It is shown that the strong isospin dependence observed in pion absorption on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ can be described by the two-body mechanism $\pi \mathrm{NN}\rightleftarrows \Delta N\rightleftarrows \mathrm{NN}$. The effect of the three-body mechanism is also calculated but found to be negligibly small. The $\Delta N\rightleftarrows \mathrm{NN}$ dynamics consistently determined from $\mathrm{NN}$ inelasticities is shown to be essential to understand the isospin dependence.

• Received 4 August 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.1079