Solar Capture Tests of Inelastic Dark Matter after the LZ High-Recoil Event

Abstract

The LUX-ZEPLIN (LZ) Collaboration has reported a $248$ keV nuclear-recoil candidate for which endothermic dark matter (DM) gives some of the largest local significances. We study Solar-capture constraints on three simple interpretations: a thermal Higgsino, a thermal pseudo-Dirac fermion with off-diagonal vector interactions, and neutron-philic endothermic spin-dependent scattering through ${\mathcal{O}}_4$. The canonical full-density thermal Higgsino is excluded: its LZ-preferred splitting near $0.38,\mathrm{MeV}$ lies below the IceCube requirement, $\delta \gtrsim 0.51$--$0.56,\mathrm{MeV}$. For the pseudo-Dirac benchmark we find $C_{\odot }=1.48\times 10^{20},{\mathrm{s}}^{-1}$, but two-state kinetics strongly suppresses annihilation. Including thermally assisted ${\chi }_1\to {\chi }_2$ re-excitation gives a maximum ${\Gamma }_A\simeq 4.1\times 10^{14},{\mathrm{s}}^{-1}$, about $1.8\times 10^5$ below the adopted IceCube $b\bar{b}$ limit. For neutron-philic ${\mathcal{O}}_4$ scattering at $m_{\chi }=1$ TeV and $\delta =300$ keV, finite-temperature capture gives $C_{\odot }=3.98\times 10^{17},{\mathrm{s}}^{-1}$, with spin-response range $(1.77$--$9.72)\times 10^{17},{\mathrm{s}}^{-1}$. Even in equilibrium, the upper edge remains about a factor $154$ below the adopted $b\bar{b}$ limit, while an aggressive no-finite-$q$ suppression plus upper-spin stress test still leaves a factor about $19$. The $b\bar{b}$ mapping is used as a representative hadronic template for the two non-Higgsino benchmarks. Solar capture therefore excludes the thermal-Higgsino interpretation but does not generically exclude endothermic explanations of the LZ candidate.

Comments: 33 pages, 13 figures and 3 tables. Comments are welcome