低$Q^2$时核子电磁形状因子的味分解
Flavor decomposition of the nucleon electromagnetic form factors at low $Q^2$
摘要：
质子内的电荷和磁化的空间分布被编码在弹性形状因子中。这些已经在弹性电子散射中被精确地测量，并且质子和中子的形状因子的组合允许分离上夸克和下夸克的贡献。在这项工作中，我们提取的质子和中子的形状因子从世界的数据，重点是精确的新数据覆盖低动量区，这是敏感的大尺度结构的核子。从这些，我们分开的质子形状因子的上夸克和下夸克的贡献。我们结合横截面和极化测量的弹性电子-质子散射分离质子形状因子和双光子交换（TPE）的贡献。我们结合质子形状因子与参数化的中子形状因子数据和不确定性分离的上和下夸克贡献的质子的电荷和磁形状因子。提取的TPE校正相比，以前的唯象提取，TPE计算，和直接测量的电子和正电子散射的比较。味道分离的形状因子提取和比较模型的核子结构。通过纳入精确的新数据，提取的TPE贡献在低$Q^2$下显示出明显的符号变化，这对于解释高$Q^2$形状因子差异是必要的，同时与已知的$Q^2 \to 0$限值一致。我们发现，新的美因茨数据产生一个显着不同的结果质子磁形状因子和它的味道分离的贡献。我们还观察到上夸克和下夸克分布的均方根半径都小于质子的均方根电荷半径。
英文摘要：
The spatial distribution of charge and magnetization within the proton is encoded in the elastic form factors. These have been precisely measured in elastic electron scattering, and the combination of proton and neutron form factors allows for the separation of the up- and down-quark contributions. In this work, we extract the proton and neutron form factors from world's data with an emphasis on precise new data covering the low-momentum region, which is sensitive to the large-scale structure of the nucleon. From these, we separate the up- and down-quark contributions to the proton form factors. We combine cross section and polarization measurements of elastic electron-proton scattering to separate the proton form factors and two-photon exchange (TPE) contributions. We combine the proton form factors with parameterization of the neutron form factor data and uncertainties to separate the up- and down-quark contributions to the proton's charge and magnetic form factors. The extracted TPE corrections are compared to previous phenomenological extractions, TPE calculations, and direct measurements from the comparison of electron and positron scattering. The flavor-separated form factors are extracted and compared to models of the nucleon structure. With the inclusion of the precise new data, the extracted TPE contributions show a clear change ofsign at low $Q^2$, necessary to explain the high-$Q^2$ form factor discrepancy while being consistent with the known $Q^2 \to 0$ limit. We find that the new Mainz data yield a significantly different result for the proton magnetic form factor and its flavor-separated contributions. We also observe that the RMS radius of both the up- and down-quark distributions are smaller than the RMS charge radius of the proton.
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