Naturalness and light higgsinos: A powerful reason to build the ILC

A core prediction of natural Supersymmetry is the existence of four light higgsinos not too far above the mass of the $Z$ boson. The small mass splittings amongst the higgsinos -- typically 5-20\,GeV -- imply very little visible energy release from decays of heavier higgsinos. In particular, if other SUSY particles are quite heavy, as can be the case in SUSY with radiatively-driven naturalness, the higgsinos are extremely hard to detect at hadron colliders. The clean environment of electron-positron colliders with $\sqrt{s} > 2m_{\mathrm{higgsino}}$, however, would allow for a decisive search for the required light higgsinos. Thus, $e^+e^-$ colliders should either discover or exclude natural SUSY. We present a detailed study of higgsino pair production at the proposed International Linear $e^+e^-$ Collider which is under consideration for construction in Japan. A variety of precision measurements should allow for extraction of underlying parameters and provide a window onto physics at the grand unified scale.