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

在我们寻求理解物质的基本结构时，物理学家通过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称为粒子物理学标准模型的理论描述。自20世纪70年代中期成立以来，粒子物理学标准模型已经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广泛物理现象。然而，有令人信服的理论理由来预期标准模型会崩溃，以及一些精确测量的实验提示。因此，人们普遍认为，标准模型只是一个近似的低能描述，产生于更基本的物理学，在目前实验无法达到的能量下表现出来。精确的测量为在低能电子加速器上检验粒子物理学的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）是目前世界领先的标准模型精密测量测试实验室。该项目支持在杰斐逊实验室的两个大型合作实验中对质子和电子的弱电荷进行精确测量。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，将使我们能够限制标准模型之外的物理模型。研究人员正在努力通过具体的招聘和推广工作来增加物理学的经济和种族多样性。其中一名研究人员通过一个全国性的APS附属组织积极改善物理学中的性别多样性，并正在威廉玛丽大学开发一个新的工程物理和应用设计课程&，其目标是吸引和提高有色人种妇女和学生对STEM学科的保留率。在标准模型中，混合角随能量标度变化，将测量值与预测值进行比较，可以灵敏地搜索标准模型之外的物理。在Qweak实验（质子）和MOLLER实验（电子）中，弱电荷是通过宇称违反电子散射来获得的。Qweak实验已经取得了数据，目前正在准备签名出版物。这将构成对质子弱电荷的第一次精确测量。在公布主要结果后，该实验将最终确定十几个辅助结果。研究人员将继续在模拟和数据分析工作中发挥核心作用，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。MOLLER实验目前处于设计阶段，将通过测量电子的弱电荷，使用宇称破坏电子-电子散射来提取电弱混合角，其精度可与现有最佳高能测量相媲美。研究人员将在MOLLER实验的径迹重建探测器的开发和测试中发挥主导作用，并且还负责检测系统以测量来自电和光产生的π介子的背景。

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.