The Production of Positive Mesons by Photons

Abstract

Experimental results are presented on the production of positive mesons in hydrogen and carbon, as a function of meson angle and energy. The principal results are as follows: The angular distribution in the case of hydrogen is rather flat in the region 45\ifmmode^\circ\else\textdegree\fi{}-135\ifmmode^\circ\else\textdegree\fi{} in the laboratory system, slightly peaked near 90\ifmmode^\circ\else\textdegree\fi{}. This demonstrates that the pion is not a scalar. The excitation function at 90\ifmmode^\circ\else\textdegree\fi{} rises approximately linearly to a plateau which starts at photon energies of about 260 Mev. This rise is slower than expected on the basis of a phase space argument. The total cross section for 255-Mev photons in hydrogen is 1.9\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}28}$ ${\mathrm{cm}}^{2}$. The cross sections in carbon are lower, per proton, by a factor of approximately three. With respect to hydrogen, the meson energy distribution is shifted to lower energies and the angular distribution is shifted to the backward angles.