| Description: |
Initial results are presented from the recently-completed Q(weak) experiment at Jefferson Lab. The goal is a precise measurement of the proton's weak charge Q(w)(p), 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 (e) over right arrowp scattering at low momentum transfer, Q(2) = 0.025 GeV2. A 180 mu A longitudinally-polarized 1.16 GeV electron beam was scattered from a 35 cm long liquid hydrogen at small angles, 6 degrees < theta < 12 degrees. 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 Q(W)(p) = 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. |