Freezing In Vector Dark Matter Through Magnetic Dipole Interactions

We study a simple model of vector dark matter that couples to Standard Model particles via magnetic dipole interactions. In this scenario, the cosmological abundance arises through the freeze-in mechanism and depends on the dipole coupling, the vector mass, and the reheat temperature. To ensure cosmological metastability, the vector must be lighter than the fermions to which it couples, but rare decays can still produce observable 3$\gamma$ final states; two-body decays can also occur at one-loop with additional weak suppression, but are subdominant if the vector couples mainly to light fermions. For sufficiently heavy vectors, induced kinetic mixing with the photon can also yield additional two body decays to lighter fermions and predict indirect detection signals through final state radiation. We explore the implications of couplings to various flavors of visible particles and emphasize leptophilic dipoles involving electrons, muons, and taus, which offer the most promising indirect detection signatures through 3$\gamma$, $e^+ e^- \gamma$, and $\mu^+ \mu^- \gamma$ decay channels. We also present constraints from current and past telescopes, and sensitivity projections for future missions including e-ASTROGAM and AMEGO.