High-EnergynpCharge-Exchange Scattering and One-Pion Exchange

It is shown that the "droplet" model with long-range $\pi$ exchange included can account for the observed $\mathrm{np}$ charge-exchange differential cross sections in the GeV region for $0<-t<\mathrm{}0.5\mathrm{} {\left(\frac{\mathrm{GeV}}{c}\right)}^2$. The very steep rise in $\frac{d\sigma }{\mathrm{dt}}$ for $-t<\mathrm{}0.02\mathrm{} {\left(\frac{\mathrm{GeV}}{c}\right)}^2$ is due to long-range $\pi$ exchange and is similar to the steep rise near $t=0\mathrm{}$ in elastic scattering of charged particles due to the Coulomb interaction. From the droplet-model view-point, $\pi$ exchange can be identified only in large-impact-parameter collisions. For smaller-impact-parameter collisions, the model differs from the absorptive OPE model; however, the resulting amplitudes also have the one-pion pole at $t={\mu }^2$. The reaction $\mathrm{np}\to \mathrm{pn}$ in the GeV region is remarkable in that it may be the only reaction in which the effect of long-range $\pi$ exchange can be easily seen. In elastic scattering, long-range ${\pi }^0$ exchange scattering is small compared to diffraction scattering, and is masked by Coulomb effects. It might be observed in high-precision measurements of $\mathrm{np}$ elastic scattering. In $\overline{p}p\to \overline{n}n$, the effect of long-range ${\pi }^{-}$ exchange is likely to be more accessible to experimental observation. The anomaly near $t=0\mathrm{}$ due to $\pi$ exchange may tend to vanish at high energies, due to the rapid decrease of the $\pi$-exchange amplitude with increasing energy. Fits to the data in the region $0.1<-t<\mathrm{}0.5\mathrm{} {\left(\frac{\mathrm{GeV}}{c}\right)}^2$ indicate strong spin dependence in $\mathrm{np}$ and $\overline{p}p$ charge exchange.