Peripheral NN-Scattering: Role of Delta-Excitation, Correlated Two-Pion and Vector Meson Exchange

Abstract

We evaluate, within one-loop chiral perturbation theory, the two-pion exchange diagrams with single and double delta-isobar excitation contributing to elastic NN-scattering. We find that virtual $\Delta$-excitation processes (in the static limit) produce the correct amount of isoscalar central attraction as needed in the peripheral partial waves with $L\ge 3$. Furthermore we compute the two-loop diagrams involving the $\pi \pi$-interaction (so-called correlated $2\pi$-exchange). Contrary to common believe these processes lead to negligibly small and repulsive corrections to the NN-potential. The exchange of vector mesons ($\rho ,\omega$) turns out to be important for the F-wave phase shifts above $T_{lab}=150$ MeV. Without adjustable parameters we are able to reproduce the empirical NN phase shifts up to 350 MeV for $L\ge 3$ and up to about (50--80) MeV for the D-waves. This is therefore the characteristic window in which the NN-interaction is basically governed by chiral symmetry. Not surprisingly, the lower partial waves require non-perturbative methods and additional short-distance parametrizations of the NN-dynamics.

Comments: 25 pages, 9 figures in ps, submitted to Nuclear Physics A