Analytic study of dark photon and gravitational wave production from axion

TL;DR

This paper provides an analytic framework for understanding how axion-like fields coupled with dark photons can produce chiral gravitational waves during the early universe, offering a way to probe such models observationally.

Contribution

It derives analytic expressions for dark photon production and gravitational wave spectra in axion-dark photon models, facilitating observational tests.

Findings

Analytic formula for dark photon number density during tachyonic resonance.

Analytic expression for gravitational wave spectrum in terms of model parameters.

Potential to probe axion-dark photon interactions through gravitational wave observations.

Abstract

Axion-like fields heavier than about $10^{-27}$eV are expected to oscillate in the radiation dominated epoch when the Hubble parameter drops below their mass. Considering the Chern-Simons coupling with a dark gauge boson, large amount of dark photons are produced during a short time interval through tachyonic resonance instability. The produced dark photons then source gravitational tensor modes leading to chiral gravitational waves. Through this process, one can indirectly probe a large parameter space of coupled axion-dark photon models. In this work we first find an analytic expression for the number density of the dark photons produced during the tachyonic resonance regime. Second, by using the saddle point approximation we find an analytic expression for the gravitational wave spectrum in terms of the mass, coupling and misalignment angle. Our analytic results can be used for the observational analysis of these types of scenarios.