Threshold pion absorption in nuclei

We attempt to explain threshold pion absorption in nuclei using simple pion rescattering mechanicsms. While the model is successful in the case of the deuteron it leads to 10-30% too small absorption rates in other nuclei. We also study a $\rho \omega$ exchange absorption mechanism that does not act in the deuteron, but find that it can only remove a small part of the remaining discrepancy. The two-nucleon absorption mechanism accounts for 75% of the imaginary part of the $S$-wave pion nucleus optical potential.