The Qweak Experiment -- A search for new physics at the TeV Scale
Abstract
A new precision measurement of the parity violating analyzing power in longitudinally polarized electron scattering from the proton at very low Q2 at an incident energy of 1.16 GeV is in the final stages of preparation for execution at Jefferson Laboratory (JLab). A 2200 hour measurement of the parity violating asymmetry in elastic electron-proton scattering at Q2 = 0.03 (GeV/c)2 employing 180 microamp of 85% polarized beam on a 0.35 m long liquid hydrogen target will determine the weak charge of the proton, Qw = 1 - 4sin2(thetaW), with 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of Qw, based on the `running' of the weak mixing angle sin2(thetaW) from the Z-pole down to lower energies. Any significant deviation of sin2(thetaW) from its Standard Model prediction at low Q2 would constitute a signal of new physics. In the absence of new physics, the envisaged experiment will provide a 0.3% determination of sin2(thetaW), making this a very competitive measurement of the weak mixing angle. Complementary to the present experiment is a measurement of the weak charge of the electron in parity violating Moller scattering at 11 GeV, currently under consideration, with the upgraded CEBAF at JLab. The objective of that experiment would be a measurement of sin2(thetaW) with a precision comparable to or better than any individual measurement at the Z-pole.