Probing for New Physics at the LHC: Unraveling the Higgs Mechanism through Polarisation and Hadronic Decays

欧洲核子研究中心的大型强子对撞机（LHC）以实验室中产生的最高能量对撞质子。ATLAS实验每秒收集4000万张碰撞“图片”，使用快速电子系统选择一小部分（最有趣的），并每秒存储1gb数据以供进一步分析。像我这样的粒子物理学家分析这些海量的数据，调查构成我们宇宙的粒子，研究它们之间的相互作用。2012年，希格斯玻色子被发现，证实了粒子物理标准模型（SM）的一个预测，该理论准确预测了广泛的观测现象。然而，宇宙学观测表明，SM只解释了4%的宇宙，它的“可见”部分，96%是未知的，因此被称为暗物质和暗能量。粒子物理学的目标是尽可能精确地测量SM，并寻找可能构成暗物质的SM之外的物理现象。绝大多数的质子碰撞都涉及到它的组成部分：夸克和胶子（强作用力的载体），它们将夸克和胶子结合在一起。我的工作包括研究质子发射的（弱）矢量玻色子（W和Z，电弱力的载体）的罕见碰撞，使用LHC作为弱玻色子对撞机！这些过程被称为矢量玻色子散射（VBS），对新物理学非常敏感，可以揭示那些不与夸克和胶子相互作用的未被发现的粒子。通过VBS更好地了解电弱力是解决暗物质之谜的一个非常有希望的途径，一些理论预测暗物质是一种弱相互作用的粒子，另外，还可以理解基本粒子是如何获得质量的（以及希格斯玻色子是否在这个过程中是唯一的）。研究电弱力可能是解释中微子微小质量的关键。但探测VBS是非常具有挑战性的，因为这些只是碰撞的一小部分，任何新粒子的细微迹象都很难发现。我的研究重点是从质子-质子碰撞数据中提取罕见VBS事件的技术。利用这些数据，我在数十亿次事件中只选择了60次，我在观察两个W玻色子产生相同电荷的小组中担任领导角色，这是一个非常罕见的过程，每20万亿次碰撞才发生一次（在大型强子对撞机中通常每天一次）。通过收集更多的数据，可以进一步提高测量的精度。但更大的影响可能来自新的分析技术，比如识别强子W玻色子衰变（当W衰变为夸克时）。这很难做到，因为许多其他更频繁的过程也会产生夸克。为了实现这一点，我的研究将涉及复杂的机器学习算法，类似于那些允许自动面部识别或无人驾驶汽车的算法。同样令人感兴趣的是希格斯玻色子，它与弱玻色子的相互作用，以及它的自相互作用（HH），比VBS更罕见，需要升级的大型强子对撞机。这些是非常重要的SM探测，可能会产生新物理学的线索，因为与SM的小偏差会对事件率产生很大的影响。为了提高测量的精度，我对用于收集数据的探测器进行了研发，特别是硅像素探测器，它类似于数码相机中的传感器。在粒子探测器的核心，它们是碰撞产物首先遇到的。我设计和研究新的探测器概念，以应对未来粒子对撞机的挑战。高精度硅探测器对于识别碰撞事件特征至关重要，这是揭开宇宙奥秘的第一步。最后，我对数据保存很敏感，这样未来的理论就可以用ATLAS数据来检验，这样我们今天所做的工作就可以帮助后代了解自然。

The Large Hadron Collider (LHC) at CERN collides protons at the highest energy produced in the laboratory. The ATLAS experiment collects 40 million collision "pictures" per second, selects a small fraction (the most interesting ones) using fast electronic systems, and stores 1 GB of data per second for further analysis. Particle physicists like myself analyse these huge amounts of data, investigating the particles that make up our Universe and studying their interactions.In 2012, the Higgs boson was discovered, confirming a prediction of the Standard Model of particle physics (SM), a theory that accurately predicts a wide range of observed phenomena. Yet, cosmological observations suggest that the SM only explains 4% of the Universe, its "visible" part, with 96% being unknown, and for this reason called dark matter and dark energy. The goal of particle physics is to measure the SM as accurately as possible and search for physics phenomena Beyond the SM (BSM) that dark matter is possibly made of.The vast majority of proton collisions involve its constituents: quarks and the gluons (carriers of the strong force) which hold them together. My work involves the study of the much rarer collision of (weak) vector bosons (W and Z, carriers of the electroweak force) emitted by the protons, using the LHC as a weak boson collider! These processes, known as vector boson scattering (VBS), are highly sensitive to new physics, and could shed light on undiscovered particles that don't interact with quarks and gluons. Gaining a better understanding of the electroweak force through VBS is one very promising path to solving the mystery of dark matter, that some theories predict to be a weakly-interacting particle, and separately, to understanding how fundamental particles acquire mass (and whether the Higgs boson is alone in this process). Studying the electroweak force could be key to explaining the tiny mass of the neutrinos.But probing VBS is very challenging as these are a tiny fraction of collisions, and any subtle sign of new particles is difficult to uncover. My research focuses on techniques to extract the rare VBS events from proton-proton collision data. Using these to select only 60 events among many billions, I had a lead role in the group that observed the production of two W bosons of the same charge, a very rare process that happens once per 20 000 billion collisions (typically once per day at the LHC).The precision of the measurements can be further improved by collecting more data. But an even bigger impact can come from new analysis techniques, such as the identification of hadronic W boson decays (when the W decays to quarks). This is very difficult to do, as many other, more frequent, processes also produce quarks. To achieve this, my research will involve complex machine learning algorithms, similar to those that allow for automatic face recognition or driver-less cars.Also of great interest is the Higgs boson, its interaction with weak bosons, and its self-interaction (HH), even rarer than VBS and which requires an upgraded LHC. These are very important probes of the SM that could yield hints of new physics, as small deviations from the SM can have a large impact on event rates.To improve the precision of measurements, I perform R&D on the detectors used to collect the data, in particular silicon pixel detectors, which are similar to the sensors in digital cameras. At the heart of particle detectors, they are the first that the collision products encounter. I devise and study new detector concepts to cope with the challenges of future particle colliders. High precision silicon detectors are essential to identify the collision event characteristics, the first step towards unraveling the mysteries of our Universe.Finally, I am sensitive to data preservation so that future theories can be tested against ATLAS data, so that the work we do today can help future generations shed light on Nature.