The Qweak experiment – A search for physics and the TeV scale

Abstract

Abstract A new precision measurement of the parity violating analyzing power in longitudinally polarized electron scattering from the proton at very low Q 2 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, Q W p = 1 − 4 sin 2 θ W , 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 Q 2 = 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 Q W p , based on the ‘running’ of the weak mixing angle sin 2 θ W from the Z o pole down to lower energies. Any significant deviation of sin 2 θ W from its Standard Model prediction at low Q 2 would constitute a signal of new physics. In the absence of new physics, the envisaged experiment will provide a 0.3% determination of sin 2 θ W , 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 sin 2 θ W with a precision comparable to or better than any individual measurement at the Z o pole.