First Determination of the Weak Charge of the Proton

The Qweak experiment has measured the parity-violating asymmetry in →ep elastic scattering at Q²=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.