Leptogenesis in crossing and runaway regimes

TL;DR

This paper investigates how effective thermal masses and widths influence resonant leptogenesis, revealing that the crossing and runaway regimes affect asymmetry generation differently, with off-shell effects being negligible.

Contribution

It introduces a detailed analysis of crossing and runaway regimes in leptogenesis, demonstrating that mass degeneracy does not suppress asymmetry and clarifying the role of off-shell effects.

Findings

Effective thermal masses impact leptogenesis differently in crossing and runaway regimes.

Mass degeneracy does not necessarily suppress the lepton asymmetry.

Off-shell contributions are negligible even at the crossing point.

Abstract

We study the impact of effective thermal masses and widths on resonant leptogenesis. We identify two distinct possibilities which we refer to as crossing and runaway regimes. In the runaway regime the mass difference grows monotonously with temperature, whereas it initially decreases in the crossing regime, such that the effective masses become equal at some temperature. Following the conventional logic the source of the asymmetry would vanish in the latter case. Using non-equilibrium quantum field theory, we analytically demonstrate that the vanishing of the difference of the effective masses does however neither imply a suppression nor a strong enhancement of the source for the lepton asymmetry. In the vicinity of the crossing point the asymmetry calculated in an (improved) Boltzmann limit develops a spurious peak, which signals the breakdown of the quasiparticle approximation. In the exact result this spurious enhancement is compensated by coherent transitions between the two mass shells. Despite the breakdown of the quasiparticle approximation off-shell contributions remain negligibly small even at the crossing point.