Sigma Photoproduction from CH2

The first photoproduced sigmas have been observed by placing nuclear emulsions within a few mm of a C${\mathrm{H}}_{2}$ target. One dozen in-flight decays (10 $\ensuremath{\Sigma}'\mathrm{s}$ and 2 ${\ensuremath{\Lambda}}^{0}'\mathrm{s}$ have been identified. The background which previously would have prevented useful exposures has been sharply reduced by applying a 160 KG field along the photon beam. The bulk of the electrons are held in small orbits while the heavier charged sigmas are relatively unaffected. Both of the sigma decay modes have been observed, as well as the charged mode for ${\ensuremath{\Lambda}}^{0}$ decay. The sign of the charged-pion sigma decays cannot be distinguished. The sigma proton decay can only come from the ${\ensuremath{\Sigma}}^{+}$ and this experiment provides the first measurement of the reaction, $\ensuremath{\gamma}+p\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{+}+{K}^{0}$. No event has been included with less than 10% difference in ionization between primary and secondary. All observed events had at least one track with ionization greater than 2\ifmmode\times\else\texttimes\fi{} minimum. If one corrects for the assumed loss of lighter events, the average cross section for photon energies between 1100 and 1200 MeV and for kaon c.m. angles between 30\ifmmode^\circ\else\textdegree\fi{} and 120\ifmmode^\circ\else\textdegree\fi{} (\ensuremath{\sim}60\ifmmode^\circ\else\textdegree\fi{} c.m.) in $\ensuremath{\gamma}+p\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{+}+{K}^{0}$ is (4\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}31}$ ${\mathrm{cm}}^{2}$/sr.