Stacking the Deck: Gambling on a Light QCD Axion

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.

Comments: 15 pages, 6 figures