Axionic Electroweak Baryogenesis

TL;DR

This paper proposes that axions can induce a strongly first-order electroweak phase transition and serve as a CP phase for baryogenesis, with observable implications for future experiments.

Contribution

It introduces a novel mechanism where axions facilitate electroweak baryogenesis, independent of their coupling strength, and explores experimental prospects for detection.

Findings

Axions can enable a strong first-order electroweak phase transition.

The axion can act as a CP phase via coupling to the top quark.

Baryon asymmetry can be explained for axion decay constants between TeV and 10 TeV.

Abstract

An axion can make the electroweak phase transition strongly first-order as required for electroweak baryogenesis even if it is weakly coupled to the Higgs sector. This is essentially because the axion periodicity naturally allows the structure of phase transition to be insensitive to the axion decay constant that determines the strength of axion interactions. Furthermore, the axion can serve as a CP phase relevant to electroweak baryogenesis if one introduces an effective axion coupling to the top quark Yukawa operator. Then, for $f$ between about TeV and order $10$~TeV, the observed baryon asymmetry can be explained while avoiding current experimental constraints. It will be possible to probe the axion window for baryogenesis in future lepton colliders and beam-dump experiments.