The QCD Axion as a hot relic and the H_0 tension

by Alessio Notari (UB)

Tuesday 2 Jul 2019, 12:00 → 13:00 Europe/Madrid

IFAE seminar room (IFAE)

I will discuss the possibility of detecting the QCD axion as a hot relic from the early universe, if it is directly coupled to quarks or leptons. For sufficiently low decay constant the dominant production mechanism at temperatures 1GeV≲T≲100GeV is obtained via scatterings with heavy quarks or leptons. This leads to a contribution to the effective neutrino number, N_eff, which can be larger than the one obtained when thermalization happens only above the electroweak phase transition, ΔNeff≲0.027. Such a prediction is within reach of future CMB S4 experiments, thus opening an alternative window to detect the QCD axion and to test the early universe at such temperatures. Moreover, in the case of production via muon scattering, the contribution can be so large that it alleviates the cosmological H_0 tension. Finally I discuss production with cross-sections through coupling to gauge bosons and Higgs.