Search for New Physics at the TeV Scale via a Measurement of the Weak Charge of the Proton

Willem van Oers

Department of Physics and Astronomy, University of Manitoba
and the QWEAK Collaboration

A low energy search for new physics beyond the Standard Model is proposed at Jefferson Laboratory. The experiment calls for a precise absolute measurement of the weak charge of the proton via a measurement of the parity violating asymmetry in elastic electron-proton scattering at a Q^2 value of about 0.03(GeV/c)^2. The weak charge of the proton is a fundamental quantity which has not been measured. It is the neutral current analog of the vector coupling G_V, which enters neutron and nuclear beta-decay. The proposed measurement in the semi-leptonic sector will be complementary to SLAC experiment E158, which is carrying out parity violating asymmetry measurements in the purely leptonic sector at similar Q^2 in Moeller scattering. The proposal is similar to, but has different theoretical uncertainties than, recently completed and proposed atomic parity experiments, which measure the weak charge of complex nuclei. The present reference design calls for a single toroidal magnet to measure electrons scattered at small angles. A 2000 hour measurement with a 180 microA 80% polarized electron beam incident on a 0.35 m liquid hydrogen target will determine the weak charge of the proton with about a 4% combined statistical and systematic error. In the absence of physics beyond the Standard Model, this would be equivalent to a 0.2% measurement of sin^2(theta_W), making this a truly competitive measurement of this quantity at low Q^2.