Freeze-in dark matter in neutron stars

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 $χ$ from the SM particles in such events, SM $\toχ\barχ$, for the freeze-in range of couplings, $α_{\rm FI} \sim O(10^{-26}) $, finding that $O(10^{-6})$ $χ$'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, $χ\barχ\to$ 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 $σ_{χ\,\rm SM} \propto O(10^{-70})\,\rm cm^2$.

Figures (4)

• Figure 1: Schematic diagram showing in- and out- reaction rates that can be relevant for the FI DM production, and for the late-time heating of NS.
• Figure 2: Main Feynman diagram responsible for the production of DM pairs. The reverse process leads to the annihilation. The freeze-in sub-processes that regulate the rate of production are colored differently. For scenario A, $\alpha_{\rm FI} \propto g_L^2 \varepsilon^2$, and for B, $\alpha_{\rm FI} \propto g_d^2$. The reverse annihilation of cooled $\chi\bar{\chi}$ pairs occurs via an off-shell mediator, and has $\sigma_{ann}\propto g_d^2 g_L^2 \varepsilon^2$ scaling.
• Figure 3: Total number of DM particles produced and the DM energy in MeV per baryons (dashed curves). Realistic curves that take into account the retained fraction are shown in solid red (dashed green for DM energy retained per baryons). The coupling constant $\alpha_{\rm FI}$ is set to a value (\ref{['a_FI_cosmo']}) that reproduces observed DM abundance.
• Figure 4: NS heating constraints as exclusion plots for certain projections of the parameter space. Top panel: NS heating constraints on $\{m_\chi,\alpha_d\}$ parameter space when $\alpha_{\rm FI}$ is fixed to reproduce observed amount of DM, and $m_V/m_\chi$ is varied for three representative values. Bottom panel: dark coupling constant is fixed, $\alpha_d =10^{-10}$, and constraints are shown on $\{m_\chi,\alpha_{\rm FI}\}$ plane. Dashed lines represent "optimal value" consistent with $\chi$ saturating DM abundance.