Qweak: A precision measurement of the proton’s weak charge

Abstract

The Q_weak experiment at Jefferson Lab will measure the parity-violating asymmetry in e–p elastic scattering at very low Q² 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, Q _w , to an accuracy of 4%. Q _w 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^o ± 2^o, corresponding to Q² ~ 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 Q² and the dilution factor of background events. The theoretical context of the experiment and the status of its design are discussed.

PACS: 24.80.+y Nuclear tests of fundamental interactions and symmetries – 25.30.Bf Elastic electron scattering