Search for the Higgs Boson in its decay to two muons

标准模型（SM）以高精度描述了广泛的粒子物理现象。在SM中，Brout-Englert-Higgs（BEH）机制自发地打破了电弱（EW）规范对称性，并通过Yukawa耦合为W和Z规范玻色子以及带电费米子产生质量。寻找由BEH机制预测的希格斯玻色子。一个质量约为125.5 GeV的类希格斯玻色子现在已经在大型强子对撞机（LHC）的ATLAS和CMS实验中得到了实验证实。到目前为止，已经观察到衰变为光子对、W+-、Z玻色子对和τ轻子对。这些衰变的速率被发现与粒子物理学的标准模型（SM）一致。这些数据也与希格斯玻色子的自旋宇称分配为0+一致。其他重要的性质是与其他基本粒子的耦合，如美夸克、魅夸克、μ子和电子，它们的SM分支比与费米子质量的平方成正比。对于125 GeV的希格斯玻色子质量（mH），H -> mu+mu-的SM分支比为2.2x10-4。一些新的物理扩展的SM，如最小超对称标准模型（MSSM）修改这些预测。H -> mu+mu-衰变有一个干净的终态特征，允许测量希格斯玻色子与第二代费米子的耦合。占主导地位的不可约背景是Z/gamma -> mu+mu-过程，与预期信号的产生速率相比，其具有大约三个数量级的高产生速率。本文将描述使用2015年至2018年期间收集的数据在LHC上使用ATLAS探测器搜索H -> mu+mu-衰变。数据集将足够大，以排除SM的某些新物理扩展。该搜索是为了寻找窄H -> mu+mu-共振的存在，其信号宽度由实验分辨率确定。它是通过拟合在110-160 GeV区域的不变质量分布。这个范围允许确定背景形状和标准化，并设置一个限制的SM希格斯玻色子的质量为125.5 GeV的dimuon衰变。论文的第一章是对标准模型和希格斯玻色子的理论介绍。第2章和第3章将介绍ATALS实验的每个子系统以及触发和数据采集，并总结用于模拟信号过程和开发背景过程分析模型的数据样本和蒙特卡罗（MC）模拟样本。第4章和第5章将描述事件的选择和分类，用于重建μ子的软件，μ子效率的数据驱动确定，接受的蒙特卡罗确定，以及用于描述信号和背景过程的不变质量分布的分析模型。第六章将详细描述系统不确定性的方法和评估。最后，在第7章中，我们将介绍这些结果，并将其与（a）ATLAS和CMS的先前上限以及CMS的竞争测量（或上限）（B）与理论预测进行比较。

The Standard Model (SM) describes a wide range of particle physics phenomena with high precision. In the SM, the Brout-Englert-Higgs (BEH) mechanism spontaneously breaks the electroweak (EW) gauge symmetry and generates masses for the W and Z gauge bosons as well as for the charged fermions via Yukawa couplings. In searches for the Higgs boson predicted by the BEH mechanism. A Higgs-like boson with a mass of about 125.5 GeV is now experimentally well established by the ATLAS and the CMS experiments at the Large Hadron Collider (LHC). So far, decays to pairs of photons, W+-, and Z boson pairs and tau lepton pairs have been observed. The rates of these decays are found to be consistent with the Standard Model (SM) of particle physics. The data is also consistent with a spin parity assignment of Higgs Boson of 0+. Other important properties are the couplings to other elementary particles such as the beauty quark, charm quark, muon and electron, with their SM branching ratios proportional to the squares of the fermion masses. The SM branching ratio for the H -> mu+mu- is 2.2x10-4 for a Higgs boson mass (mH ) of 125 GeV. Some new physics extensions of the SM like the Minimal Supersymmetric Standard Model (MSSM) modify these predictions. The H -> mu+mu- decay has a clean final state signature that allows a measurement of the Higgs boson coupling to second-generation fermions. The dominant irreducible background is the Z/gamma -> mu+mu- process, which has an approximately three orders of magnitude higher production rate compared to that of the expected signal. This thesis will describe the search for H -> mu+mu- decays using data collected between 2015 and 2018 with the ATLAS detector at the LHC. The dataset will be large enough to rule out certain new physics extensions of the SM. This search is for the presence of a narrow H -> mu+mu- resonance, with a signal width determined by experimental resolution. It is performed by fitting the invariant mass distribution in the 110-160 GeV region. This range allows determination of the background shape and normalization and setting a limit on the dimuon decay of the SM Higgs boson with a mass of 125.5 GeV. Chapter 1 of the thesis will be a theoretical introduction to the Standard Model and the Higgs Boson. Chapter 2 & 3 will provide a description of each of the subsystems of the ATALS experiment and the trigger and data acquisition and summarizes the data sample and Monte Carlo (MC) simulation samples used to model the signal process and to develop an analytical model for the background processes. Chapter 4 & 5 will describe the event selections and categorization, the software used to reconstruct muons, data driven determinations of muon efficiency, Monte Carlo determination of the acceptance, and analytical models used to describe invariant mass distributions for signal and background processes. Chapter 6 will describe in detail the methodology and evaluation of the systematic uncertainties. Finally, in Chapter 7 the results will be presented and compared to (a) previous upper limits from ATLAS and CMS and from the competing measurement (or upper limit) from CMS (b) to theoretical predictions.