The origin of the proton radius puzzle

Abstract

AbstractThe 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 $$R_E^\alpha $$ R E α stands for the root-mean-square charge radius of the proton $$\sqrt{\langle r_E^2\rangle }$$ ⟨ r E 2 ⟩ , obtained by the experimental or theoretical Collaboration $$\alpha $$ α .) $$R_E^\mathrm{CLAS}= (0.831\pm 0.007_\mathrm{stat}\pm 0.012_\mathrm{syst})$$ R E CLAS = ( 0.831 ± 0 . 007 stat ± 0 . 012 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 $$R_E^\mathrm{MAINZ}= (0.879\pm 0.005_\mathrm{stat}\pm 0.004_\mathrm{syst}\pm 0.002_\mathrm{model}\pm 0.004_\mathrm{group})$$ R E MAINZ = ( 0.879 ± 0 . 005 stat ± 0 . 004 syst ± 0 . 002 model ± 0 . 004 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.