Spontaneous Baryogenesis from Axions with Generic Couplings

TL;DR

This paper demonstrates that axion-like particles with generic couplings can naturally generate the Universe's baryon asymmetry through spontaneous baryogenesis, providing a comprehensive framework for analyzing these processes.

Contribution

It introduces a general formalism for calculating baryon asymmetry from axion couplings, applicable to various operators and simplifying the analysis.

Findings

Coupling axions to gluons can produce baryon asymmetry with electroweak sphalerons.

A unified transport equation framework is developed for different axion couplings.

The formalism avoids common pitfalls and clarifies theoretical subtleties.

Abstract

Axion-like particles can source the baryon asymmetry of our Universe through spontaneous baryogenesis. Here we clarify that this is a generic outcome for essentially any coupling of an axion-like particle to the Standard Model, requiring only a non-zero velocity of the classical axion field while baryon or lepton number violating interactions are present in thermal bath. In particular, coupling the axions only to gluons is sufficient to generate a baryon asymmetry in the presence of electroweak sphalerons or the Weinberg operator. Deriving the transport equation for an arbitrary set of couplings of the axion-like particle, we provide a general framework in which these results can be obtained immediately. If all the operators involved are efficient, it suffices to solve an algebraic equation to obtain the final asymmetries. Otherwise one needs to solve a simple set of differential equations. This formalism clarifies some theoretical subtleties such as redundancies in the axion coupling to the Standard Model particles associated with a field rotation. We demonstrate how our formalism automatically evades potential pitfalls in the calculation of the final baryon asymmetry.