Speaker
Description
Thermal production of hot axions in the early universe has attracted much attention recently, owing to the fact that
it will be sensitive to upcoming precision constraints on $N_{\mathrm{eff}}$. The theoretical uncertainty associated with
such calculations is known to come primarily from the axion production rate, which has been
studied across a broad range temperatures and axion momenta using perturbative methods [1,2,3]. Coupled
with existing measurements of $N_{\mathrm{eff}}$, these
calculations seem to suggest that the hot axions freeze out from the primordial plasma around or above the QCD crossover, where
the application of perturbative QCD is not well-justified. This motivates the study of the
axion production rate using strong-coupling methods.
In this talk, I will present a first determination of the axion production
rate from a strongly coupled plasma, using the AdS/CFT correspondence.
Our calculation extends the well-known strong-coupling computation of the sphaleron rate [4] to finite axion momenta,
and is hence under control for all momenta, unlike its perturbative counterpart. I will argue how,
from our result of the corresponding correlation function in $\mathcal{N}=4\,\text{SYM}$, we can estimate the
strongly coupled QCD axion production rate. Finally, I will discuss
the impact of strong-coupling effects on $N_{\mathrm{eff}}$.