The College of William and Mary, Williamsburg, Virginia 23187, USA.
Phys Rev Lett. 2011 Apr 1;106(13):132501. doi: 10.1103/PhysRevLett.106.132501. Epub 2011 Mar 29.
Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, G(E)/G(M), obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H(e[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5 GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(ℓ) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.
在过去十年中，人们进行了密集的理论和实验研究，旨在解释从截面和极化转移测量中分别获得的质子电-磁形状因子比 G(E)/G(M)数据之间的差异。这种差异的一个可能解释是双光子交换贡献。为了寻找超越单光子交换或玻恩近似的效应，我们报告了在 Q(2)=2.5 GeV(2)处三个不同束能下 H(e[over →],e(')p[over →])弹性反应中极化转移可观测量的测量结果，该能区涵盖了广泛的动力学参数 ε。在玻恩近似下，比值 R 等于 μ(p)G(E)/G(M)，我们发现它在 1.5%的水平上与 ε 无关。纵向极化转移分量 P(ℓ)的 ε 依赖性相对于大 ε 时的玻恩近似增强了 (2.3±0.6)%。
