Fine structure of the cross sections of e+e− annihilation near the thresholds of pp¯ and nn¯ production

The energy dependence of the cross sections of pp¯, nn¯, and meson production in e+e− annihilation in the vicinity of the pp¯ and nn¯ thresholds is studied. The proton-neutron mass difference and the pp¯ Coulomb interaction are taken into account. The values of the cross sections are very sensitive to the parameters of the optical potential. It is shown that the commonly accepted factorization approach for the account of the Coulomb interaction does not work well enough in the vicinity of the threshold due to the finite size of the optical potential well.