The electromagnetic form factors of the proton in the timelike region

The reactions p¯p→e+e− and e+e−→p¯p are analyzed in the near-threshold region. Specific emphasis is put on the role played by the interaction in the initial or final antinucleon-nucleon (N¯N) system which is taken into account rigorously. For that purpose a recently published N¯N potential derived within chiral effective field theory and fitted to results of a new partial-wave analysis of p¯p scattering data is employed. Our results provide strong support for the conjecture that the pronounced energy dependence of the e+e−↔p¯p cross section, seen in pertinent experiments, is primarily due to the p¯p interaction. Predictions for the proton electromagnetic form factors GE and GM in the timelike region, close to the N¯N threshold, and for spin-dependent observables are presented. The steep rise of the effective form factor for energies close to the p¯p threshold is explained solely in terms of the p¯p interaction. The corresponding experimental information is quantitatively described by our calculation.