GAIA meets LZ: a high velocity tail-tale sign for dark matter

Abstract

The interpretation of high-energy nuclear recoils in direct-detection experiments can depend sensitively on the high-velocity tail of the galactic dark matter (DM) distribution. The LUX-ZEPLIN (LZ) experiment has recently reported an event, LZ230616, with nuclear recoil energy 248 $\pm$ 23 (stat) $\pm$ 23 (sys) keV, while the Gaia mission has provided an unprecedented view of the kinematics and mass distribution of the Milky Way. We bring these two probes together to investigate the inelastic DM interpretation of LZ230616, using a velocity distribution inferred from Gaia-informed galactic model. Compared to the Standard Halo Model, the resulting distribution has a lower escape speed and a significantly suppressed high-velocity tail, with a higher local density. We find that while the exothermic interpretation is largely insensitive to this change, the endothermic interpretation requires substantially larger scattering cross sections and smaller mass splittings. Furthermore, using simulation-based study of the gravitational effects of the Large Magellanic Cloud, we show that the high-velocity tail can extend the allowed endothermic parameter space towards lower DM masses and larger splittings. Our results demonstrate how combining direct searches for DM detection with precision measurements of galactic dynamics can qualitatively affect the particle-physics interpretation of high-energy nuclear recoils.