Meson-exchange Currents in Pp-bremsstrahlung Typeset Using Revt E X

The long-standing discrepancy between pp-bremsstrahlung data and calculations based on the relativistic impulse approximation current is substantially reduced by the inclusion of the PVγ and intermediate-state ∆-resonance iso-scalar meson-exchange currents. The success of the standard procedures adopted here shows that pp-bremsstrahlung provides a powerful tool for the further study of T =1 isoscalar exchange currents. The need for addition high-precision data is stressed. 1 In earlier work [1] we calculated a number of pp-bremsstrahlung observables within a relativistic meson-baryon model. We included initial-and final-state interactions, as well as the numerically cumbersome rescatter contributions, as shown in Fig 1(a-c). We placed special emphasis on a consistent approach to both the 1-and 2-body problems, i.e., the strong interaction potential [2,3] and the form factors it contains [3,4], were both microscopically calculated from a consistently parameterized meson field-theoretic Lagrangian. However, we considered the electromagnetic interaction only in relativistic impulse approximation and our results shared the discrepancy with experiment [5] reported elsewhere [6–12]. In Fig 2 we present results of these calculations, now extended to provide the first fully consistent comparison of the Paris [13], Nijmegen [14], Bonn [15] and RuhrPot [2] interactions. The selected geometries essentially correspond to best-and worse-case comparison with experiment. It may be observed that, when the data is not dominated by uninteresting phase space variations, the differences between the results from these models is smaller than their collective discrepancy with experiment. We conclude that the failure of all recent attempts to describe the pp-bremsstrahlung data has little to do with the model-dependent choice of the half-off-shell t-matrices, but indicates a sensitivity to the T = 1 isoscalar meson-exchange currents. Indeed, some 16 years ago Taitor et al [16] showed that the pp-bremsstrahlung observ-ables at photon energies as low as 80 MeV may take large contributions from meson-exchange currents involving an intermediate-state ∆-resonance. Such results must be taken as indicative , rather than conclusive, because the off-shell direct amplitudes shown in Fig 1(a-c) were included as the measured elastic scattering cross section data. This not only implies that the direct amplitudes are taken in the notoriously unreliable soft-photon approximation, it also means that all interferences between the meson-exchange and direct amplitudes are necessarily neglected. Not even the relative sign of these contributions can be properly defined and must simply be chosen to optimize the correspondence with experiment. Nonetheless, the calculations by Taitor et al are useful and interesting because they …