Neutron star cooling with lepton-flavor-violating axions

TL;DR

This paper investigates how axion-like particles emitted via lepton-flavor-violating processes in neutron stars can be used to set constraints on axion couplings, providing bounds comparable to laboratory experiments.

Contribution

It introduces a calculation of axion emission rates from neutron star matter via lepton-flavor-violating couplings and derives new astrophysical bounds on these couplings.

Findings

Upper limit on coupling |g_{aeμ}| ≈ 10^{-6} from neutron star cooling.

Stronger limit |g_{aeμ}| ≈ 10^{-11} from supernova observations.

Axion emission via lepton-flavor violation is suppressed by plasma temperature to muon mass ratio.

Abstract

The cores of dense stars are a powerful laboratory for studying feebly coupled particles such as axions. Some of the strongest constraints on axionlike particles and their couplings to ordinary matter derive from considerations of stellar axion emission. In this work we study the radiation of axionlike particles from degenerate neutron star matter via a lepton-flavor-violating coupling that leads to muon-electron conversion when an axion is emitted. We calculate the axion emission rate per unit volume (emissivity) and by comparing with the rate of neutrino emission, we infer upper limits on the lepton-flavor-violating coupling that are at the level of $|g_{ae\mu}| \lesssim 10^{-6}$. For the hotter environment of a supernova, such as SN 1987A, the axion emission rate is enhanced and the limit is stronger, at the level of $|g_{ae\mu}| \lesssim 10^{-11}$, competitive with laboratory limits. Interestingly, our derivation of the axion emissivity reveals that axion emission via the lepton-flavor-violating coupling is suppressed relative to the familiar lepton-flavor-preserving channels by the square of the plasma temperature to muon mass ratio, which is responsible for the relatively weaker limits.