First result from Qweak
Physics Dept., College of William & Mary, Williamsburg, VA 23187, USA
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Published online: 9 June 2014
Initial results are presented from the recently-completed Qweak experiment at Jefferson Lab. The goal is a precise measurement of the proton's weak charge Qwp, to yield a test of the standard model and to search for evidence of new physics. The weak charge is extracted from the parity-violating asymmetry in elastic scattering at low momentum transfer, Q2 = 0.025 GeV2. A 180 μA longitudinally-polarized 1.16 GeV electron beam was scattered from a 35 cm long liquid hydrogen at small angles, 6∘ < θ < 12∘. Scattered electrons were analyzed in a toroidal magnetic field and detected using an array of eight Cerenkov detectors arranged symmetrically about the beam axis. The initial result, from 4% of the complete data set, is QWp = 0.064 ± 0.012, in excellent agreement with the standard model expectation. Full analysis of the data is expected to yield a value for the weak charge to about 5% precision.
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