"Efficiency SRA 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.

本文报告了一项针对顶超对称夸克（top squark, $ ilde{t}$）直接成对产生的搜寻结果，所用数据由大型强子对撞机（Large Hadron Collider, LHC）上的ATLAS探测器采集，来自质心系能量$sqrt{s}=13$ TeV的质子-质子碰撞，积分亮度为36.1 fb$^{-1}$，分析对象为包含两个轻子（电子或μ子）的事例。为覆盖顶超对称夸克与轻质超对称粒子间的各类质量差区间，研究针对顶超对称夸克的四种衰变模式设计了专属事例选择方案：其一为$ ilde{t}
ightarrow b ilde{chi}_{1}^{pm}$衰变，即顶超对称夸克衰变为b夸克与最轻荷微子（lightest chargino, $ ilde{chi}_{1}^{pm}$）；其二为$ ilde{t}
ightarrow t ilde{chi}_{1}^{0}$衰变，即顶超对称夸克衰变为在壳顶夸克与最轻中性微子（lightest neutralino, $ 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}$。针对所有选择方案，均未观测到超出标准模型（Standard Model）本底的显著事例超出，随后基于顶超对称夸克与最轻中性微子的质量，对顶超对称夸克设置了相应限制。本结果在95%置信度下排除了最高约720 GeV的最轻中性微子质量区间，拓展了此前搜寻覆盖的超对称参数空间排除区域。