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

Beojan将分析2015年至2018年数据采集期间的ATLAS数据，以寻找粒子物理学标准模型（SM）之外的新物理学证据，并在2 H-> 4 b最终状态下设定SM希格斯对产生的横截面上限，其中两个希格斯玻色子都衰变为一对b夸克。该末态在可能的双Higgs末态中具有最大的速率，因此是未来Higgs自耦合测量的重要通道。如果大型强子对撞机（LHC）的性能符合预期，Beojan将分析包含高达150 fb-1的数据集，比目前的结果高出一个数量级。他将与牛津ATLAS Exotics和Higgs小组密切合作，并将在ATLAS合作的各个分析小组中进行合作和工作。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，以提高对这种衰变通道的灵敏度，以便在LHC进行未来的希格斯自耦合测量。他将使用Monte Carlo模拟和数据中的控制区域来理解这一过程的运动学及其背景。他将为这一过程制定选择标准，以使信号（S）相对于背景（B）和发现意义S/sqrt（B）最大化。我们的目的是制定一个分析策略，这将使他能够联合收割机的搜索在不同的运动学制度的这一进程连贯一致（解决，调解和推动制度）。这些制度是由两个希格斯玻色子的动量和最终状态产物的准直水平定义的。它们的结合将提高分析灵敏度，特别是对于更高质量的新共振。这种最终状态的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。他将需要开发新的实验技术，以确定高能希格斯玻色子在这些不同的运动学制度的两个希格斯玻色子的4 b最终状态。这种最终状态的挑战将是减轻信号效率的损失，因为随着衰变的希格斯玻色子的能量增加，b夸克的准直性会更强。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。他将需要校准和开发新的和先进的b夸克识别算法，使用可变R锥大小，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，他将使用算法和多变量分析技术来选择希格斯喷流。由于Beojan的最终状态补充了圣地亚哥帕雷德斯的论文，因为两者都有一个希格斯玻色子衰变为b夸克对，所以两个学生将能够在H-> bb衰变识别方面共同努力。Beojan将专注于开发先进的b标记算法，其校准和系统的不确定性，而圣地亚哥将专注于喷流能量和质量校准和b夸克喷流的不确定性。如果他有了新发现，他将专注于测量新状态的质量及其产生速度。在这种情况下，没有发现他将计算新物理模型的排斥极限和SM di-Higgs截面的截面上限。目标是在他博士期间以ATLAS论文的形式发表他的分析和新开发的实验技术。他将在CERN度过他的第二年，尽可能接近实验室和数据采集，并在ATLAS合作中获得知名度。

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.