Aligning a Majorana fermion’s anapole moment with an external current through photon emission mediated by the fermion’s generalized polarizabilities

The sole static electromagnetic property of a spin-$\frac{1}{2}$ Majorana fermion is its anapole moment. Though they cannot couple to single real photons, these particles can interact with electric currents through virtual photons. If immersed in background current, there an energy difference between the spin states fermion; higher state has moment antialigned current. In this paper, we address ability system initially unpolarized fermions achieve some degree polarization relative considering processes that allow fermion's flip lower-energy state, focus upon two irreversible processes: spontaneous emission photons and photon emitted Compton scattering. Both involve coupling via polarizaibilities. We compute spin-flip transition rates for using low-energy expansion Hamiltonian construct toy model showcase how depend underlying parameters within model. Applying ideas thermal dark matter (DM) model, find when DM thermally decouples from Standard Model plasma early universe, two-photon negligible but partial medium proceed scattering if sufficient exist.