For nuclear processes involving pions, such as pion production/absorption or exchange currents, it is shown that a natural organization involves adoption of the $\pi \mathrm{NN}$ vertex function $g_{\pi }\left(q^2\right){\gamma }_5{\tau }_{\alpha }$, where $q$ is the pion four-momentum and $g_{\pi }\left({m_{\pi }}^2\right)\simeq 13.4$ is the renormalized pion-nucleon coupling constant. The result is independent of the form and details of any fundamental interaction Lagrangian. The difference between $g_{\pi }\left(q^2\right){\gamma }_5{\tau }_{\alpha }$ and the fully-dressed $\pi \mathrm{NN}$ vertex function contributes to amplitudes involving more bosons, e.g., scattering amplitudes ($\pi N\to \pi N$), production amplitudes ($\pi N\to \pi \pi N$), etc. Such contributions are mediated through either seagull-like terms or $P11\mathrm{}$ intermediate states. It is pointed out that it is neither necessary nor practical to calculate these partial contributions to scattering and production amplitudes. One can always exploit other approaches, theoretical and phenomenological, to make reasonably good models for the full amplitudes. The usual procedure of including the effects of negative energy parts of the nucleon propagator in interaction potentials, exchange currents, etc., is easily included in the present organization. The result is that from scattering amplitudes, etc; the positive energy nucleon poles are deleted but $Z$ graphs are included.

NUCLEAR REACTIONS Theoretical, organization scheme, pion reactions with nuclei, effective $\pi \mathrm{NN}$ vertex.

• Received 26 May 1982

DOI:https://doi.org/10.1103/PhysRevC.27.657