EPJ Web of Conferences 73, 07008 (2014)
https://doi.org/10.1051/epjconf/20147307008

First result from Q_weak

Physics Dept., College of William & Mary, Williamsburg, VA 23187, USA

^a e-mail: armd@physics.wm.edu

Published online: 9 June 2014

Abstract

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 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 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.