Qweak: A precision measurement of the proton’s weak charge
The Qweak experiment at Jefferson Lab will measure the parity-violating asymmetry in e-p elastic scattering at very low Q 2 using a longitudinally polarized electron beam and a liquid hydrogen target. The experiment will provide the first measure of the weak charge of the proton, Qw, to an accuracy of 4%. Qw is simply related to the weak mixing angle θ w, providing a precision test of the Standard Model. Since the value of sin2 θ w is approximately 1/4, the weak charge of the proton Q W P = 1 − 4 sin2 θ w is suppressed in the Standard Model, making it especially sensitive to the value of the mixing angle and also to possible new physics. The experiment employs an 85% polarized, 180 µA, 1.2 GeV electron beam, a 35 cm liquid hydrogen target; and a toroidal magnet to focus electrons scattered at 8° ± 2°, corresponding to Q 2 ∼ 0.03 (GeV/c)2. With these kinematics the systematic uncertainties from hadronic processes are strongly suppressed. To obtain the necessary statistics this 2200 hours experiment must run at an event rate of over 6 GHz. This requires current (integrating) mode detection of the scattered electrons, which will be achieved using synthetic quartz Cherenkov detectors. A tracking system will be used in a low-rate counting mode to determine the average Q 2 and the dilution factor of background events. The theoretical context of the experiment and the status of its design are discussed.
KeywordsStandard Model Extension Weak Charge Atomic Parity Violation Liquid Hydrogen Target Polarize Electron Beam
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