Inelastic Dirac dark matter in U(1)−

The new gauge boson Z′ in U(1)B−L is widely considered as the mediator of dark matter. In this paper, we propose the inelastic Dirac dark matter in U(1)B−L. The dark sector contains two Dirac fermions χ˜1 and χ˜2 with U(1)B−L charge 0 and −1, respectively. A Z2 symmetry is also introduced to ensure the stability of dark matter. The mass term δmχ˜¯1χ˜2 induces the mixing of dark fermion. Then the lightest dark fermion χ1 becomes the dark matter candidate, whose coupling to Z′ is suppressed by the mixing angle θ. Instead of freezing-out via pair annihilation as χ1χ¯1→ff¯, we show that the observed relic abundance can be obtained through the processes as coannihilation χ1χ2→ff¯ and conversion χiχ2→χjχ1. We then explore the feasible parameter space of inelastic Dirac dark matter in U(1)B−L. Under various experimental constraints, the inelastic Dirac dark matter prefers the region with 3×10−6≲g′≲2×10−4 and 0.02GeV≲mZ′≲10 GeV, which is within the reach of future Belle II, FASER and SHiP.