An international collaboration of scientists working at Jefferson Lab has completed 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 existed to carry out the first precision measurement of the proton’s weak charge, QpW=1 – 4sin2θW, 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 parity violating electron scattering (PVES) experiments to constrain hadronic corrections. Under analysis are the data collected during a multi-year measurement of the parity violating asymmetry in elastic ep scattering at Q2=0.025 (GeV/c)2 employing 180 µA of 89% polarized beam on a 35 cm liquid Hydrogen target to precisely determine the proton’s weak charge.
The Standard Model of electro-weak interactions makes a firm prediction of QpW, based on the running of the weak mixing angle sin2θW from the Z0 pole down to low energies, corresponding to a 10σ effect in our experiment. Any significant deviation of sin2θW 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 absence of physics beyond the Standard Model, our experiment will provide a ≅0.3% measurement of sin2θW, making this a very competitive standalone measurement of the weak-mixing angle.
Bates Laboratory was responsible for the design, procurement, assembly and testing of the QTOR Toroid and support frame used for the Qweak experiment. Bates also designed, procured, and tested the power supply (9000A, 200V) for the toroid. The toroid was field mapped at Bates, then disassembled and transported to JLab for the experiment.
More info can be found at this website: https://www.jlab.org/qweak/