Freeze-in dark matter in neutron stars

TL;DR

This paper investigates how neutron stars can produce and retain freeze-in dark matter particles during their formation, leading to potential observable effects and providing unique constraints on dark matter models with extremely small interaction cross sections.

Contribution

It introduces a novel method of constraining dark matter models through neutron star energy injection signatures, focusing on the freeze-in production mechanism during supernova events.

Findings

Neutron stars can produce about 10^{-6} dark matter particles per nucleon during formation.

Retained dark matter can cause late-time heating of neutron stars, offering observable signatures.

Constraints on dark matter interaction cross sections are improved to around 10^{-70} cm^2.

Abstract

Every neutron star is born in the process of core-collapse supernova explosion that, for a brief moment, reproduces conditions of the early Universe with temperatures $T\sim O(30\rm\,MeV)$. We calculate the production of Dark Matter $\chi$ from the SM particles in such events, SM $\to\chi\bar\chi$, for the freeze-in range of couplings, $\alpha_{\rm FI} \sim O(10^{-26}) $, finding that $O(10^{-6})$ $\chi$'s per nucleon is produced. The strong gravitational potential well of the neutron star retains a substantial fraction of these particles that will eventually undergo the reverse process of energy injection, $\chi\bar\chi\to$ SM. This may lead to the abnormal energy injection creating observable signatures such as late-time heating of the neutron stars. To demonstrate the power of this method, we construct a set of simple dark matter models coupled to lepton currents, and show that neutron stars provide unique constraints on parameter space that otherwise cannot be accessed by other means, probing effectively the scattering cross sections with the SM in the ballpark of $\sigma_{\chi\,\rm SM} \propto O(10^{-70})\,\rm cm^2$.