Maximal axion misalignment from a minimal model

TL;DR

This paper introduces a minimal dynamical model that achieves maximal axion misalignment, allowing QCD axion dark matter with a lower decay constant and higher mass, impacting dark matter detection and collider experiments.

Contribution

The authors present a simple dynamical model that realizes large-misalignment for the QCD axion, enabling lower decay constants and higher masses than previously possible.

Findings

QCD axion with decay constant as low as 6×10^9 GeV can produce dark matter.

The model allows axion masses up to 1 meV.

Implications for dark matter experiments and collider searches are discussed.

Abstract

The QCD axion is one of the best motivated dark matter candidates. The misalignment mechanism is well known to produce an abundance of the QCD axion consistent with dark matter for an axion decay constant of order $10^{12}$ GeV. For a smaller decay constant, the QCD axion, with Peccei-Quinn symmetry broken during inflation, makes up only a fraction of dark matter unless the axion field starts oscillating very close to the top of its potential, in a scenario called "large-misalignment". In this scenario, QCD axion dark matter with a small axion decay constant is partially comprised of very dense structures. We present a simple dynamical model realising the large-misalignment mechanism. During inflation, the axion classically rolls down its potential approaching its minimum. After inflation, the Universe reheats to a high temperature and a modulus (real scalar field) changes the sign of its minimum dynamically, which changes the sign of the mass of a vector-like fermion charged under QCD. As a result, the minimum of the axion potential during inflation becomes the maximum of the potential after the Universe has cooled through the QCD phase transition and the axion starts oscillating. In this model, we can produce QCD axion dark matter with a decay constant as low as $6\times 10^9\,{\rm GeV}$ and an axion mass up to 1 meV. We also summarise the phenomenological implications of this mechanism for dark matter experiments and colliders.