Pion absorption onHe3at low energies

The reactions ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(${\mathrm{\pi }}^{+}$,pp)p and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(${\mathrm{\pi }}^{\mathrm{-}}$,pn)n were studied at 37.0 MeV by coincidence detection of two nucleons. The differential cross sections were separated to two-nucleon (${\mathrm{\sigma }}_{2\mathit{N}}$), three-nucleon (${\mathrm{\sigma }}_{3\mathit{N}}$), and final-state interaction (${\mathrm{\sigma }}_{\mathrm{FSI}}$) components. For ${\mathrm{\pi }}^{+}$, the ${\mathrm{\sigma }}_{2\mathit{N}}$ angular distribution is symmetric about 90°, and the total cross section is 1.5 times the cross section measured for d(${\mathrm{\pi }}^{+}$,pp). For ${\mathrm{\pi }}^{\mathrm{-}}$, the angular distribution is asymmetric (backward peaked). The asymmetry increases with decreasing energy, indicating increasing pion s-wave contribution at lower energies. The fraction of the cross section induced by s-wave pions as calculated by a partial wave amplitude analysis is 13%. The measured total cross sections are ${\mathrm{\sigma }}_{2\mathit{N}}$(${\mathrm{\pi }}^{\mathrm{-}}$)=0.85±0.08 mb and ${\mathrm{\sigma }}_{2\mathit{N}}$(${\mathrm{\pi }}^{+}$)=7.9±0.5 mb; ${\mathrm{\sigma }}_{3\mathit{N}}$(${\mathrm{\pi }}^{\mathrm{-}}$)=1.6±0.7 mb and ${\mathrm{\sigma }}_{3\mathit{N}}$(${\mathrm{\pi }}^{+}$)=1.3±0.3 mb. A new evaluation of ${\mathrm{\sigma }}_{3\mathit{N}}$ at ${\mathit{T}}_{\mathrm{\pi }}$=62.5 and 82.8 MeV is given, using data from an earlier experiment. The cross sections leading to the two-nucleon final-state interaction at ${\mathit{T}}_{\mathrm{\pi }}$=37.0 MeV are also estimated. © 1996 The American Physical Society.