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

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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.