Seasonal dark matter from the LUX-ZEPLIN high-energy event

Abstract: The LUX-ZEPLIN experiment has reported a single nuclear recoil at $248\pm23({\rm stat})\pm23({\rm sys})\keV$ in an extended search window. Inelastic dark matter, in which the scattering is endothermic, is a natural mechanism for producing a signal at such high energy. We fit the event to the dark matter mass and the mass splitting between two dark matter states, and find that the data favour a splitting pushed close to the kinematic limit set by the escape speed of the halo. The signal is then supplied by dark matter in the tail of the halo speed distribution. This drives a strongly seasonal signal: annual modulation rises above $50\%$ across the favoured region of parameter space and even reaches $100\%$ for some parameters, implying the signal vanishes for part of the year. Such a strong modulation distinguishes this interpretation sharply from a non-modulating signal, and we quantify the number of events needed to establish it. Experiments with larger target masses than LZ's, notably XLZD and PandaX, will be able to test this directly.