Production of Thermal Axions across the ElectroWeak Phase Transition
Light axions can potentially leave a cosmic background just like the neutrinos. We complete the study of thermal axion production above the QCD phase transition by providing a smooth and continuous treatment across the electroweak phase transition. Focusing on flavour conserving axion couplings, we quantify axion production due to scattering of thermal bath particles and we include for the first time the contributions from the longitudinal components of theWand Z bosons. We performa model independent analysis in terms of axion effective coupling to heavy quarks, and we also study the predictions for particular QCD axion scenarios, the so-called Minimal Flavour Violating Axion, as well as the invisible axion KSVZ and DFSZ models. Finally, we explore the possibility of testing with cosmological observations the solar axion interpretation of the XENON1T excess. We study minimal scenarios with a detectable signal in future CMB surveys: axions coupled democratically to all fermions, axion-electron coupling generated radiatively and the DFSZ framework for the QCD axion.
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