Q-weak: First Direct Measurement of the Weak Charge of the Proton

Understand

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.