Inelastic Dark Matter Scattering: Kinematic Suppression, Maximum Splittings and the Targets That Keep a Split Candidate Visible

This study computes the kinematics of inelastic scattering, in which the incoming candidate converts to a slightly heavier state, and derives the maximum mass splitting accessible for each target and candidate mass. The splitting raises the minimum speed required for a recoil of given energy, so scattering is suppressed by the falling tail of the halo velocity distribution and can vanish entirely when the required speed exceeds the galactic escape speed in the Earth frame. Heavy targets and heavy candidates tolerate larger splittings; a splitting of 100 keV is accessible in xenon for candidates near 100 GeV but is closed for light targets and light candidates. Inelastic kinematics naturally produces strong annual modulation and target-dependent rates, which are signatures, not inconveniences. All results are computed, not measured.