Natural Supersymmetry: Status and Prospects

The realization that supersymmetry (SUSY), if softly broken at the weak scale, can stabilize the Higgs sector led many authors to explore the role it may play in particle physics. It was widely anticipated that superpartners would reveal themselves once the TeV scale was probed in high energy collisions. Experiments at the LHC have not yet revealed any sign for direct production of superpartners, or for any other physics beyond the Standard Model. This has led to some authors to question whether weak scale SUSY has a role to play in stabilizing the Higgs sector. We show that SUSY models with just the minimal particle content may well be consistent with data and simultaneously serve to stabilize the Higgs sector, if model parameters generally regarded as independent turn out to be appropriately correlated. In our view, it would be premature to ignore this possibility, given that we do not understand the underlying mechanism of SUSY breaking. We advocate using the electroweak scale quantity, $delew$, to determine whether a given SUSY spectrum might arise from a theory with low fine-tuning, even when the parameters correlations mentioned above are present. We find that all such models contain light higgsinos and that this leads to the possibility of new strategies for searching for SUSY. We discuss phenomenological implications of these models for SUSY searches at the LHC and its luminosity and energy upgrades, as well as at future electron-positron colliders. We conclude that natural SUSY, defined as no worse than a part in 30 fine-tuning, will not escape detection at a $pp$ collider operating at 27~TeV and an integrated luminosity of 15~ab$^{-1}$, or at an electron-positron collider with a centre-of-mass energy of 600~GeV.