"Figure 2a" of "Search for direct top squark pair production in events with a Higgs or $Z$ boson, and missing transverse momentum in $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector"

A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a $Z$ boson or a pair of jets compatible with a Standard Model (SM) Higgs boson ($h$) is presented. Requirements on additional leptons, jets, jets identified as originating from b-quarks, and missing transverse momentum are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at sqrt{s} = 13 TeV collected with the ATLAS detector at the LHC in 2015-2016, corresponding to an integrated luminosity of 36.1 fb$^{−1}$. No excess is observed in the data with respect to the expected SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ($\tilde{t}_1$) with long decay chains involving $Z$ or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ($\tilde{t}_2$) decaying via $\tilde{t}_2 \rightarrow Z \tilde{t}_1$ or $\tilde{t}_2 \rightarrow h \tilde{t}_1$. The results exclude at 95% confidence level $\tilde{t}_2$ and $\tilde{t}_1$ masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.

本研究针对顶超对称夸克（top squark）直接成对产生的过程展开搜寻，该过程产生的末态事件包含两类特征：一类是同味异号双轻子对，其不变质量与Z玻色子（Z boson）相符；另一类是与标准模型（Standard Model, SM）希格斯玻色子（Higgs boson, h）相符的喷注对。为靶向顶超对称夸克对的其余衰变产物，研究对额外轻子、喷注、被识别为起源于b夸克的喷注以及横向缺失动量施加了筛选条件。本分析采用大型强子对撞机（Large Hadron Collider, LHC）上ATLAS探测器（ATLAS detector）在2015至2016年采集的质心系能量√s=13 TeV的质子-质子碰撞数据，对应的积分亮度为36.1 fb⁻¹。相较于预期的标准模型预言，实验数据中未观测到显著超出。本结果在两类模型框架下进行解释：第一类模型考虑轻质量顶超对称夸克（$ ilde{t}_1$）的直接成对产生，其衰变具有包含Z玻色子或希格斯玻色子的长衰变链；第二类模型包含重质量顶超对称夸克对（$ ilde{t}_2$）的直接成对产生，其衰变方式为$ ilde{t}_2
ightarrow Z ilde{t}_1$ 或 $ ilde{t}_2
ightarrow h ilde{t}_1$。本结果在95%置信度下排除了质量最高约800 GeV的$ ilde{t}_2$与$ ilde{t}_1$参数空间，拓展了此前大型强子对撞机搜寻所覆盖的超对称参数空间排除区域。