Speaker
Description
The axion is a hypothetical particle first proposed by Peccei, Quinn, Weinberg and Wilczek to solve the strong CP problem. It was later realized that ultra-relativistic (or "hot") axions thermally produced in the early Universe can contribute measurably to the energy density of the Universe as dark radiation.
The effective number of neutrinos Neff, parametrizing this dark radiation fraction of the energy density, has already been measured by experiments like Planck or ACT and will be determined to a greater degree of precision by future CMB telescopes like the Simons Observatory. Precise theoretical determinations of the axion contribution to Neff as a function of the axion-Standard Model couplings can be used to place constraints on those couplings.
Specifically, this talk discusses new results obtained in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) model where the axion only interacts with gluons, and the only parameter is the axion scale fPQ.
I will show how the main source of theoretical uncertainty is the implementation of finite-temperature collective effects. Comparing three leading-order equivalent implementations of those thermal corrections allow us to quantify the uncertainty on the axion rate and -- through solving the Boltzmann equation -- on Neff.
For a value of fPQ close to the current limit from astrophysical observations, we find that the axion contribution to Neff is around 0.0281, with a theoretical uncertainty of the order of 0.0003.
We also critically examine previous determinations of the rate in the literature.
The results presented in this talk are published in 2404.06113, written by myself and Jacopo Ghiglieri.
