QCD Axion Hybrid Inflation

TL;DR

This paper proposes a hybrid inflation model where the QCD axion acts as the inflaton, with mixing effects from heavy axions influencing the inflation dynamics and initial conditions, linking dark matter and early universe inflation.

Contribution

It introduces a novel scenario where the QCD axion serves as the inflaton through mixing with heavy axions, providing a detailed numerical analysis of the effects.

Findings

Initial misalignment angle of QCD axion is close to π.

Hybrid inflation occurs with low inflation scale below QCD scale.

Heavy axion may be detectable in future experiments.

Abstract

When the inflaton is coupled to the gluon Chern-Simons term for successful reheating, mixing between the inflaton and the QCD axion is generally expected given the solution of the strong CP problem by the QCD axion. This is particularly natural if the inflaton is a different, heavier axion. We propose a scenario in which the QCD axion plays the role of the inflaton by mixing with heavy axions. In particular, if the energy scale of inflation is lower than the QCD scale, a hybrid inflation is realized where the QCD axion plays the role of the inflaton in early stages. We perform detailed numerical calculations to take account of the mixing effects. Interestingly, the initial misalignment angle of the QCD axion, which is usually a free parameter, is determined by the inflaton dynamics. It is found to be close to $\pi$ in simple models. This is the realization of the pi-shift inflation proposed in previous literature, and it shows that QCD axion dark matter and inflation can be closely related. The heavy axion may be probed by future accelerator experiments.