Stacking the Deck: Gambling on a Light QCD Axion

TL;DR

This paper explores a novel mechanism involving a $ ext{Z}_N$-axion and explicit symmetry-breaking to produce lighter axions, addressing tuning issues and explaining reheating and misalignment behaviors in the early universe.

Contribution

It introduces a small explicit symmetry-breaking coupling that broadens the light axion parameter space and explains reheating and misalignment phenomena.

Findings

Enables populating a large $m_a$-$f_a$$ plane for light axions.

Removes the $1/N$ tuning in $ ext{Z}_N$-axion models.

Describes two types of axion misalignment: rigged and shuffled.

Abstract

We consider axions lighter than what their QCD couplings might otherwise suggest. Starting with a $\mathbb{Z}_N$-axion, we introduce a small explicit $\mathbb{Z}_N$ symmetry-breaking coupling between the Standard Model Higgs boson and a reheaton. This small explicit breaking allows us to populate a large portion of the light axion $m_a$-$f_a$ plane, removes the $1/N$ tuning in the $\mathbb{Z}_N$-axion, and explains why only our sector was reheated. Due to finite temperature effects, axions of this sort undergo either ``rigged" misalignment, where the axion misalignment angle is effectively $\pi$ regardless of its initial value; or ``shuffled" misalignment, where the initial angle is effectively randomized.