Light Stop Searches at the LHC in Events with two b-Jets and Missing Energy

TL;DR

The paper investigates discovering a light stop in the MSSM within the co-annihilation region, where the LSP is bino-like and the stop is the NLSP, by studying stop pair production in association with two $b$-jets and the decay $ ilde{t}_1 o c ilde{ abla}_1^0$. It shows that mixed QCD-EW contributions, especially with a higgsino-like chargino, can significantly enhance the signal cross section, improving the LHC discovery potential in the stop-neutralino mass plane. Using a benchmark point and a realistic set of cuts (including a lepton veto and MET requirements), the study finds a 5σ reach up to $m_{ ilde{t}_1} oughly 270$ GeV at 100 fb$^{-1}$ at $ ext{LHC14}$, with EW contributions boosting significance by more than a factor of two; a $2σ$ exclusion could extend to about 340 GeV. The work also discusses enhancements via tau-vetoes and possible charm tagging to further suppress backgrounds and potentially test the SUSY coupling $ ilde{t}_1- ilde{ abla}_1^ abla-b$, offering a complementary avenue to monojet and direct stop searches for probing the SUSY parameter space.

Abstract

We propose a new method to discover light top squarks (stops) in the co-annihilation region at the Large Hadron Collider (LHC). The bino-like neutralino is the lightest supersymmetric particle (LSP) and the lighter stop is the next-to-LSP. Such scenarios can be consistent with electroweak baryogenesis and also with dark matter constraints. We consider the production of two stops in association with two b-quarks, including pure QCD as well as mixed electroweak-QCD contributions. The stops decay into a charm quark and the LSP. For a higgsino-like light chargino the electroweak contributions can exceed the pure QCD prediction. We show the size of the electroweak contributions as a function of the stop mass and present the LHC discovery reach in the stop-neutralino mass plane.

Figures (10)

• Figure 1: Example diagram for QCD stop pair production in association with two $b-$jets via gluon fusion.
• Figure 2: Example diagram for EW stop pair production in association with two $b-$jets via gluon fusion. The chargino, $\tilde{\chi}_1^-$, might be on--shell.
• Figure 3: Number of isolated leptons for the signal and SM backgrounds assuming an integrated luminosity of $1\,\text{fb}^{-1}$ at $\sqrt{s}=14$ TeV. For the signal we assumed the benchmark scenario of Sect. \ref{['sec:benchmark_sceanrio']}, i.e.$m_{\tilde{\chi}_1^0} = \unit[100]{GeV}$, $m_{\tilde{t}_1} = \unit[120]{GeV}$ and $m_{\tilde{\chi}_1^+} = \unit[140]{GeV}$. The distributions are stacked on top of each other.
• Figure 4: Same as Fig. \ref{['fig:no_leptons']}, but now for the $p_T$ distribution of the hardest $b$--jet.
• Figure 5: Same as Fig. \ref{['fig:no_leptons']}, but now for the $p_T$ distribution of the second hardest $b$--jet.