University of Zagreb, Zagreb HR-10002, Croatia.
Phys Rev Lett. 2013 Oct 4;111(14):141803. doi: 10.1103/PhysRevLett.111.141803. Epub 2013 Oct 2.
The Q(weak) 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 A(ep)=-279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. The small Q(2) of this experiment has made possible the first determination of the weak charge of the proton Q(W)(p) by incorporating earlier parity-violating electron scattering (PVES) data at higher Q(2) to constrain hadronic corrections. The value of Q(W)(p) obtained in this way is Q(W)(p)(PVES)=0.064±0.012, which is in good agreement with the standard model prediction of Q(W)(p)(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 Q(W)(n)(PVES+APV)=-0.975±0.010.
Q(弱)实验在 Q(2)=0.025(GeV/c)(2)处测量了 ep 弹性散射中的宇称破坏不对称性，实验采用了 145μA 的 89%纵向极化电子，在杰斐逊实验室的 34.4 厘米长的液氢靶上进行。实验的调试运行结果，约占实验中收集数据的 4%，在此报告。从这些初步结果来看，测量得到的不对称性为 A(ep)=-279±35 (stat) ± 31 (syst) ppb，这是 ep 散射中测量到的最小且最精确的不对称性。该实验的小 Q(2)使得通过结合之前更高 Q(2)处的宇称破坏电子散射 (PVES)数据来约束强子修正，首次确定质子的弱电荷 Q(W)(p)成为可能。这种方法得到的 Q(W)(p)的值为 Q(W)(p)(PVES)=0.064±0.012，与标准模型预测的 Q(W)(p)(SM)=0.0710±0.0007 非常吻合。当将该结果与 Cs 原子宇称破坏 (APV)测量进一步结合时，也可以提取出上夸克和下夸克的弱电荷的重要限制。PVES+APV 分析表明，中子的弱电荷为 Q(W)(n)(PVES+APV)=-0.975±0.010。
