Dirac-Phase Thermal Leptogenesis in the extended Type-I Seesaw Model

TL;DR

This paper explores how the Dirac CP phase in neutrino oscillations can drive leptogenesis in a low-scale extended type-I seesaw model, highlighting the conditions under which it can generate the observed matter-antimatter asymmetry.

Contribution

It demonstrates that Dirac-phase leptogenesis is viable in a low-scale extended type-I seesaw model with linear seesaw terms, providing a predictive framework for CP violation-driven leptogenesis.

Findings

Pure inverse-seesaw fails to produce sufficient asymmetry due to wash-out effects.

Dirac-phase leptogenesis is possible with linear seesaw terms and resonance effects.

Both degenerate and hierarchical spectra can achieve leptogenesis, with hierarchical being more constrained.

Abstract

Motivated by the fact that the Dirac phase in the PMNS matrix is the only CP-violating parameter in the leptonic sector that can be measured in neutrino oscillation experiments, we examine the possibility that it is the dominant source of CP violation for leptogenesis caused by the out-of-equilibrium decays of heavy singlet fermions. We do so within a low-scale extended type-I seesaw model, featuring two Standard Model singlet fermions per family, in which lepton number is approximately conserved such that the heavy singlet neutrinos are pseudo-Dirac. We find that this produces a predictive model of leptogenesis. Our results show that for low-scale thermal leptogenesis, a pure inverse-seesaw scenario fails to produce the required asymmetry, even accounting for resonance effects, because wash-out processes are too efficient. Dirac-phase leptogenesis is, however, possible when the linear seesaw term is switched on, with the aid of the resonance contributions naturally present in the model. Degenerate and hierarchical spectra are considered -- both can achieve Dirac-phase leptogenesis, although the latter is more constrained. Finally, although unable to probe the parameter space of Dirac-phase leptogenesis, the contributions to unitarity violation of the PMNS matrix, collider constraints and charged-lepton flavour-violating processes are calculated and we further estimate the impact of the future experiments MEG-II and COMET for such models.