Stop on top: SUSY parameter regions and fine-tuning constraints

Abstract

We analyze minimal supersymmetric models in order to determine in what parameter regions with what amount of fine-tuning they are capable of accommodating the LHC-allowed top-stop degeneracy window. The stops must be light enough to enable Higgs naturalness yet heavy enough to induce a 125 GeV Higgs boson mass. These two constraints imply a large mass splitting. By an elaborate scan of the parameter space, we show that the stop-on-top scenario requires at least Delta(CMSSM) similar or equal to O(10(4)) fine-tuning in the constrained minimal supersymmetric Standard Model (CMSSM). By relaxing the CMSSM parameter space with nonuniversal Higgs masses, we find that Delta(NUHM1) similar or equal to O(10(4)). The CMSSM with a gravitino lightest supersymmetric particle works slightly better than the nonuniversal Higgs mass model. Compared to all these, the CMSSM with mu < 0 and nonuniversal gauginos yields a much smaller fine-tuning Delta(mu,g) similar or equal to O(100). Our results show that the gaugino sector can pave the road toward a more natural stop-on-top scenario.