Flavor blocking of dark matter thermalization in neutron stars

TL;DR

This paper explores how dark matter interactions involving lepton flavor violation in neutron stars can lead to observable heating signatures, offering a novel way to probe flavor-violating dark matter beyond traditional methods.

Contribution

It introduces the concept of flavor blocking in dark matter thermalization within neutron stars, highlighting its impact on heating and detection prospects for flavor-violating dark matter models.

Findings

Dark matter can induce neutron star heating via flavor-violating inelastic scattering.

Flavor blocking prevents low-energy DM from thermalizing, affecting heating efficiency.

Neutron star surface temperatures can reach detectable levels due to DM interactions.

Abstract

Neutron stars (NSs) provide exceptional laboratories for probing dark matter (DM) interactions beyond the reach of terrestrial experiments. We investigate a scenario in which DM couples to electrons and muons through a lepton-flavor-violating (LFV) coupling. In the strong gravitational field of NSs, infalling DM attains semi-relativistic velocities that activate inelastic transitions $\chi e \leftrightarrow \chi \mu$, leading to efficient energy deposition through scattering and annihilation. We show that this latter heating mechanism remains efficient even for $p$-wave suppressed annihilations. This is due to \textsl{flavor blocking} of DM thermalization with the NS, as LFV interactions become forbidden for low kinetic energies of $\chi$. The resulting DM-induced heating can sustain NS surface temperatures of $T_s \gtrsim 2 \times 10^3~\mathrm{K}$, providing an observable signature for future infrared searches. This establishes NS heating as a powerful probe of flavor-violating DM portals, capable of probing thermal targets beyond the reach of direct, indirect, and accelerator-based searches, as we illustrate for the axion-like particle (ALP) mediator.