Lukewarm inflation and the QCD axion

Abstract

We show that a late period of warm inflation, at temperatures just above the QCD scale, can dilute the abundance of the QCD axion produced by the misalignment mechanism, thus avoiding the need to fine-tune the initial misalignment angle in scenarios with an axion decay constant close to the GUT scale. We develop a concrete realization of this lukewarm inflation stage involving right-handed neutrinos in 0.1 - 1 GeV mass range and an associated scalar sector, showing that coherent axion oscillations are damped during this period. In this scenario, a previously generated large baryon asymmetry may also be diluted to yield the observed value, and lukewarm inflation is followed by an early-matter era dominated by the lightest right-handed neutrino, before it decays into Standard Model states. This model can be tested via changes to the number of relativistic species and the induced small-scale enhancements of the primordial curvature power spectrum, leading to primordial black holes and scalar-induced gravitational waves.