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

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, Qweak(p), through a precision measurement of the parity-violating asymmetry in elastic e-p scattering at low momentum transfer Q^2= 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 Qweak(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, sin^2 theta_?W, with a relative uncertainty of < 0.5% that will provide a competitive measurement of the running of sin^2 theta_?W to low Q^2. 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.