The impact of electric currents on Majorana dark matter at freeze out

Abstract

Thermal relics with masses in the GeV to TeV range remain possible candidates for the Universe's dark matter (DM). These neutral particles are often assumed to have vanishing electric and magnetic dipole moments so that they do not interact with single real photons, but the anapole moment can still be nonzero, permitting interactions with single virtual photons. This anapole moment allows for p-wave annihilation of DM into standard model particles, and the DM interacts with external electric currents via the anapole moment. Moving beyond their static electromagnetic properties, these particles generically have non-zero polarizabilities which mediate interactions with two real photons; in particular, spin-dependent polarizibilities admit s-wave annihilation of the DM into two photons. When the Universe cools from a temperature on the order of the DM mass to freeze out, the DM is in thermal equilibrium with the background plasma of particles, but the comoving DM density decreases due to annihilation. If a collection of initially unpolarized DM particles were subjected to an electric current, then the DM medium would become partially polarized, according to the Boltzmann distribution, with a slight excess of anapole moments aligned with the current, relative to those anti-aligned. For this region of partially polarized DM particles, the s-wave annihilation mode becomes partially suppressed because it requires a state of vanishing angular momentum. As a consequence, the decreased DM annihilation rate in this region will result in an excess of DM density, relative to an unpolarized region, as DM drops out of thermal equilibrium. We explored this relative change of DM density for DM that is subjected to an electric current through freeze out.