Exclusive differential cross sections of virtual bremsstrahlung in proton-proton scattering below the pion-production threshold have been measured for the first time. The total cross section integrated over photon invariant masses of 15 to $80\mathrm{MeV}/c^2$ and integrated over the acceptance of the detector is $3.2\pm 0.1\pm 0.5\mathrm{pb}$. The data are compared to a low-energy calculation and a fully relativistic microscopic model, which predict a virtual bremsstrahlung cross section of 3.4 and 5.9 pb, respectively. Over the entire experimentally covered phase space, the low-energy calculation gives a better description of the data than the microscopic model.

• Received 25 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2649