Two-Nucleon Emission Following Absorption of Stopped Negative Pions

A beam of ${\pi }^{-}$ was brought to rest in a variety of light-element targets. The angular distributions of nucleon pairs emitted in the subsequent pion absorption were measured. The angular distributions peak strongly at 180°, supporting the two-nucleon model of the absorption process. The emission of an $n-p (n-n)$ pair implies capture of the ${\pi }^{-}$ by a $p-p (n-p)$ pair inside the nucleus. The ratio of $n-n$ to $n-p$ emission has been measured to be 3.3±0.9 averaged over $p$-shell nuclei. This is compared with predictions of this ratio which are sensitive to the two-nucleon correlation within the nucleus. Measurements of the proton spectra have been made in the $n-p$ emission and are in agreement with the two-nucleon mechanism. The branching ratio of ${\pi }^{-}$ absorption yielding two correlated nucleons is typically 40%.