Cosmological abundance of the QCD axion coupled to hidden photons

TL;DR

This paper investigates how coupling the QCD axion to hidden photons affects its cosmological abundance, revealing that such interactions can significantly alter the axion's relic density and relax existing bounds on its decay constant.

Contribution

The study provides the first detailed lattice simulation analysis of the nonlinear dynamics of the QCD axion coupled to hidden photons, including backreaction effects.

Findings

Axion abundance can be suppressed by up to two orders of magnitude due to hidden photon production.

Strong coupling can dampen axion motion, increasing its relic abundance.

The cosmological upper bound on the axion decay constant can be relaxed by a few hundred times.

Abstract

We study the cosmological evolution of the QCD axion coupled to hidden photons. For a moderately strong coupling, the motion of the axion field leads to an explosive production of hidden photons by tachyonic instability. We use lattice simulations to evaluate the cosmological abundance of the QCD axion. In doing so, we incorporate the backreaction of the produced hidden photons on the axion dynamics, which becomes significant in the non-linear regime. We find that the axion abundance is suppressed by at most ${\cal O}(10^{2})$ for the decay constant $f_a = 10^{16}$ GeV, compared to the case without the coupling. For a sufficiently large coupling, the motion of the QCD axion becomes strongly damped, and as a result, the axion abundance is enhanced. Our results show that the cosmological upper bound on the axion decay constant can be relaxed by a few hundred for a certain range of the coupling to hidden photons.