"SR kinematics 8" of "Search for direct top squark pair production in final states with two leptons in $\sqrt{s} = 13$ TeV $pp$ collisions with the ATLAS detector"

The results of a search for direct pair production of top squarks in events with two leptons (electrons or muons) are reported, using 36.1 fb$^{-1}$ of integrated luminosity from proton--proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark $\tilde{t}$ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay $\tilde{t} \rightarrow b \tilde{\chi}_{1}^{\pm}$ into a $b$-quark and the lightest chargino, the decay $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ into an on-shell top quark and the lightest neutralino, the three-body decay $\tilde{t} \rightarrow b W \tilde{\chi}_{1}^{0}$ and the four-body decay $\tilde{t} \rightarrow b \ell \nu \tilde{\chi}_{1}^{0}$. No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the $\tilde{t}$ and $\tilde{\chi}_{1}^{0}$ masses. The results exclude at 95% confidence level $\tilde{\chi}_{1}^{0}$ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.

本文报道了利用大型强子对撞机（Large Hadron Collider）ATLAS探测器采集的、质心系能量$sqrt{s}=13$ TeV的质子-质子对撞数据（积分亮度为36.1 fb⁻¹），针对包含两个轻子（电子或μ子）的事件开展顶超对称夸克（top squark，$ ilde{t}$）直接对产生的搜索结果。
为覆盖顶超对称夸克与较轻超对称粒子之间的各类质量差区间，研究针对顶超对称夸克的四种可能衰变模式设计了专属的事件选择判据：一是衰变$ ilde{t}
ightarrow b ilde{chi}_{1}^{pm}$，即顶超对称夸克衰变为底夸克与最轻荷微子（chargino，$ ilde{chi}_{1}^{pm}$）；二是衰变$ ilde{t}
ightarrow t ilde{chi}_{1}^{0}$，即顶超对称夸克衰变为在壳顶夸克与最轻中性微子（neutralino，$ ilde{chi}_{1}^{0}$）；三是三体衰变$ ilde{t}
ightarrow b W ilde{chi}_{1}^{0}$；四是四体衰变$ ilde{t}
ightarrow b ell
u ilde{chi}_{1}^{0}$。
所有事件选择判据均未观测到显著超出标准模型本底的事件超额，据此设置了以$ ilde{t}$与$ ilde{chi}_{1}^{0}$质量为变量的顶超对称夸克质量限制。
本研究在95%置信度下将最高约720 GeV的$ ilde{chi}_{1}^{0}$质量区间纳入排除范围，拓展了此前搜索覆盖的超对称参数空间排除区域。