the ratio of energy densities of the neutrino to photons (ργ), and it has different values while changing the NSI parameters. A first approximate study has been done considering the qualitative and analytic study of the effects that the delayed decoupling of the three neutrino species from the primordial plasma could have had: I compared the evolution of the plasma temperature considering the degrees of freedom that come from photons and the couples e± with the evolution of the plasma temperature considering the previous degrees of freedom and neutrinos too. This will allow us to see that the plasma undergoes a lower heating in the second case because neutrinos are still coupled with it, sharing entropy. On the other hand, the study of the continuity equation allows us to understand the changes in the Neff parameter too, which grows if the three neutrino species decouple later. In the second part of my thesis, a more detailed study of the Neff parameter was conducted by inserting NSI parameters into the continuity equation: the magnitude of these parameters can be constrained from the analysis of data from neutrino-electron scattering experiments. Moreover, since the study done until now assumes some approximations (such as me → 0, no QED corrections to the plasma, no effect of the active neutrino oscillations, etc.), more accurate results will follow in a fully numerical and momentum-dependent calculation obtained with FortEPiaNO (Fortran-Evolved Primordial Neutrino Oscillations), a code used to interpret the NSI for the neutrino decoupling scene.