Model-Independent Sideband Constraints on Inelastic Dark Matter at the LZ High-Recoil Candidate

Abstract

The LUX-ZEPLIN experiment has reported one nuclear-recoil event at $248\pm 23_{\mathrm{stat}}\pm 23_{\mathrm{sys}}\mathrm{keV}$ in a $2.84\mathrm{tonne}\text{-}\mathrm{year}$ exposure, with a local significance of $3.4\sigma$. Inelastic (endothermic) dark matter is a natural candidate: its kinematic threshold suppresses low-energy recoils and shifts the signal toward higher recoil energies. We perform a model-independent analysis of the LZ data using two sidebands, without specifying an ultraviolet completion, so our results apply broadly to inelastic dark matter with coherent spin-independent nuclear scattering. For each $(m_{\chi },\delta )$ we compute $N_{\mathrm{low}}/N_{\mathrm{sig}}$ and $N_{\mathrm{hi}}/N_{\mathrm{sig}}$---predicted count ratios in the sidebands $[14,225)\mathrm{keV}$ and $(271,800]\mathrm{keV}$ relative to the signal region $[225,271]\mathrm{keV}$. The two ratios vary in opposite directions with $\delta$ and are simultaneously optimized at a unique splitting ${\delta }^{\star }$. For $m_{\chi }=1\mathrm{TeV}$, ${\delta }^{\star }=339\mathrm{keV}$ with a combined Poisson tension of $\approx 1.9\sigma$; over $500\mathrm{GeV}$--$3\mathrm{TeV}$, the tension at ${\delta }^{\star }$ rises from $\lesssim 1.3\sigma$ to $\approx 2.5\sigma$. Additional suppression arises from the Helm form factor: $|F_{\mathrm{Xe}}^{\mathrm{Helm}}{|}^2\simeq 3\times 10^{-4}$ at $248\mathrm{keV}$, near its second diffraction zero. A dedicated LZ search over $271<E_R<800\mathrm{keV}$ would provide a decisive test of inelastic dark matter with $\delta \gtrsim 330\mathrm{keV}$.

5 pages, 2 figures