Search for physics beyond the Standard Model in Higgs pair final states which decay into two pairs of b-quarks with the ATLAS detector

使用 ATLAS 探测器在希格斯对最终态中寻找超出标准模型的物理，这些最终态衰变成两对 b 夸克

• 批准号: 1793420
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2016
• 资助国家: 英国
• 起止时间: 2016 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-1793420
• 关键词: Search; physics; beyond; Standard; Model

Beojan will analyse ATLAS data from the data-taking period of 2015 to 2018 to search for evidence of new physics beyond the Standard Model (SM) of Particle Physics and set upper cross-section bounds on SM Higgs pair production in the 2H -> 4b final state, where both Higgs bosons decay into a pair of b-quarks. This final state has the largest rate among the possible di-Higgs final states and consequently is an important channel for the future Higgs self-coupling measurement. If the Large Hadron Collider (LHC) performs as expected Beojan will analyse datasets containing up to 150 fb-1, comprising an order of magnitude more than current results. He will work closely with the Oxford ATLAS Exotics and Higgs groups in the department and also will collaborate and work in the respective analysis groups within the ATLAS collaboration. He will also work on feasibility studies with phenomenologists in Oxford and at CERN to improve the sensitivity to this decay channel for a future Higgs self-coupling measurement at the LHC. He will work on the understanding of the kinematics of this process and its backgrounds using Monte Carlo simulations and control regions in data. He will develop selection criteria for this process to maximize the signal (S) over background (B) and the discovery significance S/sqrt(B). The aim is to develop an analysis strategy which will enable him to combine the searches in the different kinematic regimes of this process coherently (resolved, intermediated and boosted regimes). These regimes are defined by the momentum of the two Higgs bosons and the level of collimation of their final state products. Their combination will improve analysis sensitivity, especially for new resonances at higher masses. One of the most important backgrounds for this final state is the multi-jet background and he will develop data-driven background estimation methods in order to minimize the uncertainty on the backgrounds. The theoretical uncertainties on multi-jets are too large and would degrade the sensitivity of the analysis. He will need to develop new experimental techniques to identify highly energetic Higgs bosons in these different kinematic regimes for both Higgs bosons of the 4b final state. The challenges of this final state will be to mitigate the signal efficiency loss due to the fact that the b-quarks collimate more strongly as the energy of the decaying Higgs bosons increases. The collimation of the b-quarks of the H->bb decay will also be an important problem to overcome. He will need to calibrate and develop new and advanced b-quark identification algorithms using variable-R cone sizes and also re-optimise the multivariate b-tagging algorithms. In order to maximise the Higgs boson discrimination against background he will use algorithms and multivariate analysis techniques to select the Higgs jets. As Beojan's final state complements that of Santiago Paredes' thesis in that both share a Higgs boson decaying into a b-quark pair, both students will be able to work together on the H-> bb decay identification aspects. Beojan will focus on the development of advanced b-tagging algorithms, their calibration and systematic uncertainties, whereas Santiago will focus on the jet energy and mass calibration and uncertainties for b-quark jets. If he makes a discovery, he will focus on measuring the mass of the new state and the rate of its production. In the case, there is no discovery he will compute exclusion limits on new physics models and upper cross-section bounds on the SM di-Higgs cross section. The goal is the publication of his analysis and newly developed experimental techniques in the form of ATLAS papers during his PhD. He will spend his second year at CERN to be as close as possible to the laboratory and data-taking, and to gain visibility within the ATLAS collaboration.

Beojan 将分析 2015 年至 2018 年数据采集期间的 ATLAS 数据，以寻找超出粒子物理标准模型 (SM) 的新物理学证据，并为 2H -> 4b 最终状态下的 SM 希格斯对产生设定上横截面界限，其中两个希格斯玻色子衰变成一对 b 夸克。该最终态在可能的双希格斯最终态中具有最大的速率，因此是未来希格斯自耦合测量的重要通道。如果大型强子对撞机 (LHC) 按预期运行，Beojan 将分析包含多达 150 个 fb-1 的数据集，比当前结果高出一个数量级。他将与该系的牛津 ATLAS 奇异物和希格斯小组密切合作，并将在 ATLAS 合作范围内的各自分析小组中合作和工作。他还将与牛津大学和欧洲核子研究中心的现象学家一起进行可行性研究，以提高对大型强子对撞机未来希格斯自耦合测量的衰变通道的敏感性。他将利用蒙特卡罗模拟和数据控制区域来理解这一过程的运动学及其背景。他将为该过程制定选择标准，以最大化背景 (B) 上的信号 (S) 和发现显着性 S/sqrt(B)。目的是开发一种分析策略，使他能够连贯地结合该过程的不同运动学机制（解析、中介和增强机制）中的搜索。这些体系由两个希格斯玻色子的动量及其最终状态产物的准直水平定义。它们的组合将提高分析灵敏度，特别是对于更高质量的新共振。这个最终状态最重要的背景之一是多射流背景，他将开发数据驱动的背景估计方法，以最大限度地减少背景的不确定性。多射流的理论不确定性太大，会降低分析的灵敏度。他需要开发新的实验技术来识别 4b 最终态希格斯玻色子在这些不同运动学机制中的高能希格斯玻色子。这个最终状态的挑战将是减轻信号效率损失，因为随着衰变希格斯玻色子能量的增加，b夸克准直得更加强烈。 H->bb衰变的b夸克的准直也将是一个需要克服的重要问题。他需要使用可变 R 锥尺寸来校准和开发新的先进 b 夸克识别算法，并重新优化多元 b 标记算法。为了最大限度地提高希格斯玻色子对背景的辨别力，他将使用算法和多元分析技术来选择希格斯喷流。由于 Beojan 的最终状态补充了 Santiago Paredes 的论文，因为两者共享一个希格斯玻色子衰变成 b 夸克对，因此两个学生将能够在 H-> bb 衰变识别方面合作。 Beojan 将专注于先进 b 标记算法的开发、其校准和系统不确定性，而 Santiago 将专注于 b 夸克射流的射流能量和质量校准以及不确定性。如果他有发现，他将专注于测量新状态的质量及其产生速率。在这种情况下，没有发现他将计算新物理模型的排除极限和 SM di-Higgs 横截面的横截面上限。目标是在攻读博士学位期间以 ATLAS 论文的形式发表他的分析和新开发的实验技术。他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 合作中获得知名度。