Proton-proton bremsstrahlung below and above pion-threshold: the influence of the $Δ$-isobar

Abstract

The proton-proton bremsstrahlung is investigated within a coupled-channel model with the $Δ$ degree of freedom. The model is consistent with the $NN$ scattering up to 1 GeV and the $γNΔ$ vertex determined in the study of pion photoproduction reactions. It is found that the $Δ$ excitation can significantly improve the agreements with the $pp \rightarrow ppγ$ at $E_{lab}=280$ MeV. Predictions at $E_{lab}=550$ and $800$ MeV are presented for future experimental tests.