"Cutflow 2" 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-质子碰撞积分亮度36.1 fb⁻¹的数据，针对包含两个轻子（电子或μ子）的事件开展顶夸克超伴子（top squark，$ ilde{t}$）直接成对产生的搜索结果。为覆盖顶夸克超伴子与较轻超对称粒子间的各类质量差区间，研究针对顶夸克超伴子的四种可能衰变模式设计了专属筛选方案：衰变$ ilde{t}
ightarrow b ilde{chi}_{1}^{pm}$（产生一个底夸克与最轻带电微子（chargino））、衰变$ ilde{t}
ightarrow t ilde{chi}_{1}^{0}$（产生一个在壳顶夸克与最轻中性微子（neutralino））、三体衰变$ ilde{t}
ightarrow b W ilde{chi}_{1}^{0}$以及四体衰变$ ilde{t}
ightarrow b ell
u ilde{chi}_{1}^{0}$。所有筛选方案均未观测到显著超出标准模型本底的事件，据此针对顶夸克超伴子设置了以$ ilde{t}$与$ ilde{chi}_{1}^{0}$质量为变量的实验限制。本研究在95%置信水平下排除了最高约720 GeV的$ ilde{chi}_{1}^{0}$质量区间，拓展了以往搜索覆盖的超对称参数空间排除区域。