Phenomenology of leptogenesis: From cosmological phase transitions to new mechanisms and experimental probes

Abstract

This thesis addresses several aspects of leptogenesis, a theoretical paradigm that attempts to resolve two prominent open questions in high-energy phenomenology: the origin of neutrino masses and the observed baryon asymmetry of the universe (BAU).

The thesis is structured into three parts, with the first part focusing on first-order phase transitions (FOPTs) and cosmic strings (CSs) within the context of gauged $U(1)_{B-L}$ extensions of the Standard Model (SM). Initially, we re-examine the FOPT and gravitational-wave (GW) production in the classically conformal $U(1)_{B-L}$ model, identifying regions of parameter space that provide optimal conditions for bubble-assisted leptogenesis. Subsequently, we investigate the production of right-handed neutrinos (RHNs) from CSs and assess the magnitude of the $B-L$ asymmetry that they may source.

The second part of this thesis is dedicated to the recently proposed wash-in leptogenesis (WILG) mechanism, which does not require any $CP$ violation in the RHN sector. Specifically, we derive a lower bound on the mass of the lightest RHN by requiring WILG to successfully account for the observed BAU. Moreover, we introduce a novel ultraviolet mechanism for WILG based on non-minimal gravitational interactions.

In the final part of this thesis, we present novel strategies to search for new physics (NP) in rare Kaon and $B$-meson decays, focusing on the channels $K\rightarrow \pi+\slashed{E}$, $B\rightarrow K(K^*)+\slashed{E}$, and $B\rightarrow X_s+\slashed{E}$. We demonstrate that careful measurements of the kinematic distributions of missing energy can serve as a powerful tool to distinguish between the effective operators contributing to NP. Particular attention is devoted to lepton-number-violating (LNV) NP, and how observing LNV NP in rare meson decays could put leptogenesis under tension.