Regge-Pole Models for Pion Exchange Reactions

The problem of obtaining a satisfactory Regge-pole model for the pion-exchange reactions $\mathrm{np}\to \mathrm{pn}$, $\pi N\to \rho N$, $\pi N\to \rho N^{*}$, $\gamma p\to {\pi }^{+}n$, and $\overline{p}p\to {\overline{N}}^{*}{N}^{*}$, is discussed with particular emphasis on the dynamics as seen in the $s$ channel. It is pointed out that (i) satisfactory models can be obtained using a modified conspiracy model; (ii) pion exchange should provide the dominant contribution to the forward cross sections for incident momenta up to 30-50 GeV/c; and (iii) the energy dependence of the differential cross sections should change from that characteristic of pion exchange for $t\sim 0$, to that characteristic of the exchange of vector or tensor mesons at large momentum transfers $t\lesssim -0.5\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$.