On dark matter self-interaction via single neutrino exchange potential

Abstract

Neutrinos – amongst the lightest known particles – can mediate a force driving dark matter self-interaction and the small scale structure of the universe. We explore such a possibility in the simplest neutrino portal dark sector model where neutrino has a Yukawa coupling with a scalar ϕ and fermion χ that are degenerate in mass and together comprise 100% of dark matter in the universe. We derive the non-relativistic potential generated by single-neutrino exchange which is of the monopole–dipole form and explore χϕ→χϕ scattering based on phase shift analysis. Our result shows that Born approximation continues to be valid in the low energy regime and the scattering cross section scales as 1/v2 over a wide range of dark matter velocities. Such a velocity-dependent self-interacting cross section can be large enough to explain the shallow density of dwarf galaxy cores and consistent with the upper limit from colliding galaxy clusters. The 1/v2 behavior persists down to rather low velocities v∼mν/m where m is the dark matter mass, leaving the opportunity for further astrophysical probes. Through the neutrino portal, the self-interacting dark matter parameter space can be tested by searches for Z-boson and light mesons decaying into the dark sector, as well as low-mass dark matter direct detection experiments.