Leptogenesis in the presence of density perturbations

TL;DR

This paper introduces 'acoustically driven freeze-out,' a new effect where density perturbations in the early universe significantly influence the decoupling of heavy particles, impacting leptogenesis and relaxing neutrino mass constraints.

Contribution

It identifies and analyzes a novel non-linear effect on particle freeze-out caused by density perturbations, with implications for leptogenesis models.

Findings

Perturbations enhance the final lepton asymmetry.

Constraints on heavy neutrino masses are relaxed due to perturbations.

The effect becomes significant at density fluctuations of order 1%.

Abstract

We point out a new effect on the freeze-out process of heavy particles induced by density perturbations in the early universe, which we call ``acoustically driven freeze-out.'' This beyond-linear effect is caused by the exponential decoupling of heavy particles from the thermal bath in the presence of density perturbations, and already at moderately large values $\delta T / \bar{T} = O (10^{-2})$ it cannot be captured by linear perturbation theory. We illustrate this effect with leptogenesis taking the decay and inverse decay of heavy neutrinos into account, and discuss its phenomenological implications. We found that perturbations always enhance the (spatially averaged) values of the final lepton asymmetry, and as a result, constraints on the mass of heavy neutrinos are found to be relaxed in the presence of perturbations.