Vector fermion-portal dark matter: Direct detection and Galactic Center gamma-ray excess

We investigate a neutral gauge boson X-mu that originates from a hidden U(1) extension of the standard model as the particle dark matter candidate. Vector dark matter interacts with the standard model fermions through heavy fermion mediators. The interactions give rise to a t- channel annihilation cross section in the XX -> f (f) over bar process, which dominates the thermal relic abundance during thermal freeze- out and produces measurable gamma- ray flux in the galactic halo. For a light vector dark matter, if it predominantly couples to the third- generation fermions, this model could explain the excess of gamma rays from the Galactic Center. We show that vector dark matter with a mass of similar to 20- 40 GeV and which annihilates into the b (b) over bar and t _ t final states provides an excellent description of the observed gamma- ray excess. The parameter space aimed at explaining the gamma- ray excess could also provide the correct thermal relic density and is compatible with the constraints from electroweak precision data, Higgs invisible decay, and collider searches. We show the dark matter couplings to the nucleon from the fermion portal interactions are loop suppressed, and only contribute to the spin- dependent cross section. Therefore vector dark matter could easily escape the stringent constraints from the direct- detection experiments.