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.