Higgs and $Z$ Assisted Stop Searches at Hadron Colliders

Current searches for the light top squark (stop) mostly focus on the decay channels of $\tilde{t} \rightarrow t \chi_1^0$ or $\tilde{t} \rightarrow b \chi_1^\pm \rightarrow bW \chi_1^0$, leading to $t\bar{t}/bbWW+\met$ final states for stop pair productions at the LHC. However, in supersymmetric scenarios with light neutralinos and charginos other than the neutralino lightest supersymmetric particle (LSP), more than one decay mode of the stop could be dominant. While those new decay modes could significantly weaken the current stop search limits at the LHC, they also offer alternative discovery channels for stop searches. In this paper, we studied the scenario with light Higgsino next-to-LSPs (NLSPs) and Bino LSP. The light stop decays primarily via $\tilde t_1 \to t \chi_2^0/\chi_3^0$, with the neutralinos subsequent decaying to a $Z$ boson or a Higgs boson: $\chi_2^0/\chi^0_3 \to \chi_1^0 h/Z$. Pair production of light stops at the LHC leads to final states of $t \bar t hh\met$, $t \bar t hZ\met$ or $t \bar t ZZ\met$. We consider three signal regions: one charged lepton (1$\ell$), two opposite sign charged leptons (2 OS $\ell$) and at least three charged leptons ($\ge 3 \ell$). We found that the 1$\ell$ signal region of channel $t \bar t hZ\met$ has the best reach sensitivity for light stop searches. For 14 TeV LHC with 300 ${\rm fb}^{-1}$ integrated luminosity, a stop mass up to 900 GeV can be discovered at 5$\sigma$ significance, or up to 1050 GeV can be excluded at 95\% C.L. Combining all three decay channels for $1 \ell$ signal region extends the reach for about 100$-$150 GeV. We also studied the stop reach at the 100 TeV $pp$ collider with 3 ${\rm ab}^{-1}$ luminosity, with discovery and exclusion reach being 6 TeV and 7 TeV, respectively.