Implications of exclusive photon leptoproduction measurements for the proton charge-radius puzzle

Abstract

In the present study, we extend our previous analysis of the proton electromagnetic form factors (FFs) extracted from exclusive photon leptoproduction (EP) measurements in kinematic regions where the Bethe-Heitler (BH) process dominates the cross section by including all currently available high-precision EP data from the CLAS and Hall~A Collaborations. Using the same phenomenological framework, we investigate the consistency among the different data sets, determine the proton electromagnetic FFs within several fitting scenarios, and extract the corresponding charge and magnetic radii. A significant tension is observed between the CLAS 2018 measurements and the remaining EP data. We show that excluding this data set, or restricting its kinematic coverage by imposing suitable low-$|t|$ cuts, leads to stable fits with good quality and consistent FFs. For all analyses, the extracted proton charge radius is smaller than the Particle Data Group average and most determinations based on elastic electron-proton scattering. However, the results are consistent, within uncertainties, with the PRad measurement and muonic hydrogen spectroscopy. In contrast, the magnetic radius is found to be compatible with the current world average. These results demonstrate that BH-dominated EP measurements provide an independent and complementary approach to determine the electromagnetic structure of the proton and offer additional support for the small-radius solution of the proton charge-radius puzzle.