Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments

Abstract

We consider the inelastic dark matter interpretation of the recent 248 keV event observed by LZ and identify different experiments that could test it. Details of the underlying model and the ability of complementary experiments to test it depend significantly on the high-velocity tail of the dark matter distribution. We consider several example velocity distributions motivated by the impact of the Large Magellanic Cloud (LMC) in order to map out the effects of this dependence. For example, we illustrate the type of high-velocity tail that would make the thermal Higgsino viable despite stringent constraints from solar neutrinos: Reducing the flux of the tail relative to simulation results improves the Higgsino fit, while extending the tail to higher velocities at fluxes below current simulation resolution evades the solar neutrino bound. This motivates more detailed simulations at higher resolution to understand the tail better. Even if xenon experiments confirm the signal event, complementary experiments are still required to test whether the underlying model is inelastic dark matter and to break degeneracies in the measurement of dark matter parameters such as mass and mass splitting. Heavy-element paleodetectors could cover most of the parameter space relevant to the LZ event. Their use of different elements could determine the dark matter mass and splitting, while their unique sensitivity to the history of the dark matter distribution could test the signal's LMC origin. Luminous detection in large neutrino detectors with heavy-element supplements could directly test the inelastic nature of the signal and measure the mass splitting, while its sensitivity to the radiative decay rate could provide strong evidence for the Higgsino interpretation.