The LHC higgsino discovery plane for present and future SUSY searches

Considerations from electroweak naturalness and stringy naturalness imply a little hierarchy in supersymmetric models where the superpotential higgsino mass parameter mu is of order the weak scale whilst the soft SUSY breaking terms may be in the (multi-) TeV range. In such a case, discovery of SUSY at LHC may be most likely in the higgsino pair production channel. Indeed, ATLAS and CMS are performing searches in the higgsino mass discovery plane of m((chi) over tilde 20) vs. Delta m(0) equivalent to m((chi) over tilde 20) - m((chi) over tilde 10). We examine several theoretical aspects of this discovery plane in both the gravity-mediation NUHM2 model and the general mirage-mediation (GMM') models. These include: the associated chargino mass m((chi) over tilde1)+/-, the expected regions of the bottom-up notion of electroweak naturalness Delta(EW), and the expected regions of stringy naturalness. While compatibility with electroweak naturalness allows for mass gaps Delta m(0) similar to 4-20GeV, stringy naturalness exhibits a clear preference for yet smaller mass gaps of 4-10GeV. For still smaller mass gaps, the plane becomes sharply unnatural since very large gaugino masses are required. This study informs the most promising SUSY search channels and parameter space regions for the upcoming HL-LHC runs and possible HE-LHC option. (C) 2020 The Authors. Published by Elsevier B.V.