Q_weak: First Direct Measurement of the Weak Charge of the Proton

Hampton University, Hampton VA, 23668, USA

^a e-mail: nur@jlab.org

Published online: 29 April 2014

Abstract

The Q_weak experiment at Hall C of Thomas Jefferson National Accelerator Facility has made the first direct measurement of the weak charge of the proton, Q_W^p, through a precision measurement of the parity-violating asymmetry in elastic e-p scattering at low momentum transfer Q²= 0.025 (GeV/c)² with incident electron beam energy of 1.155 GeV. The Q_weak experiment, along with earlier results of parity violating elastic scattering experiments, is expected to determine the most precise value of Q_W^p which is suppressed in the Standard Model. If this result is further combined with the ¹³³Cs atomic parity violation (APV) measurement, significant constraints on the weak charge of the up quark, down quark, and neutron can be extracted. This data will also be used to determine the weak-mixing angle, sin² θ_W, with a relative uncertainty of < 0.5% that will provide a competitive measurement of the running of sin² θ_W to low Q². An overview of the experiment and its results using the commissioning dataset, constituting approximately 4% of the data collected in the experiment, are reported here.