Light Z^prime and Dirac fermion dark matter in the B-L model

Abstract

We consider a U(1)_{B-L} model with a Z^prime portal Dirac fermion dark matter (DM) chi of low mass which couples very weakly to the B - L gauge boson Z^prime . An arbitrary B-L charge Qne pm 1, pm 3 of the DM chi ensures its stability. Motivated by the sensitivity reach of forthcoming “Lifetime Frontier” experiments, we focus on the Z^prime mass, m_{Z^prime }, in the sub-GeV to few GeV range. To evaluate the DM relic abundance, we examine both the freeze-out and freeze-in DM scenarios. For the freeze-out scenario, we show that the observed DM abundance is reproduced near the Z^prime resonance, m_chi simeq m_{Z^prime }/2, where m_chi is the DM mass. For the freeze-in scenario, we focus on m_chi ll m_{Z^prime }. We show that for a fixed value of m_{Z^prime }, g_{BL} values roughly scale as 1/Q to reproduce the observed DM abundance. For various Q values in the range between 10^{-6} and 10^2, we show that the gauge coupling values g_{BL} needed to reproduce the observed DM abundance lie in the search reach of future planned and/or proposed experiments such as FASER, Belle-II, LDMX, and SHiP. In the freeze-in case, the Q values to realize observable g_{BL} values are found to be much smaller than that in the freeze-out case.