2020 Impact factor 3.043
Hadrons and Nuclei
Open Access
Eur. Phys. J. A 24, s2.155-s2.158 (2005)
DOI: 10.1140/epjad/s2005-04-039-5

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

Gregory R. Smith

Jefferson Lab, Newport News, VA 23606, USA

(Received 15 October 2004 / Published online 08 February 2005)


The Qweak 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, Qw, to an accuracy of 4%. Qw is simply related to the weak mixing angle θw, providing a precision test of the Standard Model. Since the value of sin2 θw is approximately 1/4, the weak charge of the proton Qwp = 1-4 sin2 θw 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 8o ± 2o, 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

The Author, for the Qweak Collaboration

© Società Italiana di Fisica, Springer-Verlag 2005