Coupled-channel interaction for intermediate-energy nucleon-nucleon reactions

General effects of coupling a system to higher-mass channels are reviewed. The dominant effect is the addition of an attraction which increases with energy up to and, when the coupled channel is not in an $S$ state, beyond inelastic threshold. This can produce resonances whose widths and inelasticities are largely determined by the resonance position. A model is presented for nucleon-nucleon channels coupled to $N\Delta$, $N{N}^{*}$, or $\Delta \Delta$ channels. It uses the Feshbach-Lomon nucleon-nucleon interaction and one-pion-exchange potentials plus phenomenological cores for the transition interactions. The width of the isobars is included. Good fits are obtained to $\mathrm{NN}$ channels for $E_L\lesssim 1$ GeV, including the "resonant" ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ partial waves. A structure may be present in the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ partial wave. In those partial waves where one-pion exchange predicts the largest transition potentials, the fit is improved by the long-range potential tails.