Closing up a light stop window in natural SUSY at LHC

Archil Kobakhidze,Mengchao Zhang,Lei Wu,Ning Liu,Jin Min Yang

doi:10.1016/j.physletb.2016.02.003

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.

Highlights

Weak scale supersymmetry is a leading candidate for solving the naturalness problem of the Standard Model, i.e. explaining the radiative stability of the hierarchy between the electroweak scale and high energy scales, such as Planck mass
In a small strip of parameter space with mχ01 190 GeV, the stop mass can still be as light as 210 GeV and compatible with the Higgs mass measurement and the monojet bound
For mt1 ≫ mt, we found that the stop mass is excluded up to about 540 (430) GeV for μ ≃ 100 (250) GeV by the 8 TeV LHC direct searches in 1l + jets + ETmiss (2b + ETmiss) channel

Summary

INTRODUCTION

Weak scale supersymmetry is a leading candidate for solving the naturalness problem of the Standard Model, i.e. explaining the radiative stability of the hierarchy between the electroweak scale and high energy scales, such as Planck mass. In some popular grand unification models, supersymmetry breaking is usually assumed to transmit to the visible sector at a certain high energy scale, and Yukawa contributions to the renormalization group evolution tend to reduce stop masses more than other squark masses Another one is that the chiral mixing for certain flavor squarks is proportional to the mass of the corresponding quark, and is more sizable for stops. In the compressed region, where ∆mt1−χ01 ≈ mt, the kinematics of the top quarks from stop decay are similar to those in the top pair production and the standard search strategies often suffer from a poor sensitivity For this case, one way is to compare the top pair production cross section measurement with the theoretical prediction, which can rule out stop masses below ∼ 180 GeV for a light neutralino LSP [15, 24, 25].

CALCULATIONS, RESULTS AND DISCUSSION

Direct Constraints

14 TeV 20 fb

CONCLUSIONS