First Determination of Weak Charge of Proton

Abstract

The Qweak experiment has measured the parity-violating asymmetry in ep elastic scattering at $Q^2=0.025 (GeV/c)^2$, employing 145  μA of 89% longitudinally polarized electrons on a 34.4 cm long liquid hydrogen target at Jefferson Lab. The results of the experiment’s commissioning run, constituting approximately 4% of the data collected in the experiment, are reported here. From these initial results, the measured asymmetry is Aep=-279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. The small Q2 of this experiment has made possible the first determination of the weak charge of the proton QWp by incorporating earlier parity-violating electron scattering (PVES) data at higher Q2 to constrain hadronic corrections. The value of QWp obtained in this way is QWp(PVES)=0.064±0.012, which is in good agreement with the standard model prediction of QWp(SM)=0.0710±0.0007. When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted. That PVES+APV analysis reveals the neutron’s weak charge to be QWn(PVES+APV)=-0.975±0.010.