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 order hundreds of keV. The mass splitting required is very sensitive to the extreme high speed tail of the local dark matter velocity distribution, and thus susceptible to astrophysical uncertainties: the standard halo model prefers ~350 keV, while a model incorporating Large Magellanic Cloud effects prefers ~500 keV (a rough estimate based on partial information about a particular simulation). A much better understanding of the high speed tail is needed to evaluate whether the parameter space is consistent with an IceCube constraint on the annihilation of dark matter captured by the Sun. In any case, the mass splitting of several hundred keV implies that the electroweak gauginos and heavy Higgs scalars in supersymmetry have to be at a much higher scale ~10^{3-4} 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 $\mu$. 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.