$\eta$ and ${\eta }^{\prime }$ Production near Thresholds and Backward Charge-Exchange Differential Cross Section in ${\pi }^{-}-p$ Reactions

We have performed an experiment to study the reaction ${\pi }^{-}+p\to \eta +n$ near threshold, preliminary to a forthcoming measurement of charge asymmetry in $\eta$-meson decay. The $\eta$ was identified by the velocity of the associated neutron. We detected neutrons produced in the forward hemisphere in the center-of-mass system corresponding to the most energetic neutrons in the laboratory. Data were taken at ${\pi }^{-}$ momenta between 670 and 805 $\frac{\mathrm{MeV}}{c}$. The four neutron detectors made it possible to detect neutrons at angles of 0° to 21° from the incident pion beam. We present backward differential cross sections for both pion charge exchange and $\eta$ production calculated from the data. We looked for ${\eta }^{\prime }$ at pion momenta of 1.5 $\frac{\mathrm{BeV}}{c}$ and observed none. We obtained $\sigma ({\pi }^{-}p\to n{\eta }^{\prime })\le 60$ μb.