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

A new precision measurement of parity violation in electron scattering from the proton at very low Q2 and forward angles is being prepared for execution at Jefferson Laboratory (JLab). The experiment is a direct challenge to the predictions of the Standard Model of quarks and leptons and is a search for new physics. There exists a unique opportunity to carry out the first precision measurement of the weak charge of the proton, , by building on technical advances that have been made at Jefferson Laboratory's world-leading parity violating electron scattering program and by using the results of earlier experiments to constrain hadronic corrections. A 2200 hour measurement of the parity violating asymmetry in elastic electron-proton scattering at Q2=0.03 (GeV/c)2 employing 180μA of 85% polarized beam on a 0.35 m long 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 , based on the 'running' of the weak mixing angle sin2(θW) from the Z0 pole down to lower energies, corresponding to a 9 σ effect at the envisaged experiment. Any significant deviation of sin2(θW) from the Standard Model prediction at low Q2 would be a signal of new physics, where as agreement would place new and strict constraints on possible Standard Model extensions. In the absence of new physics the envisaged experiment will provide a 0.3% determination of sin2(θW), making this a very competitive standalone measurement of the weak mixing angle indeed.