Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event

Recently, the LUX-ZEPLIN (LZ) collaboration reported the observation of a single dark matter (DM)-nucleus scattering event at a nuclear recoil energy of 248 plus or minus23 stat plus or minus23 sys keV, corresponding to an exposure of 2.84 tonne-year. The absence of events at lower nuclear recoil energies in the predicted spectrum is naturally explained if the underlying process is inelastic DM-nucleus scattering. Motivated by this, we investigate the singlet-doublet fermion DM model, in which the DM consists of two pseudo-Dirac states: the Majorana nature of the lighter state forbids tree level Z-mediated elastic scattering identically, while the same states enable inelastic DM-nucleus scattering via Z exchange, with any residual elastic scattering proceeding only through a suppressed Higgs-mediated channel. We further extend the model with a Z 2-even scalar triplet, which is responsible both for generating the pseudo-Dirac splitting and for realizing Majorana neutrino masses via the Type-II seesaw mechanism.