Capture of leptophilic dark matter in neutron stars

Abstract

Dark matter particles will be captured in neutron stars if they undergo scattering interactions with nucleons or leptons. These collisions transfer the dark matter kinetic energy to the star, resulting in appreciable heating that is potentially observable by forthcoming infrared telescopes. While previous work considered scattering only on nucleons, neutron stars contain small abundances of other particle species, including electrons and muons. We perform a detailed analysis of the neutron star kinetic heating constraints on leptophilic dark matter. We also estimate the size of loop induced couplings to quarks, arising from the exchange of photons and Z bosons. Despite having relatively small lepton abundances, we find that an observation of an old, cold, neutron star would provide very strong limits on dark matter interactions with leptons, with the greatest reach arising from scattering off muons. The projected sensitivity is orders of magnitude more powerful than current dark matter-electron scattering bounds from terrestrial direct detection experiments.Neutron stars provide particularly powerful DM constraints, because their high density leads to efficient capture. Indeed, for DM-nucleon cross sections above a threshold value of order 10 −45 cm 2 , the capture probably saturates at the geometric limit, σ ∼ πR 2 * m n /M * , such that all dark matter incident on the star is captured. This conclusion holds irrespective of whether DM scattering interactions are spin independent (SI) or spin dependent (SD). Given that conventional direct detection experiments currently have sensitivity to DM-nucleon cross sections below 10 −45 cm 2 only in a limited DM mass range, and only for SI interactions, NS techniques are clearly very useful. Moreover, velocity or momentum dependent interactions are completely inaccessible to terrestrial direct detection experiments, being severely suppressed in the non-relativistic regime applicable for scattering on Earth. In comparison, DM particles are accelerated to quasi-relativistic velocities upon NS infall, effectively erasing such kinematic suppression.When dark matter is gravitationally captured by a NS, the kinetic energy transferred in the collisions heat up the star. The captured dark matter will then undergo a series of further collisions, eventually transferring almost all of its initial kinetic energy, to reach a state of thermal equilibrium with the star. As shown in Refs. [18,19] this can heat neutron stars up to 1700K. 1 Because old isolated neutron stars can cool to temperatures below 1000K, such heating (or its absence) can be used to place limits on the strength of dark matter interactions. 2 Importantly, this kinetic heating may be within reach of forthcoming infrared telescopes [18,19]. Provided NSs are nearby, faint and sufficiently isolated, they are likely to be discovered by existing radio telescopes such as the Fivehundred-meter Aperture Spherical radio Telescope (FAST) [37], or the future Square Kilometer Array (SKA) [38]. Their thermal emission can then be m...

Probing muonic forces with neutron star binaries

“…NSs with masses of order a solar mass subsequently have 0.15%-0.75% of their mass stored in muons, providing a unique laboratory to test couplings of muons to light new degrees of freedom. This has been leveraged to place constraints on muon-philic dark matter due to its accretion in NSs [45,63,64].…”

Dark matter capture in celestial objects: improved treatment of multiple scattering and updated constraints from white dwarfs

“…Others have calculated the effect of this accumulated dark matter on cooling of celestial objects [34][35][36][37], or have compared the dark luminosity with the observed luminosity to provide stringent constraints on dark matter interactions with SM particles [35,38,39]. More recently, limits on DM-nucleon cross section have also been obtained from non-observation of collapse of massive white dwarfs [40] or from neutron star heating [41][42][43][44].…”

A little theory of everything, with heavy neutral leptons

“…In the case where DM is partially or purely symmetric, which occurs for smaller values of m N and g s in our model (recall figure 1), the accumulated DM inside the NS core can annihilate and the annihilation products might thermalize, heating up the star and contributing to its luminosity [94,99]. The latter is also increased by DM kinetic heating from multiple DM scatterings with the NS constituents, namely neutrons, electrons and muons, and this effect is independent of whether the DM is symmetric or asymmetric [99,100]. However, the expected NS surface temperature generated only by DM annihilation and scattering is too low to be detected by current infrared telescopes.…”