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Abstract: A new precision measurement of the parity violating analyzing power inlongitudinally polarized electron scattering from the proton at very low Q^2 atan incident energy of 1.16 GeV is in the final stages of preparation forexecution at Jefferson Laboratory JLab. A 2200 hour measurement of the parityviolating asymmetry in elastic electron-proton scattering at Q^2 = 0.03GeV-c^2 employing 180 microamp of 85% polarized beam on a 0.35 m long liquidhydrogen target will determine the weak charge of the proton, Q w = 1 -4sin^2theta W, with 4% combined statistical and systematic errors. TheStandard Model makes a firm prediction of Q w, based on the `running- of theweak mixing angle sin^2theta W from the Z-pole down to lower energies. Anysignificant deviation of sin^2theta W from its Standard Model prediction atlow Q^2 would constitute a signal of new physics. In the absence of newphysics, the envisaged experiment will provide a 0.3% determination ofsin^2theta W, making this a very competitive measurement of the weak mixingangle. Complementary to the present experiment is a measurement of the weakcharge of the electron in parity violating Moller scattering at 11 GeV,currently under consideration, with the upgraded CEBAF at JLab. The objectiveof that experiment would be a measurement of sin2theta W with a precisioncomparable to or better than any individual measurement at the Z-pole.



Author: Qweak Collaboration: Willem T. H. van Oers

Source: https://arxiv.org/







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