Closing up a light stop window in natural SUSY at LHC

Abstract

Top squark (stop) plays a key role in the radiative stability of the Higgs boson mass in supersymmetry (SUSY). The LHC searches for stop have made a great progress and tightly constrained the stop mass during Run-1. 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 $\tilde{\chi}^0_{1,2}$ and $\tilde{\chi}^\pm_{1}$ are light and nearly degenerate. We find that the stop mass has been excluded up to 430 GeV for $m_{\tilde{\chi}^0_1} \lesssim 250$ GeV and to 540 GeV for $m_{\tilde{\chi}^0_1} \simeq 100$ GeV by the Run-1 SUSY searches for $2b+E^{miss}_T$ and $1\ell+jets+E^{miss}_T$, respectively. In a small strip of parameter space with $m_{\tilde{\chi}^0_1} \gtrsim 190$ 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$^{-1}$ can further cover such a light stop window by monojet and $2b+E^{miss}_T$ searches and push the lower bound of the stop mass to 710 GeV. We also explore the potential to use the Higgs golden ratio, $D_{\gamma\gamma}=\sigma(pp \to h \to \gamma\gamma)/\sigma(pp \to h \to ZZ^* \to4\ell^\pm)$, 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 $e^+e^-$ colliders, the light stop can be excluded for most of the currently allowed parameter region.