Diagrammatic approach to meson production in proton-proton collisions near threshold

We evaluate the threshold T-matrices for the reactions pp→ppπ0, pnπ+, ppη, ppω, pΛK+ and pn→pnη in a relativistic Feynman diagram approach. We employ an effective range approximation to take care of the strong S-wave pN and pΛ final state interaction. We stress that the heavy baryon formalism is not applicable in the NN-system above π-production threshold due to the large external momentum, |p(vector sign)|≅√(Mmπ). The magnitudes of the experimental threshold amplitudes extracted from total cross section data, A=(2.7-0.3i) fm4, B=(2.8-1.5i) fm4, vertical bar C vertical bar =1.32 fm4, |Ω|=0.53 fm4, K=√(2 vertical bar Ks vertical bar2 + vertical bar Kt vertical bar2)=0.38 fm4 and vertical bar D vertical bar =2.3 fm4 can be reproduced by (long-range) one-pion exchange and short-range vector meson exchanges, with the latter giving the largest contributions. Pion loop effects in pp→ppπ0 appear to be small. The presented diagrammatic approach requires further tests via studies of angular distributions and polarization observables