Isospin dependence of pion absorption on nucleon pairs

The suppressed ratio of (${\pi }^{-},\mathrm{pn}$) to (${\pi }^{+},\mathrm{pp}$) pion absorption cross sections on ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}$ has an important contribution from NN′ intermediate states, where N′ denotes a $P_{11} \pi \mathrm{N}$ interaction. The order-of-magnitude energy variation of this ratio, from stopped pions to pions above the (3,3) resonance, is reproduced in calculations using a unitary isobar model. We predict a strong asymmetry in the angular distribution of the ratio of higher pion energies.