Heavy Higgsino Interpretation of the LZ Event
The LZ experiment has recently reported a $248$ keV nuclear recoil event in a region where backgrounds are expected to be low. A pure higgsino scattering inelastically through a $Z$-boson is an exciting possible interpretation of this event, but at the mass required for thermal freeze-out to account for all dark matter, $m_\chi = 1.1$ TeV, it predicts several events in the empty high-energy sideband corresponding to $E_R\gtrsim350$ keV and is further excluded by searches for high energy neutrinos coming from the Sun by the IceCube detector. We fit the higgsino mass scale and neutral state mass splitting, $m_\chi$ and $\delta$, to the event and the empty sideband. We additionally recompute solar capture bounds as a function of $m_\chi$. The LZ data alone are best fit at low mass, $m_\chi \approx (240,\,450)$ GeV, but is excluded by solar capture. The parameter space most consistent with the event, the empty sideband and the solar capture bound is $m_\chi \approx (10^5,\,10^6) $ GeV with $\delta \approx (330,\,480)$ keV, with some dependence on DM halo modeling. Such a higgsino must be produced by some other method than standard freeze-out; this can be achieved by freezing-out during a period of early matter domination which ends by reheating the universe to temperature $T_{\rm RH}\approx 1$ TeV. We give a simple proof of concept model which does this via a Kim-Nilles mechanism and therefore connects to the origin of the Higgsino mass scale and solves the strong CP problem.