Flavor blocking of dark matter thermalization in neutron stars

TL;DR

This work studies how lepton-flavor-violating DM couplings to electrons and muons, mediated by an axion-like particle, heat neutron stars in a way that seeds observable infrared signatures. In the strong gravity of neutron stars, DM gains semi-relativistic energies enabling inelastic χe→χμ upscattering, while flavor blocking prevents full thermalization with the stellar medium, boosting DM temperatures and making p-wave secluded annihilations (χχ→aa) the primary heating channel. The authors derive the capture rate, optical factor, kinetic-energy deposition, and LFV interaction rates, then analyze thermalization and annihilation, showing that old NSs could sustain surface temperatures around T_s ~ 2×10^3 K for sizable LFV couplings. They benchmark a GeV-scale axion-like mediator, demonstrating that next-generation infrared observations of old NSs could probe thermal DM targets far beyond current direct and indirect searches, thereby linking accelerator-scale LFV signals to astrophysical heating as a robust probe of LFV DM portals.

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 $χe \leftrightarrow χμ$, 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 $χ$. 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.

Figures (5)

• Figure 1: Optical factor, $\eta(r)$, for a $2\,M_\odot$ NS modeled with the BSk25 equation of state. We assume $m_\chi = 2\,m_a$, $m_a = 1~\textrm{GeV}$, and the thermal value of $g_{\chi,\textrm{th}}$. Results are presented for selected $g_{\mu e}$ coupling strengths, as indicated. The purple-shaded region, corresponds to low values of $R_{\textrm{NS}}-r$, where $R_{\textrm{NS}}$ is the NS radius and $r$ is the distance measured from the center of the star. This region marks the regime close to the star's surface where the electron chemical potential and density are too low and their upscattering to muons in the outer NS crust can be neglected. $R_\mu$ denotes the radius of the stable muon region within the star. We indicate the transition between the crust and core of the star by a vertical gray dashed line. The center of the star is toward the right-hand side, where $R_{\textrm{NS}} – r\to 0$.
• Figure 2: Evolution of the DM temperature ($T_{\mathrm{DM}}$) over time $t$ in years for $m_\chi = 2 \ m_a$ and $m_\chi = 1~\mathrm{GeV}$ (brown lines) or $20~\mathrm{GeV}$ (red). Solid lines correspond to LFV DM, while dashed lines represent the lepton flavor conserving case. The horizontal gray dashed line indicates the temperature of an old neutron star, $T_\mathrm{NS}$.
• Figure 3: Current constraints and projected exclusion bounds on the LFV DM model with the ALP mediator and the $m_\chi = 2m_a$ mass hierarchy in the $(m_a,g_{\mu e})$ plane. The green (brown, red) solid lines define the lower boundary of the regions in the parameter space where DM-induced kinetic and annihilation heating is expected to sustain neutron star temperatures above $2300~\mathrm{K}$ ($2100~\mathrm{K}$, $1900~\mathrm{K}$) even for old NSs.
• Figure 4: Lowest temperature reached by DM for case $1$ and a NS of mass of $2 \ M_\odot$ (BSk 25).
• Figure 5: Equilibration time with respect to each channel and the DM temperature for an NS of mass of $2 \ M_\odot$ (BSk 25) and DM of mass $1$ GeV.