From Supernovae to Neutron Stars: A Systematic Approach to Axion Production at Finite Density

TL;DR

This paper systematically studies how finite density and temperature environments affect QCD axion production, revealing significant modifications to axion couplings and implications for astrophysical bounds and terrestrial searches.

Contribution

It derives finite density corrections to axion-nucleon couplings within chiral perturbation theory, impacting axion phenomenology in supernovae and neutron stars.

Findings

Axion-nucleon couplings can increase by an order of magnitude near nuclear saturation density.

Supernova axion luminosity bounds are strengthened by a factor of three.

Neutron star cooling bounds are weakened by a factor of four when including density corrections.

Abstract

We present a systematic study of QCD axion production in environments with finite baryon density and temperature, implying significant changes to axion phenomenology. Within heavy baryon chiral perturbation theory, we derive the effective Lagrangian describing axion interactions with nucleons and mesons up to next-to-leading-order in the chiral expansion. We focus on corrections to the axion-nucleon couplings from higher orders and finite density. These couplings are modified by up to an order of magnitude near nuclear saturation density, significantly impacting axion production in supernovae and neutron stars. Density-dependent corrections enhance the axion luminosity in supernovae by an order of magnitude, strengthening current best bounds by a factor of three. We stress the importance of including all axion production channels up to a given chiral order for a consistent luminosity calculation and classify the missing contributions up to the third chiral order. The modified axion-nucleon couplings also affect neutron star cooling rates via axion emission. A re-evaluation of existing neutron star cooling bounds, constrained to regions where perturbative control is reliable, weakens these bounds by a factor of four. Lastly, our results have implications for terrestrial axion searches that rely on precise knowledge of axion-nucleon couplings.