Pion absorption on ³He at low energies

The reactions ³He(π⁺,pp)p and ³He(π^-,pn)n were studied at 37.0 MeV by coincidence detection of two nucleons. The differential cross sections were separated to two-nucleon (σ_2N), three-nucleon (σ_3N), and final-state interaction (σ_FSI) components. For π⁺, the σ_2N angular distribution is symmetric about 90°, and the total cross section is 1.5 times the cross section measured for d(π⁺,pp). For π^-, 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 σ_2N(π^-)=0.85 ±0.08 mb and σ_2N(π⁺)=7.9±0.5 mb; σ_3N(π^-)=1.6±0.7 mb and σ_3N(π⁺)=1.3±0.3 mb. A new evaluation of σ_3N, at T_π=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 T_π=37.0 MeV are also estimated.