Q-weak: A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering
We propose a new precision measurement of parity violating electron
scattering on the proton at very low Q2 and forward angles to
challenge predictions of the Standard Model and search for new physics. A
unique opportunity exists to carry out the first precision measurement of
the proton's weak charge,
QPW=1 -
4sin2
W, at JLab, building on
technical advances that have been made in the laboratory's world-leading
parity violation
program and using the results of earlier experiments to constrain hadronic
corrections. A 2200 hour measurement of the parity violating asymmetry in
elastic ep scattering at Q2=0.03 (GeV/c)2 employing
180 
A of 85% polarized beam on a 35 cm liquid
Hydrogen target will determine the proton's weak charge with 
4% combined statistical and systematic errors. The
Standard Model makes a firm prediction of
QpW, based on the
running of the weak mixing angle
sin2W
from the Z0 pole down to low energies,
corresponding to a 10
effect in our experiment. Any significant deviation of
sin2W from the
Standard Model prediction at low Q2 would be a
signal of new physics, whereas agreement would place new and significant
constraints on possible Standard Model extensions. In the absense of
physics beyond the Standard Model, our experiment will provide a 0.3% measurement of
sin2W,
making this a very competitive standalone measurement of the weak mixing
angle.
The Electroweak Standard Model (SM) has to date been enormously successful although it is known to be incomplete. The search for a fundamental description of nature beyond the SM is driven by two complementary experimental strategies. The first is to build increasingly energetic colliders, such as the Large Hadron Collider (LHC) at CERN, to excite matter into a new form. The second approach is to perform high precision measurements where an observed discrepancy with the SM would reveal the signature of new forms of matter. As shown in Figure 1, the upcoming "Q-weak" measurement at Jefferson Lab will lead to an extremely precise determination of the weak charges of the quarks and as shown in Figure 2, a severe constraint on or a signature of potential new physics at 2 TeV or higher. For example: If nature is cooperative, early LHC running might see new physics in the form of a directly produced Z' boson and the Q-weak measurement would constrain its properties. Alternately, the mass limits for new physics beyond the SM will be significantly raised.
Figure 1 shows the anticipated knowledge after the Q-weak measurement of the weak charges associated with an axial coupling to the electron and a vector coupling to the up and down quarks. The large grey contour displays the previous experimental limits (95% confidence level, CL) reported by the Particle Data Group, together with the prediction of the Standard Model (small red dot). The solid teal ellipse denotes the anticipated constraint provided by the upcoming Q-weak measurement on the proton, (at 1 standard deviation) while the small black contour (95% CL) indicates the full constraint obtained by combining all results. All other experimental limits shown are displayed at 1 standard deviation. Figure 2 shows the model independent mass limit (Λ/g) in TeV as a function of the "nature" of potential new physics (flavor mixing angle, θ;h). The solid red curve is the current limit which includes all recent JLAB PVES measurements, while the dashed blue curve is the anticipated improvement due to Q-weak. If a significant deviation from the SM occurs the dashed blue curve can be radically altered into a small closed ellipse at the flavour mixing angle selected by the new physics signature.