Spontaneous Baryogenesis from Axions with Generic Couplings
  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.
