Photoproduction of Positive Pions from Hydrogen near Threshold

The variation of the differential cross section for ${\pi }^{+}$ photoproduction from hydrogen, with $\gamma$-ray energy, has been examined at a laboratory angle of 58° to the $\gamma$-ray beam. A thin hydrogen target, and a counter system designed to eliminate random events, have been employed. Mean values for the differential cross section $\frac{d\sigma }{d\Omega }$ at $\gamma$-ray energies of 162, 168, 175, and 192 Mev are 5.42±0.38, 5.77±0.41, 6.74±0.47, and 8.22±0.58 μb/sr, respectively, where the error limits refer to relative values. The results substantiate the rising trend of the interaction quantity $\left\{\right(\frac{d\sigma }{d\Omega }\left)\right(\frac{{\mu }^2}{p\epsilon }\left){(1+\frac{\omega }{M})}^2\right\}$ near threshold, in accord with dispersion theory; and the absolute cross sections are compatible with a threshold value for ${a_0}^{+}$ near 20 μb/ steradian, consistent with findings in related pion work.