Inelastic dark matter mediated by natural dark photon

Abstract

At loop level, a dark $U(1)_X^{}$ gauge field for dark photon can naturally acquire a tiny kinetic mixing with the standard model $U(1)_Y^{}$ gauge field for hypercharge after a $Z_2^{}$ discrete symmetry is spontaneously broken for generating a small mass difference between two vector-like fermions carrying the same $U(1)_Y^{}$ charge and the opposite $U(1)_X^{}$ charges of equal magnitude. To avoid an unacceptable relic density with electric charge, these vector-like fermions can decay into the standard model charged leptons with a TeV-scale complex dark scalar. The real and imaginary parts of this dark scalar can elegantly obtain a mass split below the MeV scale after a dark Higgs scalar develops its vacuum expectation value for generating a dark photon mass of the GeV order. Therefore, the dark photon can mediate an exothermic inelastic scattering of dark matter off nucleus to naturally explain the recently reported event in the LUX-ZEPLIN experiment and substantially relax the stringent constraints from the IceCube neutrino searches.