ALP leptogenesis

This master thesis project explores cosmological aspects of beyond the Standard Model physics, and aims at addressing the baryon asymmetry of the Universe (BAU) and how it could be affected by axion-like particles (ALPs). It intends to provide an alternative path to the standard thermal leptogenesis mainly studied in the literature, focusing on the possibility to source non-thermal leptogenesis via decays of ALPs. ALPs are weakly-interacting pseudo Nambu-Goldstone bosons of spontaneously broken global symmetries, predicted by many high-energy extensions of the Standard Model and they are natural targets to discover new physics. We study the evolution of ALPs in the Early Universe, probing their thermal production and decay, considering their couplings with gluons, top quarks and, in addition, Majorana right-handed neutrinos, Ns. In particular, we investigate the decay a -> N N and its implications, notably the non-thermal production of Ns. In this thesis, the contribution of this non-thermal ALP leptogenesis is estimated and computed by solving the associated Boltzmann Equations. Moreover, we explore the region of viable parameter space for ALP leptogenesis where the ALP induces a matter-dominated phase, diluting significantly the pre-existing relics. Results favour values for the ALP decay constant f_a such that f_{a} > 10^{11} GeV and for the Majorana neutrino mass M_N such that M_{N} > 1 TeV. The advantages of this alternative approach are that, while standard leptogenesis would undergo strong washout regime, the non-thermal leptogenesis via ALP decays avoids this, and the secondary non-thermal population of Ns, produced via ALP decays, fuels leptogenesis and enhances the resulting baryon asymmetry.