Two-Pion Exchange in proton-proton Scattering

The cross section and the spin-correlation observable, CNN, measured in the region from 0 to 12 GeV/c Plab at 90 o provide an excellent testing ground for our theories of hadronic interactions. These data have been the focus of a number of experimental [1] and theoretical studies [2]. There is an apparent simplification of the amplitude in the region from 4 GeV/c to 8 GeV/c. At 90 ◦ the spin-correlation observable CNN has a constant value of about 0.07 which might indicate the dominance of a single mechanism. The simple quark exchange mechanism gives 1/3 for CNN . One pion exchange provides an important contribution below about 2 GeV/c. The exchange of two pions has been used as a basis for the NN interaction at low energies and it is reasonable to assume that it remains important at higher energies. We have calculated the box and crossed two-pion exchange Feynman graphs with nucleons as intermediate states. Even if this mechanism dominates one can not expect good quantitative agreement with the data due to the strong inelasticity of the NN interaction at these energies since distortion effects need to be considered. None the less, several questions can be raised and answered.