Speaker
Description
The axion can address the strong CP problem and also provide a promising dark matter candidate in the form of a condensate of zero-momentum modes. In addition, axion models feature an unavoidable “hot” ensemble of thermally-produced axions acting as dark radiation and thus subject to present and future constraints from the effective number of neutrinos $N_\mathrm{eff}$.
I will concentrate on this latter population and show how the contribution to $N_\mathrm{eff}$ and its uncertainty can be quantified from thermal QCD dynamics above its crossover transition. In more detail, I will show how the leading-order thermal axion production rate has been computed in the past for axion momenta of the order of the temperature and I will show how different schemes for the incorporation of the collective dynamics of soft gluons extrapolate differently into the regime of softer axion production, thus giving us a first quantitative handle on the theory uncertainty of the rate. I will also show how popular gauge-dependent resummations in the thermal gravitino and axion literature are actually plagued by IR divergences. Finally, I will show how, upon solving the Boltzmann equation, the theory uncertainty on the axion rate translates to an uncertainty on $N_\mathrm{eff}$ in models with a direct axion-gluon coupling.
Talk based on 2404.06113
