Virtual bremsstrahlung in proton-proton scattering below the pion-production threshold

Exclusive differential cross sections of virtual bremsstrahlung in proton-proton scattering below the pion-production threshold have been measured and their leptonic-angle dependence was exploited to determine all six nucleon-nucleon electromagnetic response functions in the timelike region. The data are compared to a low-energy calculation and a fully relativistic microscopic model. Over the entire phase space the low-energy calculation gives a better description of the cross sections than does the microscopic model. At large virtual-photon masses both models are unable to account for the observed strength of the interference response functions.