Isobar-width effects in the coupling of nucleon to isobar channels.

Abstract

The investigation of the effects of isobar coupling to two-nucleon channels has been extended to include additional physical features. A new code discretizes the mass distribution of the isobar widths and treats each mass as a separate channel. This allows the treatment of width in the presence of coupling by transition potentials, in addition to the previously permitted boundary coupling. It also produces the S-italic-matrix components required to describe the many-body final-state distributions. When indicated by the one-pion-exchange coupling strength new isobar channels are included. The new results for nucleon-nucleon scattering fit the data better, starting from more realistic models. The observed /sup 1/D/sub 2/ and /sup 3/F/sub 3/ structures are well understood as coupled-channel effects, without exotic-quark contributions. The existence of a structure in the /sup 3/P/sub 0/ channel depends on the amount of inelasticity, which differs among the phase shift solutions. The energy dependence seen in recent analyses, of the /sup 3/P/sub 2/ phase shift near T-italic/sub L-italic/ = 800 MeV, is shown to be a consequence of the isobar channel coupling. Improved models obtained for the /sup 1/S/sub 0/ and /sup 3/S/sub 1/-/sup 3/D/sub 1/ channels are being developed. They are important to accurately extrapolate those phasesmore » to higher energies where six-quark effects are expected.« less