A Precision Measurement of the Weak Mixing Angle at Low Momentum Transfer

本计划主要针对斯坦福大学直线加速器中心的E158实验，该实验将通过测量Moller（电子-电子）散射中的宇称违反不对称性来确定低动量转移时的弱混合角（温伯格角）。 该实验将为粒子相互作用标准模型的这一基本参数提供世界上最好的低能测量。通过比较E158的结果与现有的高能量测量的弱混合角，我们将建立，第一次，其动量转移依赖性，预测的标准模型。偏离预测将表明标准模型之外的新现象的存在，包括质量范围高达900 GeV的Z'玻色子介导的新中性电流和规模高达11 TeV的新接触相互作用。这种灵敏度匹配或超过了CERN和FNAL的高能对撞机实验的直接和间接搜索灵敏度。在低动量传递下测量宇称违反效应的小不对称性给实验者带来了许多挑战，特别是在控制与极化电子束相关的系统效应方面。加州大学伯克利分校的核物理小组为E158实验带来了在束流仪器和数据分析方面的丰富经验。我们主要负责实验的运行、物理分析和可能的升级。 在基础研究的最前沿，该项目还为研究生和本科生的培训和教育提供了独特的机会，使他们能够亲身体验硬件并接触现代分析技术。

This proposal concentrates on the experiment E158 at the Stanford Linear Accelerator Center, which will determine the weak mixing angle (Weinberg angle) at low momentum transfer by measuring theparity-violating asymmetry in Moller (electron-electron) scattering. The experiment will provide the world's best low energy measurement of this fundamental parameter of the Standard Model of particleinteractions. By comparing the E158 results with the existing high energy measurements of the weak mixing angle, we will establish, for the first time, its momentum transfer dependence, predicted by the Standard Model. Deviations from the prediction would signal the presence of new phenomena beyond the Standard Model, including new neutral currents mediated by Z' bosons in the mass range of up to 900 GeV and new contact interactions with a scale of up to 11 TeV. This sensitivity matches or exceeds the direct and indirect search sensitivities of the high-energy collider experiments at CERN and FNAL.Small asymmetries associated with measuring parity violating effects at low momentum transfer mount many challenges to the experimenters, in particular, in controlling systematics effects related to thepolarized electron beam. The UC Berkeley Nuclear Physics group brings an extensive experience in the beam instrumentation and data analysis to the E158 experiment. We take major responsibilities in the running of the experiment, physics analysis, and possible upgrades. While being at the forefront of the fundamental research, this project also provides unique opportunities for training and education of graduate and undergraduate students, giving them hands-on hardware experience as well as exposure to the modernanalysis techniques.