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 similar to 100-300 GeV, the lighter the better. While such light compressed spectra may be challenging to observe at the LHC, the International Linear e(+) e(-) Collider (ILC) with root s > 2m(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 root s = 500 GeV and 1000 fb(-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 similar to 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.