Higgsino Above the Sea of Fog

Abstract

Higgsino dark matter is a classic, but still viable, weakly-interacting massive particle (WIMP) scenario that could point to supersymmetry, a new spacetime symmetry of nature. It is a simple realization of the "inelastic dark matter" mechanism, where its scattering off a nucleus occurs between two nearly degenerate states. Intriguingly, very recently the LZ collaboration reported a single nuclear recoil event at a high recoil energy. Assuming that this event is a dark matter candidate, it could be explained by higgsino dark matter, e.g., a 1.1 TeV thermal higgsino with a mass splitting of about 350 keV (using the standard halo model; a much better understanding of the high speed tail in the dark matter velocity distribution is needed for a more precise determination). We map out the parameter space in the higgsino mass and splitting plane that could account for this single LZ event. The needed mass splitting implies that the electroweak gauginos and heavy Higgs scalars in supersymmetry have to be at a much higher scale $\sim (10^3\text{ to }10^4)$ TeV. We discuss possible UV completions to generate a SUSY spectrum in which all superpartners are at a common scale, except for a much lighter higgsino mass $μ$. Besides the thermal benchmark, we comment on possible non-thermal cosmological scenarios of higgsino dark matter with gravitino and modulus decays. The higgsino dark matter scenario could also be searched for at other complementary probes such as indirect detection and future colliders. One striking LZ event may herald an exciting future of further experimental developments.