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 $248\pm23_{\rm stat}\pm23_{\rm sys}\,{\rm keV}$ in a $2.84\,{\rm tonne\text{-}year}$ exposure, with a local significance of $3.4σ$. 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 $(m_χ,δ)$ we compute $N_{\rm low}/N_{\rm sig}$ and $N_{\rm hi}/N_{\rm sig}$---predicted count ratios in the sidebands $[14,225)\,{\rm keV}$ and $(271,800]\,{\rm keV}$ relative to the signal region $[225,271]\,{\rm keV}$. The two ratios vary in opposite directions with $δ$ and are simultaneously optimized at a unique splitting $δ^\star$. For $m_χ=1\,{\rm TeV}$, $δ^\star=339\,{\rm keV}$ with a combined Poisson tension of $\approx1.9σ$; over $500\,{\rm GeV}$--$3\,{\rm TeV}$, the tension at $δ^\star$ rises from $\lesssim1.3σ$ to $\approx2.5σ$. Additional suppression arises from the Helm form factor: $|F_{\rm Xe}^{\rm Helm}|^2\simeq3\times10^{-4}$ at $248\,{\rm keV}$, near its second diffraction zero. A dedicated LZ search over $271<E_R<800\,{\rm keV}$ would provide a decisive test of inelastic dark matter with $δ\gtrsim330\,{\rm keV}$.