"VR kinematics 3" 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.

本文报道了一项搜索结果：利用大型强子对撞机（Large Hadron Collider, LHC）上的ATLAS探测器（ATLAS）采集的质心系能量$sqrt{s}=13$ TeV的质子-质子碰撞数据，其积分亮度为36.1 fb$^{-1}$，针对包含两个轻子（电子或μ子）的事件中顶夸克超伴子（top squark，$ ilde{t}$）的直接成对产生过程展开搜索。为覆盖顶夸克超伴子与较轻超对称粒子间的各类质量差，研究针对顶夸克超伴子的四种可能衰变模式设计了专属筛选方案：其一为衰变$ ilde{t}
ightarrow b ilde{chi}_{1}^{pm}$，即顶夸克超伴子衰变为底夸克与最轻 chargino（chargino）；其二为衰变$ ilde{t}
ightarrow t ilde{chi}_{1}^{0}$，即衰变为在壳顶夸克与最轻中性微子（neutralino）；其三为三体衰变$ ilde{t}
ightarrow b W ilde{chi}_{1}^{0}$；其四为四体衰变$ ilde{t}
ightarrow b ell
u ilde{chi}_{1}^{0}$。所有筛选方案均未观测到超出标准模型本底的显著事件超额，研究基于顶夸克超伴子与$ ilde{chi}_{1}^{0}$的质量参数设定了顶夸克超伴子的实验限制。本结果以95%置信水平排除了最高约720 GeV的$ ilde{chi}_{1}^{0}$质量区间，拓展了既往同类搜索覆盖的超对称参数空间排除区域。