On the strong energy dependence of the e+e−↔pp¯ amplitude near threshold

We study the energy dependence of the e+e−→pp¯ cross section close to the two-nucleon threshold, recently reported by the BaBar Collaboration. Our analysis also includes the p¯p→e+e− data collected by PS170 Collaboration and the e+e−→NN¯ data from the FENICE Collaboration. We show that the near-threshold enhancement in the e+e−→pp¯ cross section can be explained by the final-state interaction between proton and antiproton in the S13 partial wave, utilizing the Jülich nucleon-antinucleon model. As a consequence, the strong dependence of the proton electromagnetic form factors on the momentum transfer close to the two-nucleon threshold is then likewise driven by this final-state interaction effect. This result is in line with our previous studies of the near-threshold enhancement of the pp¯ invariant mass spectrum seen in the J/Ψ→γpp¯ decay by the BES Collaboration and in the B+→pp¯K+ decay by the BaBar Collaboration.