Photoproduction of Positive Pions from Hydrogen near Threshold

Abstract

The variation of the differential cross section for ${\ensuremath{\pi}}^{+}$ photoproduction from hydrogen, with $\ensuremath{\gamma}$-ray energy, has been examined at a laboratory angle of 58\ifmmode^\circ\else\textdegree\fi{} to the $\ensuremath{\gamma}$-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 $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}$ at $\ensuremath{\gamma}$-ray energies of 162, 168, 175, and 192 Mev are 5.42\ifmmode\pm\else\textpm\fi{}0.38, 5.77\ifmmode\pm\else\textpm\fi{}0.41, 6.74\ifmmode\pm\else\textpm\fi{}0.47, and 8.22\ifmmode\pm\else\textpm\fi{}0.58 \ensuremath{\mu}b/sr, respectively, where the error limits refer to relative values. The results substantiate the rising trend of the interaction quantity ${(\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}})(\frac{{\ensuremath{\mu}}^{2}}{p\ensuremath{\epsilon}}){(1+\frac{\ensuremath{\omega}}{M})}^{2}}$ near threshold, in accord with dispersion theory; and the absolute cross sections are compatible with a threshold value for ${{a}_{0}}^{+}$ near 20 \ensuremath{\mu}b/ steradian, consistent with findings in related pion work.