Impact of Helical Electromagnetic Fields on the Axion Window

TL;DR

Helical electromagnetic fields in the early universe can significantly increase axion dark matter abundance, potentially closing the conventional axion decay constant window due to overproduction, especially for hidden photon couplings.

Contribution

This paper demonstrates that primordial helical electromagnetic fields can source axion velocities, leading to overproduction of axion dark matter and constraining the axion decay constant window.

Findings

Helical electromagnetic fields can source axion velocities and enhance relic abundance.

The conventional axion window can be closed due to overproduction from helical fields.

Overproduction constraints depend on gauge coupling and extra relativistic degrees of freedom.

Abstract

Primordial electromagnetic fields can strongly affect the cosmic evolution of axions, and vice versa. We show that if helical electromagnetic fields are coherently produced in the early universe, their remnants source a field velocity to the coupled axions and enhance the relic abundance of axion dark matter. We discuss the implications for the QCD axion and axion-like particles that are coupled to the SM or hidden gauge groups. For a QCD axion coupled to hidden photons, we find that the conventional window for the axion decay constant $10^{8}\, \mathrm{GeV} \lesssim f \lesssim 10^{12}\, \mathrm{GeV}$ can be completely closed due to overproduction of axion dark matter by helical electromagnetic fields as little as $\alpha \, \Delta N_{\mathrm{eff}} \gtrsim 10^{-12}$, where $\alpha$ is the gauge coupling and $\Delta N_{\mathrm{eff}}$ is the effective extra relativistic degrees of freedom of the hidden photons.