The ILC as a natural SUSY discovery machine and precision microscope: from light higgsinos to tests of unification

TL;DR

The paper demonstrates that the International Linear Collider can effectively discover and precisely measure light higgsinos predicted by natural SUSY models, providing insights into gaugino masses and unification tests.

Contribution

It shows that the ILC can detect and measure light higgsinos with high precision, even in challenging scenarios, and can test gaugino mass unification through detailed mass measurements.

Findings

Higgsino masses can be measured to 1-2% precision at the ILC.

Higgsino mass splittings inform gaugino mass extraction with 3-20% accuracy.

The ILC can test gaugino mass unification and probe natural SUSY spectra beyond HL-LHC reach.

Abstract

The requirement of electroweak naturalness in simple supersymmetric models implies the existence of a cluster of four light higgsinos with mass $\sim 100-300$\,GeV, the lighter the better. While such light compressed spectra may be challenging to observe at LHC, the International Linear $e^+e^-$ Collider (ILC) with $\sqrt{s}>2m_{\rm higgsino}$ would serve as both a SUSY discovery machine and a precision microscope. We study higgsino pair production signatures at the ILC based on full, \texttt{Geant4-}based simulation of the ILD detector concept. We examine several benchmark scenarios that may be challenging for discovery at HL-LHC due to mass differences between the higgsino states between $20$ and $4$\,GeV. Assuming $\sqrt{s}= 500$\,GeV and 1000\,fb$^{-1}$ of integrated luminosity, the individual higgsino masses can be measured to $1-2\%$ precision in 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 $\sim 3-20\%$ (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.