[论文解读] Search for new phenomena with top-quark pairs and large missing transverse momentum using 140 $\mathrm{fb}^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV with the ATLAS detector

G. Aad|arXiv (Cornell University)|Jan 1, 2024

Particle physics theoretical and experimental studies被引用 1

一句话总结

本论文利用ATLAS探测器收集的13 TeV质子-质子对撞数据（140 fb⁻¹），在顶夸克对事例中搜寻新物理，这些事例具有大缺失横向动量。通过使用先进的神经网络分类器，该研究设定了新的停粒子（stop squarks）排除极限（最高达1230 GeV）和自旋-0暗物质媒介子（最高达370 GeV）的排除极限，其灵敏度在具有挑战性的质量差区域以及多种末态中均得到提升，未发现超出标准模型背景的显著过量。

ABSTRACT

A search is conducted for new phenomena in events with a top quark pair and large missing transverse momentum, where the top quark pair is reconstructed in final states with one isolated electron or muon and multiple jets. The search is performed using the Large Hadron Collider proton--proton collision data sample at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector that corresponds to an integrated luminosity of 140 fb$^{-1}$. An analysis based on neural network classifiers is optimised to search for directly produced pairs of supersymmetric partners of the top quark (stop), and to search for spin-0 mediators, produced in association with a pair of top quarks, that decay into dark-matter particles. In the stop search, the analysis is designed to target models in which the mass difference between the stop and the neutralino from the stop decay is close to the top quark mass. This new search is combined with previously published searches in final states with different lepton multiplicities. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set. Models with neutralinos with masses up to 570 GeV are excluded, while for small neutralino masses models are excluded for stop masses up to 1230 GeV. Scalar (pseudoscalar) dark matter mediator masses as large as 350 (370) GeV are excluded when the coupling strengths of the mediator to Standard Model and dark-matter particles are both set to one. At lower mediator masses, models with production cross-sections as small as 0.15 (0.16) times the nominal predictions are excluded. Results of this search are also used to set constraints on effective four-fermion contact interactions between top quarks and neutrinos.

研究动机与目标

• 在顶夸克对与大缺失横向动量的末态中搜寻新物理，这是超对称和暗物质模型中的关键信号。
• 提升在停夸克-中性子质量差接近顶夸克质量这一具有挑战性区域的灵敏度。
• 结合单轻子、无轻子和双轻子末态的结果，以增强发现潜力。
• 通过有效场论（EFT）方法，对顶夸克与中微子之间的四费米子接触相互作用施加约束。

提出的方法

• 利用ATLAS探测器收集的140 fb⁻¹质子-质子对撞数据，能量为√s = 13 TeV。
• 采用神经网络分类器以重建强子衰变的顶夸克，并区分信号与背景。
• 针对停夸克对产生且停夸克-中性子质量差较小的区域进行优化，该区域是以往搜寻的难点。
• 对单轻子、零轻子和双轻子末态的结果进行统计组合，以提升灵敏度。
• 通过改进对象重建、背景模拟和事件分类，增强对信号的敏感度。
• 应用有效场论（EFT）框架，对顶夸克与中微子之间的四费米子接触相互作用施加约束。

实验结果

研究问题

• RQ1 当停夸克-中性子质量差接近顶夸克质量时，停夸克对产生过程的排除极限是多少？
• RQ2 自旋-0媒介子衰变为暗物质粒子时，其对灵敏度的影响如何随媒介子质量与耦合强度变化？
• RQ3 与以往针对其他轻子多重态的搜寻相比，包含单轻子末态是否能提升整体灵敏度？
• RQ4 本数据集对顶夸克与中微子之间有效四费米子接触相互作用的约束是什么？
• RQ5 本分析结果与以往ATLAS和CMS在相同末态中的搜寻结果相比如何？

主要发现

• 数据中未观测到超出标准模型背景的显著过量。
• 在中性子质量较小时，停夸克质量最高达1230 GeV在95%置信水平下被排除。
• 在停夸克-中性子质量差接近顶夸克质量的模型中，中性子质量最高达570 GeV被排除。
• 当耦合常数设为1时，标量和赝标量暗物质媒介子的质量分别最高达350 GeV和370 GeV被排除。
• 在低媒介子质量区域，生产截面低至名义预测值0.15–0.16倍的模型也被排除。
• 对顶夸克与中微子之间有效四费米子接触相互作用的约束已设定，优于以往EFT分析的限制。