Speaker
Description
The QCD axion has long been a subject of both theoretical and experimental interest due to being an elegant solution to the strong CP problem. It was also realized that axions can contribute significantly to the energy density of the Universe as dark radiation, that is a population of ultra-relativistic (or "hot") axions thermally produced in the early Universe by interacting with the Standard Model (SM) plasma.
Today, one of the ways to constrain the axion-SM interactions is to compare the theoretically predicted axion contribution to the effective number of neutrinos with measurements obtained by experiments like Planck. The perspective of the next generation of experiments like the future CMB-S4 motivates newer, more precise determinations of thermal axion production.
In this talk I will present recently obtained results on that matter, in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) model where the axion couples only to gluons. The most important step in computations of the thermal production rate, as well as the main source of theory uncertainty, is the implementation of collective effects due to the medium. Those are essential to cure would-be divergences caused by the exchange of "soft" gluons in the t channel.
I present two new computation schemes for determining the production rate and compare them to ones in the literature. Those new schemes solve or improve upon issues of production rate negativity and gauge dependence that can arise in the existing computation schemes. I also discuss how all considered computation schemes behave differently in the case of soft axion momentum and how this contributes to the uncertainty on the theoretical computation. Finally, I show results for quantities of phenomenological interest, like the aforementioned contribution to the effective number of neutrinos. I find that the theoretical contribution is in the ballpark of 0.032, with the error due to the computation scheme choice being around 0.002 at most.
As an outlook, I will also touch on the automated techniques that were used in the research that was presented, and offer some perspectives as to how they could be extended to automate the entire production rate computation.
The results presented in this talk were obtained in 2404.06113, written by myself and Jacopo Ghiglieri.
