Higgsino Dark Matter Interpretation of the LUX-ZEPLIN 248 keV Nuclear-Recoil Event

Abstract

We propose Higgsino dark matter as a potential interpretation of the $248\mathrm{keV}$ nuclear-recoil event of interest reported by the LUX-ZEPLIN (LZ) experiment. LZ studied several rare background processes and detector effects in detail, but did not identify any as a likely explanation of the event. A nearly pure Higgsino naturally realizes inelastic dark matter through the off-diagonal $Z$ coupling of two nearly degenerate neutral Majorana states separated by a mass splitting $\delta$. The same electroweak interaction fixes the inelastic Higgsino-nucleon scattering cross section, rather than leaving it as a free parameter. We show that the predicted Higgsino inelastic scattering cross section approaches the published LZ two-sided 90% confidence interval for a Higgsino mass $m_{\tilde{H}}\sim 1\mathrm{TeV}$ and a mass splitting $\delta \sim 350\mathrm{keV}$.

4 pages, 1 figure, v2: Added a discussion of the little hierarchy problem, outlined follow-up work, and updated references