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

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-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 ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{F}_3^{}{}_{}{}^{}$ structures are well understood as coupled-channel effects, without exotic-quark contributions. The existence of a structure in the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$ channel depends on the amount of inelasticity, which differs among the phase shift solutions. The energy dependence seen in recent analyses, of the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_2^{}{}_{}{}^{}$ phase shift near $T_L$=800 MeV, is shown to be a consequence of the isobar channel coupling. Improved models obtained for the ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$ ${\mathrm{-}}^3$ ${\mathrm{D}}_1$ channels are being developed. They are important to accurately extrapolate those phases to higher energies where six-quark effects are expected.