Leptogenesis: The Other Cuts

TL;DR

This paper derives and evaluates new source terms in leptogenesis from finite density effects, showing they can significantly impact the lepton asymmetry, especially when right-handed neutrino masses are comparable or hierarchical.

Contribution

It introduces finite density source terms in leptogenesis calculations, highlighting their importance over standard vacuum-based sources in certain mass regimes.

Findings

Finite density effects produce additional source terms.

These sources can dominate the lepton asymmetry under specific mass hierarchies.

Numerical evaluation confirms sizable corrections to standard calculations.

Abstract

For standard leptogenesis from the decay of singlet right-handed neutrinos, we derive source terms for the lepton asymmetry that are present in a finite density background but absent in the vacuum. These arise from cuts through the vertex correction to the decay asymmetry, where in the loop either the Higgs boson and the right-handed neutrino or the left-handed lepton and the right-handed neutrino are simultaneously on shell. We evaluate the source terms numerically and use them to calculate the lepton asymmetry for illustrative points in parameter space, where we consider only two right-handed neutrinos for simplicity. Compared to calculations where only the standard cut through the propagators of left-handed lepton and Higgs boson is included, sizable corrections arise when the masses of the right-handed neutrinos are of the same order, but the new sources are found to be most relevant when the decaying right-handed neutrino is heavier than the one in the loop. In that situation, they can yield the dominant contribution to the lepton asymmetry.