David Dunsky (LBNL) “Dark Radiation Constraints on Heavy QCD Axion Theories”

Abstract: The explicit breaking of the PQ symmetry by higher dimensional operators can spoil the dynamical relaxation of the strong CP angle to its minimum of zero. One solution to this PQ “quality problem” is to introduce heavy QCD axions. Such axions acquire a mass from physics occurring far above the QCD scale and possess a potential more robust to corrections from higher dimensional PQ breaking operators. However, in much of the (m_a, f_a) plane, heavy QCD axions can generate large amounts of dark radiation when decaying. In this talk, I will discuss the cosmological evolution of heavy QCD axions in the early Universe, their interactions with the Standard Model thermal bath, and precise dark radiation constraints arising from their decays. In addition, I will discuss how including a mirror photon – common in heavy axion theories involving mirror QCD sectors – modifies this picture and can lead to dangerous amounts of dark radiation