Resonant Forbidden CP Asymmetry from Soft Leptons

TL;DR

This paper reveals that Standard Model leptons at finite temperature can produce a resonantly enhanced CP asymmetry, providing a new mechanism for leptogenesis that requires minimal new physics and is protected by finite-temperature effects.

Contribution

It introduces a novel mechanism where soft leptons generate large CP asymmetry at finite temperature, bypassing the need for exotic particles in leptogenesis.

Findings

CP asymmetry from soft leptons can be enhanced by seven orders of magnitude.

Resonant enhancement is protected by controlled widths under finite-temperature perturbation theory.

The mechanism enables low-scale leptogenesis with minimal model extensions.

Abstract

To explain the baryon asymmetry in the early universe via leptogenesis, quantum corrections to new particles are commonly invoked to generate the necessary CP asymmetry. We demonstrate, however, that a large CP asymmetry can already arise from Standard Model leptons. The mechanism relies on resummation of soft leptons at finite temperatures. The CP asymmetry, which is kinematically forbidden in vacuum, can be resonantly enhanced from thermally resummed leptons by seven orders of magnitude. Contrary to the resonance from exotic particles, we show that the resonant enhancement from soft leptons is protected by controlled widths under finite-temperature perturbation theory. We quantify such CP asymmetries in leptogenesis with secluded flavor effects and comment on the significance and application. The mechanism exploits the maximal role of leptons themselves, featuring low-scale leptogenesis, minimal model buildings and dark matter cogenesis.