Flavour effects in gravitational leptogenesis

TL;DR

This paper explores how quantum effects in gravitational backgrounds can generate lepton asymmetry in the early universe, and examines conditions under which this mechanism can successfully explain the observed matter-antimatter imbalance.

Contribution

It demonstrates that accounting for flavor effects in gravitational leptogenesis allows for successful asymmetry generation with specific right-handed neutrino mass spectra, expanding previous constraints.

Findings

Flavor effects weaken washout processes, enabling successful leptogenesis.

A viable mass spectrum is identified with $M_2 extgreater 10^9$ GeV and $M_1 extgreater 6.3 imes 10^6$ GeV.

The mechanism impacts low-energy CP phases and neutrino mass scale.

Abstract

Within the Type-I seesaw mechanism, quantum effects of the right-handed (RH) neutrinos in the gravitational background lead to an asymmetric propagation of lepton and anti-leptons which allows a Ricci scalar and neutrino Dirac-Yukawa coupling dependent chemical potential and therefore a lepton asymmetry in equilibrium. At high temperature, lepton number violating scattering processes try to maintain a dynamically generated lepton asymmetry in equilibrium. However, when the temperature drops down, the interactions become weaker, and the asymmetry freezes out. The frozen out asymmetry can act as a pre-existing asymmetry prior to the standard Fukugita-Yanagida leptogenesis phase ($T_i\sim M_i$, where $M_i$ is the mass of $i$th RH neutrino). It is then natural to consider the viability of gravitational leptogenesis for a given RH mass spectrum which is not consistent with successful leptogenesis from decays. Primary threat to this gravity-induced lepton asymmetry to be able to successfully reproduce the observed baryon-to-photon ratio is the lepton number violating washout processes at $T_i\sim M_i$. In a minimal seesaw set up with two RH neutrinos, these washout processes are strong enough to erase a pre-existing asymmetry of significant magnitude. We show that when effects of flavour on the washout processes are taken into account, the mechanism opens up the possibility of successful leptogenesis (gravitational) for a mass spectrum $M_2\gg 10^9 {\rm GeV}\gg M_1$ with $M_1 \gtrsim 6.3 \times 10^6$ GeV. We then briefly discuss how, in general, the mechanism leaves its imprints on the low energy CP phases and absolute light neutrino mass scale.