Inelastic from the Other Side: Xenon Excitation Signals in Light of the LZ High-Recoil Event

Abstract

The LUX-ZEPLIN (LZ) experiment recently reported a single event consistent with a nuclear recoil of $E_{\rm NR}\sim248\pm23\,(\mathrm{stat})\pm23\,(\mathrm{syst})$~keV in an extended-energy search. We study the accompanying inelastic-xenon channel, $\chi+\Xe\to\chi+\Xe^{*}$, which provides a complementary test of DM interpretations of such high-recoil events. Using the non-relativistic effective-field-theory (NREFT) framework with shell-model nuclear-transition inputs, we compute the inelastic-xenon rates for representative scenarios including inelastic dark matter and accelerated DM populations. We also simulate their S1--S2 responses, with detector modeling validated against public LZ data. The nuclear de-excitation adds an electromagnetic component shifting the signal toward the electronic-recoil band and, in some cases, toward clustered backgrounds from isotopes. We therefore construct a schematic, physics-motivated background model and perform a simplified statistical analysis. For many benchmarks, the inelastic-xenon rate is $\mathcal{O}(0.1)$ of the elastic rate or below, suggesting that substantially larger exposures are required before this companion channel becomes observable. Our results illustrate the importance of isotope-induced background structures in the extended-energy region and extend the phenomenology of inelastic-xenon signatures to a wider range of DM scenarios.