Opening up the QCD axion window

TL;DR

This paper introduces a new mechanism involving dark photon coupling to significantly reduce QCD axion density, allowing higher decay constants and impacting cosmological observations and axion structure formation.

Contribution

The authors propose a novel depletion mechanism for the QCD axion density via coupling to a dark photon, enabling higher decay constants for generic initial conditions.

Findings

Axion density can be exponentially suppressed through tachyonic instability.

Dark radiation from this process may be observable in future CMB experiments.

The mechanism could lead to the formation of axion miniclusters.

Abstract

We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as $f_a \simeq 10^{17}\,\rm{GeV}$ viable for generic initial conditions. In our setup, the axion couples to a massless dark photon with a coupling that is moderately stronger than the axion coupling to gluons. Dark photons are produced copiously through a tachyonic instability when the axion field starts oscillating, and an exponential suppression of the axion density can be achieved. For a large part of the parameter space this dark radiation component of the universe can be observable in upcoming CMB experiments. Such dynamical depletion of the axion density ameliorates the isocurvature bound on the scale of inflation. The depletion also amplifies the power spectrum at scales that enter the horizon before particle production begins, potentially leading to axion miniclusters.