The Qweak Experiment: a Search for New Physics at the TeV Scale ⋆

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). There exists an unique opportunity to carry out the first ever precision measurement of the weak charge of the proton, , by making use of the technical advances that have been made at JLab's world-leading parity violating electron scattering program and by using the results of earlier experiments to remove hadronic contributions. A 2200 hour measurement of the parity violating asymmetry in elastic electron-proton scattering at Q2=0.03(GeV/c)2 employing 180 μ A of 85% polarized beam on a 0.35 m long liquid hydrogen target will determine the weak charge of the proton with 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of , based on the ‘running’ of the weak mixing angle sin2θW from the Zo pole down to lower energies. Any significant deviation of sin2θW 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 sin2θW, 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 Møller scattering at 11 GeV, currently under consideration, with the upgraded CEBAF at JLab. The objective of that experiment would be a measurement of sin2θW with a precision comparable to or better than any individual measurement at the Zo pole.