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

College Of William And Mary, Williamsburg VA

Investigators

Abstract

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.