The dimension of the proton, the basic building block of matter, is still object of controversy. The most precise electron-proton scattering data at low transferred momenta are re-analyzed and the extraction of the proton radius is discussed. A recent experiment from the JLAB-CLAS collaboration gives a small value for the radius (The symbol \(R_E^\alpha \) stands for the root-mean-square charge radius of the proton \(\sqrt{\langle r_E^2\rangle }\), obtained by the experimental or theoretical Collaboration \(\alpha \).) \(R_E^\mathrm{CLAS}= (0.831\pm 0.007_\mathrm{stat}\pm 0.012_\mathrm{syst})\) fm (Xiong et al. in Nature 575:147, 2019), in contrast with previous electron scattering experiments, in particular with the MAINZ experiment (Bernauer et al. (A1 Collaboration), Phys. Rev. C 90:015206, 2014) that concluded \(R_E^\mathrm{MAINZ}= (0.879\pm 0.005_\mathrm{stat}\pm 0.004_\mathrm{syst}\pm 0.002_\mathrm{model}\pm 0.004_\mathrm{group})\) fm. The experimental results are re-analyzed in terms of different fits of the cross section and of its discrete derivative with analyticity constraints. The uncertainty on the derivative is two orders of magnitude larger than the error on the measured observable, i.e., the cross section. The systematic error associated with the radius is evaluated taking into account the uncertainties from different sources, as the extrapolation to the static point, the choice of the class of fitting functions, and the range of the data sample.

质子的尺寸（物质的基本构成部分）仍然是有争议的对象。重新分析了低传递矩下最精确的电子-质子散射数据，并讨论了质子半径的提取。JLAB-CLAS合作进行的一项最新实验给出了较小的半径值（符号\（R_E ^ \ alpha \）表示质子的均方根电荷半径\（\ sqrt {\ langle r_E ^ 2 \ rangle} \），通过实验或理论协作\（\ alpha \）获得。）\（R_E ^ \ mathrm {CLAS} =（0.831 \ pm 0.007_ \ mathrm {stat} \ pm 0.012_ \ mathrm {syst }）\） fm（Xiong等人，Nature 575：147，2019），与之前的电子散射实验相反，特别是与MAINZ实验（Bernauer等人（A1 Collaboration），Phys。Rev.C 90：015206，2014）相反得出\（R_E ^ \ mathrm {MAINZ} =（0.879 \ pm 0.005_ \ mathrm {stat} \ pm 0.004_ \ mathrm {syst} \ pm 0.002_ \ mathrm {model} \ pm 0.004_ \ mathrm {group} ）\） 调频。根据横截面的不同拟合及其具有分析性约束的离散导数对实验结果进行了重新分析。导数的不确定性比测量的可观察到的误差（即横截面）大两个数量级。评估与半径相关的系统误差时要考虑到来自不同来源的不确定性，例如对静态点的外推，拟合函数类别的选择以及数据样本的范围。