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Thermal relics with masses in the GeV to TeV range remain possible candidatesfor the Universe's dark matter (DM). These neutral particles are often assumedto have vanishing electric and magnetic dipole moments so that they do notinteract with single real photons, but the anapole moment can still be nonzero,permitting interactions with single virtual photons. This anapole moment allowsfor p-wave annihilation of DM into standard model particles, and the DMinteracts with external electric currents via the anapole moment. Moving beyondtheir static electromagnetic properties, these particles generically havenon-zero polarizabilities which mediate interactions with two real photons; inparticular, spin-dependent polarizibilities admit s-wave annihilation of the DMinto two photons. When the Universe cools from a temperature on the order ofthe DM mass to freeze out, the DM is in thermal equilibrium with the backgroundplasma of particles, but the comoving DM density decreases due to annihilation.If a collection of initially unpolarized DM particles were subjected to anelectric current, then the DM medium would become partially polarized,according to the Boltzmann distribution, with a slight excess of anapolemoments aligned with the current, relative to those anti-aligned. For thisregion of partially polarized DM particles, the s-wave annihilation modebecomes partially suppressed because it requires a state of vanishing angularmomentum. As a consequence, the decreased DM annihilation rate in this regionwill result in an excess of DM density, relative to an unpolarized region, asDM drops out of thermal equilibrium. We explored this relative change of DMdensity for DM that is subjected to an electric current through freeze out.

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