The LZ event with a two-state dark matter halo

The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at E R 248 keV. LZ interprets it in terms of inelastic dark matter that scatters endothermically from its ground state, χ 1 N to χ 2 N. In pseudo-Dirac models the excited state χ 2 can survive to the present day, and exothermic scattering, χ 2 N to χ 1 N, is then also open. The published LZ intervals do not describe this case. We rebuild the LZ statistical inference and validate it against the published intervals. We then apply it to a halo that contains a fraction f of χ 2. For f 10-5 -- 10-2, depending on the mass splitting, the exothermic contribution dominates. For f=1/2 the cross section required by the candidate drops by factors of 80 to 1.6 times10 4 for δ= 250 -- 350 keV, and the kinematic cutoff of the endothermic signal disappears. A benchmark that explains the event with χ 1 alone lies a factor of 30 above the upper limit when half of the halo is χ 2.