A Light QCD Axion with Hilltop Misalignment

TL;DR

This paper explores a novel axion cosmology mechanism where a mirror sector influences the axion potential, enabling a hilltop misalignment that yields the correct dark matter abundance while addressing the strong CP problem.

Contribution

It introduces a new cosmological evolution scenario for the QCD axion involving a mirror sector that modifies the potential and allows for hilltop misalignment, expanding viable parameter space.

Findings

The mirror sector causes the axion to initially roll to a hilltop at π.

Post-QCD transition, the axion relaxes to the CP-conserving minimum at zero.

This mechanism predicts correct dark matter abundance in new parameter regions.

Abstract

We study the cosmological evolution of a light QCD axion and identify the parameter space to obtain the correct relic dark matter abundance. The axion potential is flattened at the origin, corresponding to the only minimum, while it is unsuppressed at $\pi$. These potential features arise by assuming a mirror sector with the strong CP phase $\bar\theta$ shifted by $\pi$ compared to the SM sector, which allows the mirror axion potential to be tuned against the usual QCD axion potential. Before the QCD phase transition, assuming the mirror sector is decoupled and much colder than the SM thermal bath, the mirror sector potential dominates, causing the axion to initially roll to a temporary minimum at $\pi$. However, after the QCD phase transition, the potential minimum changes, and the axion relaxes from the newly created "hilltop" near $\pi$ to the CP-conserving minimum at the origin. As the axion adiabatically tracks this shift in the potential minimum through the QCD phase transition, with non-adiabatic evolution near $\pi$ and 0, it alters the usual prediction of the dark matter abundance. Consequently, this "hilltop" misalignment mechanism opens new regions of axion parameter space, with the correct relic abundance while still solving the strong CP problem, that could be explored in future experiments.