Mechanism ofHe3(π−,pn) reaction

The ${\pi }^{\mathrm{-}}$ absorption by ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is investigated in a model study based on the nucleon pole mechanism πNN→NN→NN. We find that the amplitude of the three-body rescattering mechanism can have a strong interference effect with the amplitude of the direct two-body absorption process even in the region where the two-body process dominates. The main experimental signature of this important coherent effect is a sharp peak near the high energy end of the proton spectrum measured in a coincident experiment. Our results suggest that in order to extract the isospin dependence of pion absorption by a pair of nucleons from the recent ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(${\pi }^{\pm }$,NN) data, the three-nucleon rescattering amplitude must be calculated accurately and included coherently in the analysis.