Mono-dark-Higgs searches

蒂姆博士项目的目标是进行世界领先的分析，寻找暗物质（DM）的证据，以及调解暗物质与标准模型（SM）相互作用的新粒子。现在，压倒性的天体物理学证据表明暗物质的存在，但对它的粒子性质却一无所知：它无法用暗物质来解释，这仍然是物理学中最大的未决问题之一。SM的几个扩展假设稳定的、电中性的、弱相互作用的大质量粒子作为DM候选者，它们可以在大型强子对撞机的高能碰撞中产生。一旦产生，这种DM将逃避检测，在探测器测量的横向动量（丢失ET）中产生不平衡。在大型强子对撞机中探索的一类广泛的模型假设了一个或多个SM粒子对DM产生反冲的过程，导致“SM +缺失ET”的特征。最近，人们对一套“暗希格斯”模型产生了浓厚的兴趣，在这种模型中，一种类似的机制和玻色子——暗希格斯玻色子——负责赋予暗物质巨大的质量，而且几乎不受观测到的暗物质遗迹密度的限制。Tim将在该部门的牛津ATLAS Exotics小组工作，并将与各自的ATLAS分析小组合作和工作。他将专注于分析LHC Run-2（2015-2018）的ATLAS数据，寻找暗希格斯粒子产生的证据。他将研究暗物质希格斯玻色子衰变为一对希格斯玻色子，这与通过探测器中丢失的ET推断出的暗物质有关。这是一种最终状态，直到现在才开始被实验学家研究，除了暗希格斯模型，它对许多具有扩展希格斯扇区或额外标量的模型很敏感。最初的工作将集中在设计和优化第一个ATLAS分析这些共振的双希格斯加MET特征，建立在现有的超对称希格斯玻色子搜索的基础上，其中两个希格斯玻色子在一个事件中独立产生。他还将研究暗希格斯结果的组合——主要暗希格斯衰变通道的组合：一对b夸克、矢量玻色子和希格斯玻色子，这取决于它们的质量，预计将被包含在ATLAS暗物质总结论文中，该论文将扩展希格斯模型。这将是一个巨大的努力，涵盖许多ATLAS Run-2分析通道，这些通道对这些模型的各个方面都很敏感，并将向一般粒子物理学界总结LHC Run-2为我们对暗物质的理解带来了什么。为了最大限度地提高我们对包含希格斯（或其他重玻色子）衰变成b-夸克对的最终态的灵敏度，高动量b-夸克喷流的识别将是重要的。蒂姆将需要理解和开发新的，更先进的机器学习算法，用于识别b-夸克对衰变。在高希格斯或玻色子动量下，两个b夸克靠得很近，可以在一个大喷流中被捕获。最好的方法是对大喷流进行“双重标记”——将其识别为具有两个与重玻色子衰变相一致的b-夸克喷流，而不是顶夸克衰变或QCD过程。有许多微妙之处，从确保b-夸克对不变质量（衰变玻色子的指示）是无偏的，到确保和微调这些标记器的稳健校准方法（例如从高动量胶子分裂到b-夸克对），而传统的标记器无法使用。这项工作将使他能够很好地分析2021年底或2022年的第一批Run-3数据，碰撞能量将达到14 TeV的新纪录。他将能够确保b标记尽可能高效，这对于在这些签名中使用大亮度Run-3数据进行世界领先的分析至关重要。

The aim of Tim's PhD project is to make world-leading analyses searching for evidence of Dark Matter (DM) and new particles mediating its interactions with the Standard Model (SM). Overwhelming astrophysical evidence now suggests the existence of DM, yet nothing is known of its particle nature: it cannot be accounted in the SM and remains one of the largest open questions in physics.Several extensions of the SM postulate stable, electrically neutral, weakly interacting massive particles as DM candidates, which could be produced in the high energy collisions of the LHC. Once produced, this DM would escape detection, producing an imbalance in the measured transverse momentum (missing ET) of the detector. A wide class of models probed at the LHC postulate processes wherein one or more SM particles are produced recoiling against DM, resulting in a "SM + missing ET" signature.Recently there has been much interest in a set of 'Dark Higgs' models, in which a similar mechanism and boson - the Dark Higgs boson - is responsible for giving dark matter its large mass, and which are little constrained by the observed DM relic density. Tim will work within the Oxford ATLAS Exotics group in the department and will collaborate and work with the respective ATLAS analysis groups. He will focus on the analysis of ATLAS data from LHC Run-2 (2015-2018) for evidence of dark Higgs production. He will work on the dark Higgs decay to a pair of Higgs bosons, in association with dark matter inferred through missing ET in the detector. This is a final state only now starting to be investigated by experimentalists and, in addition to dark Higgs models, is sensitive to many models with extended Higgs sectors or additional scalars. Initially the work will focus on designing and optimising a first ATLAS analysis for these resonant di-Higgs plus MET signatures, building on the existing searches for supersymmetric Higgsinos in which two Higgs bosons are produced independently in an event.He will also be able to work on the combination of dark Higgs results - a combination of the main dark Higgs decay channels: a pair of b-quarks, vector bosons and Higgs bosons depending on the mass is foreseen for inclusion in the ATLAS dark matter summary paper on extended Higgs models. This will be a large effort, covering the many ATLAS Run-2 analysis channels that are sensitive to aspects of these models and will summarise to the general particle physics community what LHC Run-2 has brought to our understanding of dark matter.To maximise our sensitivity to final states containing Higgs (or other heavy boson) decays to b-quark pairs, the identification of high momentum b-quark jets will be important. Tim will need to understand and develop the new, more advanced machine learning algorithms being used to identify b-quark pair decay. At high Higgs or boson momentum, the two b-quarks are close together, and can be captured in one large jet. The best approach to this is to 'double tag' the large jet - identifying it as having two b-quark jets within it consistent with a heavy boson decay, rather than a top quark decay or QCD process. There are many subtleties, from ensuring that the b-quark pair invariant mass (indicative of the decaying boson) is unbiased, to ensuring and fine-tuning a robust means of calibration of these taggers (such as from high momentum gluon splitting to a b-quark pair), where the conventional ones cannot be used. This work will leave him well placed to analyse the first Run-3 data due in late 2021 or 2022, at a new record collision energy of 14 TeV. He will be able to ensure that b-tagging is as performant as possible, which will be vital for world-leading analyses using large luminosities of Run-3 data in these signatures.