Hidden fermion as milli-charged dark matter in Stueckelberg Z′ model

We augment the hidden Stueckelberg Z' model by a pair of Dirac fermions in the hidden sector, in which the Z' has a coupling strength comparable to weak scale coupling. We show that this hidden fermion-antifermion pair could be a milli-charged dark matter candidate with a viable relic density. Existing terrestrial and astrophysical searches on milli-charged particles do not place severe constraints on this hidden fermion. We calculate the flux of monochromatic photons coming from the Galactic center due to pair annihilation of these milli-charged particles and show that it is within reach of the next generation gamma-ray experiments. The characteristic signature of this theoretical endeavor is that the Stueckelberg Z' boson has a large invisible width decaying into the hidden fermion-antifermion pair. We show that existing Drell-Yan data do not constrain this model yet. Various channels of singly production of this Z' boson at the LHC and ILC are explored.