The ILC As a Natural SUSY Discovery Machine and Precision Microscope: from Light Higgsinos to Tests of Unification

The requirement of electroweak naturalness in simple supersymmetric models implies the existence of a cluster of four light Higgsinos with a mass $\ensuremath{\sim}100--300\text{ }\text{ }\mathrm{GeV}$, the lighter the better. While such light compressed spectra may be challenging to observe at the LHC, the International Linear ${e}^{+}{e}^{\ensuremath{-}}$ Collider (ILC) with $\sqrt{s}g2{m}_{\text{Higgsino}}$ would serve as both a SUSY discovery machine and a precision microscope. We study Higgsino pair production signatures at the ILC based on a full, geant4-based simulation of the ILD detector concept. We examine several benchmark scenarios that may be challenging for discovery at the HL-LHC due to mass differences between the Higgsino states between 20 and 4 GeV. Assuming $\sqrt{s}=500\text{ }\text{ }\mathrm{GeV}$ and $1000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity, the individual Higgsino masses can be measured to 1%--2% precision in the case of the larger mass differences, and at the level of 5% for the smallest mass difference case. The Higgsino mass splittings are sensitive to the electroweak gaugino masses and allow extraction of gaugino masses to $\ensuremath{\sim}3%--20%$ (depending on the model). Extrapolation of gaugino masses via renormalization group running can test the hypothesis of gaugino mass unification. We also examine a case with natural generalized mirage mediation, where the unification of gaugino masses at an intermediate scale apparently gives rise to a natural SUSY spectrum somewhat beyond the reach of HL-LHC.