Inelastic Dark Matter Signature at High Recoil Energy in LUX-ZEPLIN and CRESST

The LUX–ZEPLIN Collaboration recently reported one event at $E_{\mathrm{n}\mathrm{r}}=248\pm 23\mathrm{k}\mathrm{e}\mathrm{V}$ with an exposure of $2.84\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{n}\mathrm{e}\cdot \mathrm{y}\mathrm{r}$. In this Letter, we interpret this feature using endothermic dark matter (DM). We consider both direct scattering of the surviving ground-state halo component and the contribution of excited states produced by terrestrial upscattering. Our calculations show that explaining the high-energy event requires $m_{\chi }\gtrsim 500\mathrm{G}\mathrm{e}\mathrm{V}$ and a mass splitting of $\mathcal{O}(300)\mathrm{k}\mathrm{e}\mathrm{V}$, for which the production of excited states inside the Earth is kinematically forbidden. For ${\bar{\sigma }}_n={10}^{-37}\mathrm{c}{\mathrm{m}}^2$, an illustrative two-bin likelihood analysis yields a representative best-fit point at $(m_{\chi },\delta )\simeq (1.105\mathrm{T}\mathrm{e}\mathrm{V},350\mathrm{k}\mathrm{e}\mathrm{V})$. The preferred parameter region may be tested by the planned CRESST upgrade.