engleski

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 (statistics) ± 31 (systematics) 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, Qp W, by incorporating earlier parity-violating electron scattering (PVES) data at higher Q2 to constrain hadronic corrections. The value of Qp W obtained in this way is Qp W(PVES) = 0.064±0.012, in good agreement with the Standard Model prediction of Qp W(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 Qn W(PVES+APV) = −0.975 ± 0.010.

weak charge; proton; parity violation asymmetry

Q_{;weak}; Collaboration.