Nuclear interference versus dark sector excitation in the 248 keV LUX-ZEPLIN recoil candidate

Abstract

The 2026 LUX-ZEPLIN (LZ) high energy nuclear recoil search reports one event at $248\pm23_{\rm stat}\pm23_{\rm sys}~\mathrm{keV}$ and finds the elastic isovector interaction $\mathcal L_6^v$ locally favored at about $3.3σ$ in the heavy dark matter regime. We examine whether this recoil scale can arise from resolved nuclear response within the elastic interaction and compare its spectral, target, and annual modulation behavior with endothermic dark sector excitation. Covariant matching correlates four Galilean operators, removes the longitudinal spin response algebraically, and fixes interference between density and orbital spin orbit amplitudes. The GCN and JJ55 xenon shell model calculations give a second natural xenon cancellation at $211$ and $214~\mathrm{keV}$. In the heavy mass regime, its position changes by less than $0.01~\mathrm{keV}$ from a dark matter mass of $200~\mathrm{GeV}$ to the asymptotic limit. A curvature weighted isotope centroid reproduces the position and residual depth of the natural xenon minimum. Direct $^{40}$Ar calculations shift the nuclear feature to about $350$ and $425~\mathrm{keV}$, whereas endothermic excitation follows reduced mass scaling set by the dark state splitting. At a dark matter mass of $1~\mathrm{TeV}$, the endothermic Ar to Xe scale ratio is $1.082$, compared with $1.63$ to $2.01$ for the nuclear calculations. Elastic scattering gives a few percent annual modulation in the adopted halo model, while endothermic solutions near the maximum laboratory halo speed show much larger seasonal variation. These scaling and timing behaviors provide tests of a target-dependent nuclear cancellation against an excitation energy set by dark sector kinematics.