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, $\delta=\delta_{\rm UV}e^{-2\pi MR}$, 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 $\sigma_{\chi n}^{Z}\simeq1.86\times10^{-39}\,\mathrm{cm^2}$. Using a two-bin Poisson diagnostic of the extended LZ recoil window, we find a shallow minimum near $(m_\chi,\delta)\simeq(1.35~\TeV,366~\keV)$, while the thermally motivated point $(1.1~\TeV,360~\keV)$ lies only $\Delta\chi^2\simeq0.21$ 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 $3.15\times10^{-2}$ 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.