Nuclear Double Radiative Capture

The underlying dynamics of $\pi\pi$ annihilation in double radiative capture has been intriguing to authors for quite some time. Ericson and Wilkin (4) suggested this reaction in 1975 as a probe of the pion field in the nucleus. Nyman and Rho (8) had explored the relation between double radiative pion capture and radiative muon capture in the context of the weak coupling $g_p$ in 1977. More recently, in 1995, Gil and Oset (6) suggested the reaction as a novel window on the $\pi \pi \rightarrow \gamma \gamma$ vertex.

In the following sections, we present a survey of the theoretical work and experiments devoted to double radiative capture on pionic atoms with light nuclei.