Inferring dark matter masses and interactions from high recoil energy events in LUX-ZEPLIN

Abstract

LUX-ZEPLIN has recently revealed a single nuclear recoil event compatible with a recoil energy of $E_R\simeq248\,\mathrm{keV}$, in a region with a small expected background. Interpreting this as a signature of dark matter, we compute the posterior probability distribution of the parameters of the dark matter scattering off xenon nuclei. We retain the exact two-body kinematics for an arbitrary mass splitting between the incoming and outgoing dark-sector states, thus encompassing both elastic and inelastic scatterings. The data select non-trivial, disconnected regions of this distribution, which correspond to physically distinct endothermic and exothermic regimes, in which the observed recoil is generated either close to a kinematic threshold or through the release of dark-sector mass energy. We determine the recoil spectra preferred by the observed data and show how the absence of accompanying events at lower recoil energies sharply constrains the viable parameter space. We find a kinematically tuned region of endothermic WIMP-like models at high mass for generic couplings, as well as exothermic models ranging from sub-GeV to heavy dark matter with sizable mass splittings. Elastic dark matter scatterings require instead momentum-suppressed and/or nuclear-spin-dependent non-relativistic interactions.