Isospin dependence of pion absorption on nucleon pairs

The suppressed ratio of (..pi../sup -/, pn) to (..pi../sup +/, pp) pion absorption cross sections on /sup 3,4/He has an important contribution from NN' intermediate states, where N' denotes a P/sub 11/ ..pi..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 at higher pion energies.