Complementary Probes of Light Higgsinos: Electroweak Precision Measurements and Dark Matter Direct Detection
Although higgsinos are well motivated to be light from the viewpoint of naturalness, they remain difficult to detect experimentally because they interact only through electroweak interactions and typically possess a compressed mass spectrum. While higgsino dark matter can be efficiently probed by direct detection experiments when gauginos are relatively light, the sensitivity rapidly deteriorates for heavier gauginos due to the suppression of higgsino-gaugino mixing. In this paper, we investigate the prospects for probing light higgsinos through future electroweak precision measurements. Focusing on scenarios in which charginos and neutralinos are the only light electroweakly interacting superparticles, we evaluate their contributions to the electroweak oblique parameters as well as to the precision observables $M_W$ and $\sin^2\theta_{\mathrm{eff}}$. We compare the projected sensitivities of future $e^+e^-$ colliders with those of dark matter direct detection experiments. We find that future electroweak precision measurements provide a powerful probe of higgsinos with masses $\lesssim 500~\mathrm{GeV}$, including parameter regions with highly compressed spectra and spin-independent scattering cross sections below the neutrino fog. On the other hand, dark matter direct detection experiments are particularly sensitive to scenarios with larger charged-neutral mass splittings induced by higgsino-gaugino mixing, and can probe higgsino dark matter all the way up to the thermal relic mass of $\simeq 1~\mathrm{TeV}$. Our results demonstrate the strong complementarity between electroweak precision measurements and dark matter direct detection experiments in exploring light higgsinos and testing supersymmetric scenarios motivated by naturalness.