It is shown that the coherent-droplet model with long-range $\pi$ exchange can account for the observed behavior with $-t$ of the high-energy, small-momentum-transfer differential cross sections for $\gamma +p\to {\pi }^{+}+n$, $\gamma +p\to {\pi }^0+p$, and $\gamma +p\to K^{+}+\Lambda$. As in $n-p$ charge-exchange scattering, the steep rise near $-t=0\mathrm{}$ in $\gamma +p\to {\pi }^{+}+n$ is the effect of long-range $\pi$ exchange. The dip near $-t=0\mathrm{}$ in $\gamma +p\to K^{+}+\Lambda$ is accounted for by the presence of large helicity-flip amplitudes. Experiments to check this are suggested.

• Received 8 November 1967

DOI:https://doi.org/10.1103/PhysRevLett.20.129