Simulation-Informed High-Velocity Dark Matter in the Effective Theory of Direct Detection

Abstract

The interpretation and reach of direct detection (DD) experiments depend sensitively on the local dark matter (DM) velocity distribution, particularly its high-velocity tail. This tail is crucial for elastic scattering at low DM masses or large momentum transfers, and for inelastic upscattering, where additional kinetic energy is needed to overcome the mass splitting. In practice, however, phenomenological studies typically rely either on the Standard Halo Model (SHM) or, when an enhanced high-velocity tail contributes to the scattering rate, on the single simulation that includes a large Magellanic Cloud (LMC) analogue, Au-25, whose debris reaches ~ 950\kms. In this work, we instead build on a previously presented empirically-motivated reconstruction method, to construct the local DM velocity distribution, using stellar kinematics from the Gaia space telescope. We combine this reconstructed Milky Way component with the best LMC analog in the FIRE-2 simulations, and find no local DM faster than ~ 900\kms. We compare the resulting distribution with the two commonly used benchmarks, the SHM and Au-25. We then study how these different velocity distributions affect the interpretation of DD searches across all operators of the non-relativistic effective field theory of DM--nucleon interactions, for both elastic and inelastic DM. As an illustrative application, we consider the high-energy event observed in the LUX-ZEPLIN extended search. Requiring the event to remain compatible with the absence of signal in the surrounding energy sidebands, we observe the inferred inelastic DM mass splitting can be shifted by up to $\sim 130$~keV depending on the assumed high-velocity tail. Higher-resolution simulations of the high-velocity DM population reaching the detector will therefore be crucial for interpreting a future signal and setting robust limits on DM interactions.