Closing up a light stop window in natural SUSY at LHC

Top squark (stop) plays a key role in the radiative stability of the Higgs boson mass in supersymmetry (SUSY). In this work, we use the LHC Run-1 data to determine the lower mass limit of the right-handed stop in a natural SUSY scenario, where the higgsinos χ˜1,20 and χ˜1± are light and nearly degenerate. We find that the stop mass has been excluded up to 430 GeV for mχ˜10≲250 GeV and to 540 GeV for mχ˜10≃100 GeV by the Run-1 SUSY searches for 2b+ETmiss and 1ℓ+jets+ETmiss, respectively. In a small strip of parameter space with mχ˜10≳190 GeV, the stop mass can still be as light as 210 GeV and compatible with the Higgs mass measurement and the monojet bound. The 14 TeV LHC with a luminosity of 20 fb−120 fb−1 can further cover such a light stop window by monojet and 2b+ETmiss searches and push the lower bound of the stop mass to 710 GeV. We also explore the potential to use the Higgs golden ratio, Dγγ=σ(pp→h→γγ)/σ(pp→h→ZZ⁎→4ℓ±)Dγγ=σ(pp→h→γγ)/σ(pp→h→ZZ⁎→4ℓ±), as a complementary probe for the light and compressed stop. If this golden ratio can be measured at percent level at the high luminosity LHC (HL-LHC) or future e+e−e+e− colliders, the light stop can be excluded for most of the currently allowed parameter region.