Long lived ALP decays

长寿的 ALP 会衰变

• 批准号: 2623894
• 金额: --
• 依托单位: University of Liverpool
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2623894
• 关键词: Long; lived; ALP; decays

The Standard Model (SM) of Particle Physics describes fundamental particles and how they interact. It leaves several open questions and experiments are searching for new physics beyond the SM. Particles in the SM have lifetimes that expand an enormous range of magnitudes. The lack of discovery at the LHC of new physics beyond the SM has led to research looking for unconventional signatures which might have eluded searches until now. Particles beyond the Standard Model (BSM) could have lifetimes that are long compared to SM particles at the weak scale. These long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision when produced at the Large Hadron Collider (LHC). For a particle to be classified as an LLP it needs to decay a macroscopic, reconstructable distance from the p-p interaction point, or be quasi-stable on the scale of the relevant detector. Long-lived particles can be light or heavy, and can travel fast or slow, and decay into anything that is detectable. The experimental signatures of LLPs at the LHC are varied, and they are typically different from SM signals. An LLP signature can be identified through localised deposits of energy inside calorimeters without associated tracks, such as Axion-like particles (ALPs) in delayed photons. Another LLP signature is displaced vertices in the inner detector or muon spectrometer, such as ALPs in leptons. The unusual signatures of BSM LLPs offer great potential for the discovery of new physics at particle colliders. ALPs are hypothetical light pseudo-Nambu-Goldstone bosons which do not necessarily address the strong CP problem and appear in the spontaneous breaking of a global symmetry. ALPs can interact with all particles of the SM. The search for ALPs varies depending on the mass of the ALP and its couplings to SM particles. Smaller couplings to SM particles result in the ALP having a long lifetime and therefore being macroscopically detectable. Larger couplings and thus a stronger bound with the SM result in the ALP decaying promptly. ALPs can be produced resonantly at the LHC and can either decay inside the detector or be very long lived. The work of this thesis focuses on the cases in which the ALP is very weakly coupled to the standard model. This means the ALP decay is long lived, but not at the point that it is leaving the detector volume without interacting and decaying. If the ALP decays into leptons that are charged they could be found in the tracker or the electromagnetic or hadronic calorimeters, assuming the decay length is not large. If the ALP decays into photons they can only be seen in the electromagnetic calorimeter. Therefore, there are different techniques to look for an ALP in the Higgs decay chain depending on its lifetime and decay mode. Studies with a-yy decaying promptly are on-going so the results of this project will be complementary.The analysis to be carried out for this thesis will use data already collected during the Run 2 of the LHC, as well as data to be collected in the Run 3, starting in summer 2022. For this new data-taking period, the ATLAS detector has been upgraded for muon and other detector components but the Inner Detector (ID) tracking system is unchanged. In each proton-proton collision the direction, momentum, and charge of the electrically charged particles produced is measured by the ID. Due to ageing and increasingly challenging beam conditions, careful studies are needed to ensure good functionalities during the new data-taking period. My work will focus on the Semiconductor Tracker (SCT), one of the components of the ID made of silicon strip detector modules. It is expected that the performance of the silicon modules might be affected by noise increase as a result of radiation damage in preparation to Run 3. I will work to ensure good functionality of the SCT during Run 3 through calibration and test and adjustment of relevant operating parameters to guarantee minimal

粒子物理学的标准模型（SM）描述了基本粒子以及它们如何相互作用。它留下了几个悬而未决的问题，实验正在寻找超越SM的新物理。SM中的粒子具有扩展了巨大幅度范围的寿命。LHC在SM之外缺乏新物理学的发现，导致研究寻找非传统的签名，这些签名可能一直逃避搜索到现在。超越标准模型（BSM）的粒子可能具有比弱尺度下的SM粒子更长的寿命。这些长寿命粒子（LLP）在大型强子对撞机（LHC）中产生时，可以在远离初级质子-质子碰撞的相互作用顶点的地方衰变。对于一个粒子被归类为LLP，它需要从p-p相互作用点衰变一个宏观的、可重构的距离，或者在相关探测器的尺度上是准稳定的。长寿粒子可以轻也可以重，可以快也可以慢，并衰变为任何可检测到的东西。LLP在LHC的实验特征是不同的，它们通常不同于SM信号。LLP特征可以通过量热计内部的局部能量沉积来识别，而没有相关的轨道，例如延迟光子中的类轴子粒子（ALP）。另一个LLP特征是内部探测器或μ子谱仪中的位移顶点，例如轻子中的ALP。BSM LLP的不寻常签名为在粒子对撞机上发现新物理提供了巨大的潜力。ALP是一种假设的轻赝南武-戈德斯通玻色子，它不一定解决强CP问题，而是出现在全局对称性的自发破缺中。ALP可以与SM的所有粒子相互作用。对ALP的搜索取决于ALP的质量及其与SM粒子的耦合。与SM颗粒的较小耦合导致ALP具有长寿命，因此可宏观检测。更大的耦合和因此与SM的更强结合导致ALP迅速衰减。ALP可以在LHC共振产生，并且可以在探测器内衰变或寿命很长。本文的工作主要集中在ALP与标准模型弱耦合的情形。这意味着ALP衰减是长寿命的，但不是在它离开检测器体积而不相互作用和衰减的时候。如果ALP衰变成带电的轻子，那么假设衰变长度不大，它们可以在跟踪器或电磁或强子量热计中找到。如果ALP衰变成光子，它们只能在电磁量热计中看到。因此，有不同的技术来寻找希格斯衰变链中的ALP，这取决于它的寿命和衰变模式。a-yy衰变的研究正在进行中，因此本项目的结果将是互补的。本论文的分析将使用LHC运行2期间已经收集的数据，以及从2022年夏季开始的运行3中收集的数据。在这一新的数据采集期间，ATLAS探测器的μ子和其他探测器组件已经升级，但内部探测器（ID）跟踪系统没有改变。在每次质子-质子碰撞中，产生的带电粒子的方向、动量和电荷都由ID测量。由于老化和越来越具有挑战性的束流条件，需要仔细研究，以确保在新的数据采集期间具有良好的功能。我的工作将集中在半导体跟踪器（SCT），硅条探测器模块制成的ID的组件之一。预计硅模块的性能可能会受到运行3准备过程中辐射损伤导致的噪声增加的影响。本人将通过校准、测试和调整相关操作参数，确保SCT在运行3期间的良好功能，