Precision Studies of the Standard Model using Parity-Violating Electron Scattering

在我们寻求了解物质的基本结构的过程中，物理学家用它已知的基本成分来描述物质：电子、夸克、光子、胶子等。所有实验观察到的基本粒子及其相互作用都由一种被称为粒子物理标准模型的理论来描述。自1970年代中期创立以来，粒子物理标准模型已成功地描述了核物理和高能物理、加速器实验室和桌面实验中的各种物理现象。然而，有令人信服的理论理由预计标准模型将被打破，以及在精确测量方面的一些实验暗示。因此，人们普遍认为，标准模型只是一种近似的低能量描述，源于更基本的物理，这在目前超出实验范围的能量上是明显的。精密测量为在低能电子加速器上测试粒子物理标准模型提供了一种强有力的方法。位于弗吉尼亚州纽波特纽斯的托马斯·杰斐逊国家加速器设施(杰斐逊实验室)目前是标准模型这些精密测量测试的子类的世界领先实验室。该项目支持在杰斐逊实验室的两个大型合作实验中精确测量质子和电子的弱电荷。与电荷类似，弱电荷描述了这些粒子通过弱核力相互作用的强度。这两个实验的结果本身，但也与欧洲核子研究组织(CERN)的大型强子对撞机的结果相结合，将使我们能够约束超出标准模型的物理模型。研究人员正在努力通过具体的招聘和推广努力，增加物理学领域的经济和种族多样性。其中一名研究人员一直在通过一个隶属于APS的国家组织积极促进物理学中的性别多样性，并在威廉和玛丽大学开发了一个新的工程物理和应用设计课程轨道，其目标是吸引和提高有色人种女性和学生对STEM学科的保持能力。弱电混合角是粒子物理标准模型中的一个基本参数，可以通过测量质子或电子的弱电荷来提取核能量。在标准模型中，混合角随能量标度运行，将实测值与预测运行进行比较，允许对标准模型之外的物理进行灵敏的搜索。在量子弱实验(对于质子)和穆勒实验(对于电子)中，使用破坏宇称的电子散射来获取弱电荷。量子弱实验已经取得了数据，签名出版物目前正在准备中。这将构成对质子弱电荷的第一次精确测量。在主要结果公布后，实验将敲定十多项辅助结果。调查人员将继续在模拟和数据分析工作中发挥中心作用，这些工作对分析电子-铝散射的背景和主要探测器的偏振灵敏度至关重要。穆勒实验目前处于设计阶段，将通过测量电子的微弱电荷，使用违反宇称的电子-电子散射来提取电弱混合角，其精度可与现有的最佳高能测量相媲美。研究人员将在为穆勒实验开发和测试径迹重建探测器方面发挥主导作用，并负责一个探测系统，以测量电子和照片产生的粒子的背景。

In our quest to understand the fundamental structure of matter, physicists describe matter by its known elementary constituents: electrons, quarks, photons, gluons, etc. All experimentally observed elementary particles and their interactions are described by a theory known as the Standard Model of Particle Physics. Since its inception in the mid-1970s, the Standard Model of Particle Physics has been successful at describing a wide range of physical phenomena in nuclear and high energy physics, at accelerator laboratories and in table-top experiments. There are, however, compelling theoretical reasons to expect the Standard Model to break down, as well as some experimental hints in precision measurements. It is therefore widely believed that the Standard Model is simply an approximate low-energy description, arising from more fundamental physics that is manifest at energies that are currently beyond experimental reach. Precision measurements present a powerful approach to test the Standard Model of particle physics at low-energy electron accelerators. The Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, is currently the world's leading laboratory for a subclass of these precision measurement tests of the Standard Model. This project supports precision measurements of the weak charge of the proton and of the electron in two large collaborative experiments at Jefferson Lab. In analogy to the electric charge, the weak charge describes how strongly these particles interact through the weak nuclear force. The results of both experiments, by themselves but also in combination with the results from the Large Hadron Collider at the European Organization for Nuclear Research (CERN), will allow us to constrain models of physics beyond the Standard Model. The investigators are working to increase economic and ethnic diversity in physics through specific recruiting and outreach efforts. One of the investigators has been active in improving gender diversity in physics through a national APS-affiliated organization, and is also developing a novel curricular track in Engineering Physics and Applied Design at William & Mary, which has a goal of attracting and improving retention of women and students of color to the STEM disciplines.The electroweak-mixing angle, a fundamental parameter in the Standard Model of Particle Physics, can be extracted at nuclear energies from measurements of the weak charge of the proton or of the electron. The mixing angle runs with energy scale in the Standard Model, and comparison of the measured value with the predicted running allows a sensitive search for physics beyond the Standard Model. The weak charges are accessed using parity-violating electron scattering in the Qweak experiment (for the proton) and the MOLLER experiment (for the electron). The Qweak experiment has taken data and the signature publication is currently in preparation. This will constitute the first precision measurement of the proton's weak charge. After publication of the main result, the experiment will finalize more than a dozen ancillary results. The investigators will continue their central role in the simulation and data analysis efforts crucial to the analysis of the backgrounds from electron-aluminum scattering and the polarization sensitivity of the main detectors. The MOLLER experiment is currently in the design stages and will use parity-violating electron-electron scattering to extract the electroweak mixing angle to a precision comparable to the best available high-energy measurements, through a measurement of the weak charge of the electron. The investigators will take a leading role in the development and testing of the track reconstruction detectors for the MOLLER experiment, and are also responsible for a detection system to measure the background from electro- and photo-produced pions.