Production and Neutral Decay of theηMeson inπ−pCollisions

The reaction ${\pi }^{-}p\to \eta n$ has been observed through the decay mode $\eta \to 2\gamma$ at $T_{{\pi }^{-}}=592, 655, 704, 875, 975, 1117, \mathrm{and} 1300$ MeV. The detection apparatus was a cubic array of six steel-plate optical spark chambers that completely surrounded a liquid-hydrogen target. We identified events attributed to the decay of an $\eta$ by the large c.m. opening angle between the two showers generated in the steel plates by the decay photons. We have calculated the total cross section for $\eta$ production, which is proportional to the number of events under the large-angle peak in the opening-angle distribution. The total cross section rises steeply from threshold to a maximum of about 2.4 mb at 650 MeV, and then falls gradually to about 0.66 mb at 1300 MeV. The differential cross section was obtained by taking the coefficients of a Legendre-polynomial fit to the angular distribution of bisectors of selected two-shower events, and converting them to the coefficients of the $\eta$ c.m. angular distribution. The differential cross section is found to be isotropic at 592 MeV, to require terms through $P_2\left({cos\theta }_{\eta }\right)$ between 655 and 975 MeV, and to have a forward peak fitted by terms through $P_3\left({cos\theta }_{\eta }\right)$ at 1117 MeV and through $P_4\left({cos\theta }_{\eta }\right)$ at 1300 MeV. It is suggested that production at threshold is predominantly through an $S$ state, with some $P_1$ and $D_3$ waves entering by 655 MeV. We suggest that all the absorption in the $S_{11}$ $\pi N$ state can be explained by the $\eta$ production.