A Dark-Dimension Origin of Geometric Inelastic Dark Matter: The LUX-ZEPLIN High-Recoil Event and Multi-Target Tests

Abstract

The extended nuclear-recoil analysis of LUX-ZEPLIN (LZ) has reported one event compatible with a recoil energy near $248$ keV, motivating endothermic dark-matter scenarios with support at large recoil energy. We study a pseudo-Dirac electroweak-doublet dark-matter model embedded in a five-dimensional Dark-Dimension framework. The Standard Model and the vectorlike electroweak doublets are localized on the visible brane, while dark-number violation is communicated through a neutral bulk state. The resulting mass splitting is exponentially suppressed, $\delta ={\delta }_{\mathrm{UV}}{e}^{-2\pi MR}$, providing a geometric origin for the few-hundred-keV scale relevant to inelastic scattering. The neutral weak current is dominantly off diagonal, fixing the neutron-scale normalization at ${\sigma }_{\chi n}^Z\simeq 1.86\times 10^{-39}\mathrm{c}{\mathrm{m}}^2$. Using a two-bin Poisson diagnostic of the extended LZ recoil window, we find a shallow minimum near $(m_{\chi },\delta )\simeq (1.35\text{TeV},366\text{keV})$, while the thermally motivated point $(1.1\text{TeV},360\text{keV})$ lies only $\Delta {\chi }^2\simeq 0.21$ higher. The same benchmark is kinematically inaccessible to light targets but remains open for xenon and tungsten, providing a characteristic multi-target test of the model. For CaWO${}_4$ we obtain an integrated tungsten rate of $3.15\times 10^{-2}$ events kg${}^{-1}$ yr${}^{-1}$ in the $95$--$150$ keV interval. These results connect the geometric origin of the pseudo-Dirac splitting with high-recoil direct-detection phenomenology.

Comments: 10 pages, 10 Figures