Effects of relativity in proton-proton bremsstrahlung

Abstract

We investigate the influence of negative-energy states in proton-proton bremsstrahlung in a fully relativistic framework using the $T$ matrix of Fleischer and Tjon. The contribution from negative-energy states in the single-scattering diagrams is shown to be large, indicating that relativistic effects are important. The rescattering contribution compensates some of the effect, which is shown to be a consequence of a low-energy theorem. The net effect of negative-energy states nevertheless is of the order of 20% at higher energies. We investigate retardation effects in the nucleon-nucleon (NN) interaction by means of a one-pion exchange model, which gives effects of the order of 15% at the pion-production threshold. We furthermore modify the NN $T$ matrix to incorporate some of these effects, and find that on the level of single-scattering contributions they are of the order of 10%. We show predictions at incoming proton energy ${T}_{\mathrm{lab}}=190\mathrm{}\mathrm{MeV},$ where high accuracy measurements are being done at KVI, and conclude that even at these relatively low energies off-shell effects in the NN interaction and contributions from negative-energy states clearly show up.