Boosted or Inelastic? Discriminating Interpretations of the LZ 248 keV Event

The LUX-ZEPLIN experiment has reported a single nuclear recoil candidate at E R = 248 plus or minus 23(stat) plus or minus 23(sys) keV, disfavouring the background-only hypothesis at a global significance of 2.6σ. The difficulty such an event poses is not the recoil energy itself but the absence of any accompanying excess at low energy as elastic scattering of halo dark matter yields a monotonically falling spectrum, and supplying the required momentum transfer q 246 MeV already demands m χ 79 GeV. We confront the event with the two kinematically distinct mechanisms that evade this limitation, endothermic inelastic dark matter, in which a mass splitting δ about O(100) keV forbids low-energy recoils and boosted dark matter, in which a light relativistic flux supplies the momentum, treating both with the same model-independent scalar--scalar, pseudoscalar--scalar and pseudoscalar--pseudoscalar effective operators. The two scenarios prove spectrally distinguishable. The inelastic spectra sit near the observed energy for every operator, placing only 6 -- 17% of events below 150 keV, whereas the boosted spectra depend critically on the operator: the scalar and pseudoscalar--scalar interactions place 99% and 92% of their events below 150 keV, while the pseudoscalar--pseudoscalar interaction places 75% above it. Momentum dependence is thus essential to the boosted interpretation, but in the inelastic case it trades against the splitting, the preferred δ decreasing monotonically from O ss to O ps to O pp. Because the scenarios differ across the whole high-energy window, a handful of additional events would separate them, placing the question within reach of the full LZ exposure.