A Consistent Meson-Field-Theoretical Description of PP-Bremsstrahlung
A parameter-free and relativistic extension of the RuhrPot meson-baryon model is used to define the dominant isoscalar meson-exchange currents. We compute pp-bremsstrahlung observables below the $π-$production threshold using a relativistic hadronic current density that includes impulse, wave function re-orthonormalization, meson-recoil, $\bar{\rm N}$N creation and annihilation, $ρπγ$ + $ωπγ$ + $ρηγ$ + $ωηγ$ vector-meson decay and N$Δγ(π,ρ)$ exchange currents. We obtain a good description of the available data. The N$Δγ(π)$ current is shown to dominate the large two-body contributions and closed-form expressions for various non-relativistic approximations are analyzed. An experimental sensitivity to the admixture of pseudo-scalar and pseudo-vector admixture of the NN$π$ interaction is demonstrated. We examine the Lorentz invariance of the NN$\rightleftharpoons$NN $t$-matrices and show a dominantly pseudo-vector NN$π$ coupling renders impulse approximation calculations without boost operators to be essentially exact. Conversely, a similar analysis of the $Δ{\rm N}\rightleftharpoons$NN transitions shows that boost operators and the two-body $NΔγ$ wave function re-orthonormalization meson-recoil currents are required in NN, $Δ$N and $ΔΔ$ coupled channel $t$-matrix applications. The need for additional data is stressed.