Quasi two body e⁺e^- annihilation

We present the helicity formalism for e⁺ e^- annihilation into two-body resonance states, and show how to systematically extract the magnitudes and relative phases of the form factors present in the one-photon exchange approximation. This entails measurement of one-particle density matrices and certain polarization correlations in the final state. We discuss, as an example, the vector meson dominance expectations for annihilation into meson resonances and show how this model can be tested.