"Efficiency SR 3-body 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.

本研究报道了利用大型强子对撞机（Large Hadron Collider）ATLAS探测器（ATLAS detector）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）直接成对产生的搜索结果。为覆盖顶夸克超伴子与更轻超对称粒子间的质量差范围，研究针对顶夸克超伴子的四种可能衰变模式设计了专属事件选择方案：其一为衰变模式$ ilde{t}
ightarrow b ilde{chi}_{1}^{pm}$，即顶夸克超伴子衰变为底夸克与最轻 chargino（$ ilde{chi}_{1}^{pm}$）；其二为衰变模式$ ilde{t}
ightarrow t ilde{chi}_{1}^{0}$，即衰变为在壳顶夸克与最轻中性微子（$ ilde{chi}_{1}^{0}$）；其三为三体衰变$ 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}$。所有选择方案下均未观测到超出标准模型背景的显著事件超额，并针对顶夸克超伴子与最轻中性微子的质量设定了约束条件。本研究以95%置信度排除了最高约720 GeV的最轻中性微子质量区间，拓展了此前同类搜索所覆盖的超对称参数空间排除区域。