Abstract
Thermal axion production in the early universe goes through several mass thresholds, and the resulting rate may change dramatically across them. Focusing on the KSVZ and DFSZ frameworks for the invisible QCD axion, we perform a systematic analysis of thermal production across thresholds and provide smooth results for the rate. The QCD phase transition is an obstacle for both classes of models. For the hadronic KSVZ axion, we also deal with production at temperatures around the mass of the heavy-colored fermion charged under the Peccei-Quinn symmetry. Within the DFSZ framework, standard model fermions are charged under this symmetry, and additional thresholds are the heavy Higgs bosons masses and the electroweak phase transition. We investigate the cosmological implications with a specific focus on axion dark radiation quantified by an effective number of neutrino species and explore the discovery reach of future CMB-S4 surveys.
Highlights
The Peccei-Quinn (PQ) mechanism [6, 7] is one of the most appealing solutions
We investigate the cosmological implications with a specific focus on axion dark radiation quantified by an effective number of neutrino species and explore the discovery reach of future Cosmic Microwave Background (CMB)-S4 surveys
The field evolution in the early universe goes through two main phases: the axion is initially stuck by Hubble friction, and once its mass becomes comparable to the expansion rate it begins oscillating around the minimum of its potential
Summary
The minimal ingredients for the KSVZ framework are an electroweak singlet complex scalar φ and a vector-like colored fermion Ψ. For any value of the transformation parameter α, the Lagrangian in eq (2.1) is invariant as long as the scalar potential VKSVZ(φ) does not change and the charges satisfy qφ = qR − qL. The Yukawa coupling is responsible for a fermion mass, mΨ = yΨvφ/ 2, which can be smaller than the symmetry breaking scale vφ if yΨ is small, but not in conflict with collider searches for heavy colored states (mΨ TeV). The PQ symmetry can be linearly realized and the axion appears as the phase of the PQ breaking scalar as in eq (2.1). The production of the KSVZ axion goes through three main cosmological phases that are separated by two mass thresholds: (i) the mass of Ψ; (ii) the confinement scale
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