Kinetic decoupling of neutralino dark matter
Xuelei Chen, Marc Kamionkowski, Xinmin Zhang
TL;DR
This paper investigates the kinetic decoupling of neutralino dark matter by computing elastic-scattering cross sections with standard-model particles, focusing on neutrinos and photons. By equating the elastic-scattering rate to the Hubble rate, it places the decoupling temperature in the range $T \sim 1~\mathrm{MeV}$ to $T \sim 1~\mathrm{GeV}$, implying neutralinos become collisionless cold dark matter early in cosmic history. It challenges prior estimates (Boehm et al.) that suggested keV decoupling and potential collisional damping by photon and neutrino scattering, arguing that kinematic effects and the rate structure imply weaker damping. The results emphasize the dependence on SUSY couplings and sparticle masses and refine expectations for small-scale structure formation under neutralino dark matter scenarios.
Abstract
After neutralinos cease annihilating in the early Universe, they may still scatter elastically from other particles in the primordial plasma. At some point in time, however, they will eventually stop scattering. We calculate the cross sections for neutralino elastic scattering from standard-model particles to determine the time at which this kinetic decoupling occurs. We show that kinetic decoupling occurs above a temperature $T\sim$ MeV. Thereafter, neutralinos act as collisionless cold dark matter.