The search for pair production of Higgs Bosons at the ATLAS experiment

伊扎博士项目的目标是完成世界领先的分析，寻找新的现象，并利用双希格斯产生来限制希格斯自耦合的价值，特别是四个b夸克的最终状态。希格斯玻色子对的产生是对许多新的物理现象和对希格斯扇区的修正敏感的关键通道，也是对希格斯自耦合和扩展希格斯势的重要测量。虽然一个完整的测量可能需要高亮度LHC的全部亮度，但大型Run-2数据集是敏感的探针，已经能够约束这种重要的耦合。Iza将在该部门的牛津ATLAS Exotics小组工作，并将在ATLAS合作的各自分析小组中合作和工作。她将专注于分析LHC Run-2（2015-2018）的ATLAS数据，寻找双希格斯粒子产生的证据。在研究二希格斯粒子衰变为四个b夸克的过程中，她将寻找新的共振和非共振现象的证据。后者的一个重要方面将是理解和改进如何使用现有的大型数据集来约束希格斯自耦合的不同值。她还将研究Run-2搜索频道的组合，其中最敏感的是4b， bbtautau， bby和bblnulnu。这些通道的组合，每个通道使用完整的140 fb-1 Run-2数据（几乎是以前最佳结果的四倍），将允许对SM耦合进行更强大的约束，或者提供非SM耦合值的证据。这是一项重要的工作，需要在每个分析通道上工作的贡献者付出大量的努力，以了解它们的相对敏感性和贡献。她需要了解不同渠道之间的系统和统计相关性，并开发工具，在这个特定的分析中适当地考虑它们。特别是，已经有了很多的理论进展，导致更精确的预测和以有效的场理论方法模拟BSM物理的能力，这将需要相当多的工作来充分理解，并优化结果的灵敏度和可视化。为了提高对包含希格斯衰变到b夸克对的最终态的灵敏度，她将需要校准和开发新的、先进的b夸克识别算法。她将研究原位校准b标记算法的方法，使用高pt胶子分裂到bb的样品直接在该拓扑中提供校正因子。使用机器学习技术的专用先进b强子标记器正在开发中，其性能显著提高。通常这些都不能用传统的方法校准，但是这些技术可能提供另一种途径，在ATLAS物理项目中改进希格斯喷射的标记。这项工作将使她能够以创纪录的14 TeV碰撞能量分析2021年和2022年的第一批Run-3数据。她将能够确保b标记尽可能高效，这对于在该签名中使用大亮度Run-3数据的世界领先分析至关重要。她将在欧洲核子研究中心度过第二年，尽可能接近实验室和数据采集，并在ATLAS协作中获得知名度。

The aim of Iza's PhD project is to accomplish world-leading analyses looking for new phenomena and constraining the value of the Higgs self-coupling using di-Higgs production, specifically the four b-quark final state.Higgs boson pair production is a critical channel, sensitive to many new physics phenomena and modifications to the Higgs sector, and also an important measurement of the Higgs self-coupling and the extended Higgs potential. Though a full measurement may need the full luminosity of the High Luminosity LHC, the large Run-2 datasets are sensitive probes, already able to constrain this vital coupling.Iza will work within the Oxford ATLAS Exotics group in the department and also will collaborate and work in the respective analysis groups within the ATLAS collaboration. She will focus on analysing ATLAS data from LHC Run-2 (2015-2018) for evidence of di-Higgs production. Working on the di-Higgs decay to four b-quarks, she will search for evidence of new resonant and non-resonant phenomena. An important aspect of the latter will be to understand and improve how different values of the Higgs self-coupling can be constrained using the large datasets now available. She will also work on the combination of the Run-2 search channels, the most sensitive of which are 4b, bbtautau, bbyy and bblnulnu. The combination of these channels, each using the full 140 fb-1 of Run-2 data (nearly four times that of the best previous result) will allow for much more powerful constraints on the SM coupling, or provide evidence of a non-SM coupling value. This is a major undertaking, requiring a great deal of effort from contributors working on each of the analysis channels to understand their relative sensitivities and contributions. She will need to understand the systematic and statistical correlations within the different channels and develop tools that properly account for them in this particular analysis. In particular, there has been a lot of theoretical progress, resulting in more precise predictions and ability to model BSM physics in an effective field theory approach, which will require considerably more work to understand fully, and optimise both the sensitivity and visualisation of the results.To improve the sensitivity to final states containing Higgs decays to b-quark pairs, she will need to calibrate and develop new, advanced b-quark identification algorithms. She will work on means of calibrating b-tagging algorithms in-situ, using samples of high-pT gluon splitting to bb to provide correction factors in this topology directly. Dedicated advanced b-hadron taggers using machine learning techniques are being developed which gain significant performance. Often these cannot be calibrated by conventional means, but these techniques may provide an alternative route, improving Higgs jet tagging across the ATLAS physics programme.This work will leave her ideally placed to analyse the first Run-3 data due in 2021 and 2022, at a new record collision energy of 14 TeV. She 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 this signature. She will spend her second year at CERN to be as close as possible to the laboratory and data-taking, and to gain visibility within the ATLAS Collaboration.