"Figure 10a" of "Constraints on spin-0 dark matter mediators and invisible Higgs decays using ATLAS 13 TeV $pp$ collision data with two top quarks and missing transverse momentum in the final state"

This paper presents a statistical combination of searches targeting final states with two top quarks and invisible particles, characterised by the presenceof zero, one or two leptons, at least one jet originating from a $b$-quark and missing transverse momentum.The analyses are searches for phenomena beyond the Standard Model consistent with the direct production of dark matter in $pp$ collisions at the LHC, using 139 fb$^{-\text{1}}$ of data collected with the ATLAS detector at a centre-of-mass energy of 13 TeV.The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition,the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs boson is producedaccording to the Standard Model in association with a pair of top quarks.For scalar (pseudoscalar) dark matter models, with all couplings set to unity, the statistical combination extends the mass range excluded by the best of the individual channels by 100 (30) GeV, excluding mediator masses up to 370 GeV.In addition, the statistical combination improves the expected coupling exclusion reach by 14\% (24\%), assuming a scalar (pseudoscalar) mediator mass of 10 GeV.An upper limit on the Higgs boson invisible branching ratio of 0.38 ($\text{0.30}^{+\text{0.13}}_{-\text{0.09}}$) is observed (expected) at 95\% confidence level.本文针对包含两个顶夸克（top quark）与不可见粒子的末态开展统计组合搜索研究，该类末态的特征为：存在0、1或2个轻子（lepton）、至少一个源自b夸克（b-quark）的喷注（jet），以及存在横向缺失动量（missing transverse momentum）。本次分析利用大型强子对撞机（Large Hadron Collider，简称LHC）上ATLAS探测器（ATLAS）采集的、总计139 fb⁻¹的13 TeV质心系能量数据，旨在寻找与质子-质子（pp）碰撞中暗物质（dark matter）直接产生过程相符的、超出标准模型（Standard Model）的新物理现象。研究结果通过两类简化暗物质模型进行诠释：一类包含自旋0标量媒介子，另一类包含自旋0赝标量媒介子。此外，研究结果还被用于约束希格斯玻色子（Higgs boson）的不可见分支比（branching ratio）上限——此时希格斯玻色子按照标准模型理论，与一对顶夸克伴随产生。对于所有耦合常数设为1的标量（赝标量）暗物质模型，本次统计组合搜索将单个最优通道的排除质量范围拓展了100（30）GeV，可排除的媒介子质量最高可达370 GeV。此外，假设标量（赝标量）媒介子质量为10 GeV时，统计组合搜索将预期的耦合常数排除灵敏度提升了14%（24%）。在95%置信度（confidence level）下，观测到的希格斯玻色子不可见分支比上限为0.38，预期上限为$0.30_{-0.09}^{+0.13}$。