Isospin dependence of pion absorption on nucleon pairs

Abstract

The suppressed ratio of (${\ensuremath{\pi}}^{\ensuremath{-}},\mathrm{pn}$) to (${\ensuremath{\pi}}^{+},\mathrm{pp}$) pion absorption cross sections on $^{3,4}\mathrm{He}$ has an important contribution from NN\ensuremath{'} intermediate states, where N\ensuremath{'} denotes a ${P}_{11} \ensuremath{\pi}\mathrm{N}$ 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.