Classification of exchange currents

Abstract

After expansion of the vector and axial vector currents in powers of ($\frac{v}{c}$), a heretofore unremarked regularity results. Meson exchange currents can be classified into types I and II, according to the way they satisfy the constraints of special relativity. The archetypes of these two categories are the impulse approximation to the vector and axial vector currents. After a brief discussion of these constraints, the ($\rho \pi \gamma$) and ($\omega \sigma \gamma$) exchange currents are constructed and classified, and used to illustrate a number of important points which are often overlooked.