Neutron stars can shine a light on elusive lepton-flavor-violating dark matter

Abstract

We investigate a scenario in which dark matter (DM) poses a challenge to conventional direct and indirect detection, making it much more elusive than typical candidates. We focus on thermally produced DM that couples to electrons and muons via a lepton-flavor-violating (LFV) axion-like particle (ALP). Given the DM kinematics and lack of muon targets on Earth, direct detection would be infeasible. Indirect detection of our DM candidate is also hampered by the dominance of $p$-wave annihilation. However, we demonstrate that neutron stars (NS) can serve as probes of such a scenario, through future dedicated observational campaigns. Infalling DM is accelerated to semi-relativistic velocities, triggering inelastic $\chi e \leftrightarrow \chi \mu$ scattering off both electrons and muonic targets within the NS. We show that ``flavor blocking'' -- the kinematic suppression of LFV interactions at low energies -- prevents DM thermalization with the cold neutron star, enabling efficient $p$-wave annihilations. The resulting NS surface temperatures ($T_s \gtrsim 2 \times 10^3~\mathrm{K}$) offer a possible signature for future infrared searches, probing thermal relics beyond the reach of direct, indirect, and accelerator experiments.