Axion dark matter from Higgs inflation with an intermediate H_∗

In order to accommodate the QCD axion as the dark matter (DM) in a model in which the Peccei-Quinn (PQ) symmetry is broken before the end of inflation, a relatively low scale of inflation has to be invoked in order to avoid bounds from DM isocurvature fluctuations, H_∗ ≲ O(10⁹) GeV . We construct a simple model in which the Standard Model Higgs field is non-minimally coupled to Palatini gravity and acts as the inflaton, leading to a scale of inflation H_∗ ∼ 10⁸ GeV . When the energy scale at which the PQ symmetry breaks is much larger than the scale of inflation, we find that in this scenario the required axion mass for which the axion constitutes all DM is m₀ ≲ 0.05 μeV for a quartic Higgs self-coupling λ_φ = 0.1, which correspond to the PQ breaking scale v_σ  ⪆ 10¹⁴ GeV and tensor-to-scalar ratio r ∼ 10^-12. Future experiments sensitive to the relevant QCD axion mass scale can therefore shed light on the physics of the Universe before the end of inflation.