Differential Cross Section of the pn → pp ( 1 S 0 ) π − Reaction Extracted from pd → ppp π −

The double differential cross section for $\mathrm{pn}\ensuremath{\rightarrow}\mathrm{pp}{(}^{1}{S}_{0}){\ensuremath{\pi}}^{\ensuremath{-}}$ at three beam energies has been extracted from the quasifree process $\mathrm{pd}\ensuremath{\rightarrow}\mathrm{ppp}{\ensuremath{\pi}}^{\ensuremath{-}}$. A comparison is carried out with single differential cross section measurements for ${}^{3}\mathrm{He}({\ensuremath{\pi}}^{\ensuremath{-}},pn)n$, where the pion is thought to be absorbed onto a $\mathrm{pp}{(}^{1}{S}_{0})$ ``diproton'' state. A significant difference is observed in the shape of the angular distribution between the production and absorption data. This difference is ascribed to the effects of the ${}^{3}\mathrm{He}$ nuclear environment characterizing the absorption process; however, an adequate theoretical explanation is not available.