The Double Radiative Capture Mode of Pionic Hydrogen

A first estimate of the $\gamma \gamma n$ capture mode of pionic hydrogen was published by Joseph (17) in 1960. In 1972, Lapidus and Musakhanov calculated a more complete two-photon production amplitude of pion capture at rest by protons (18). Ericson and Wilkin (4) studied this process in 1975 and obtained an order of magnitude estimate of the $\gamma \gamma n$ capture mode. The first complete tree-level calculation became available in 1979. In a pseudoscalar-coupled pion nucleon theory, Beder (7) presented a detailed calculation of the $\pi ^-p\rightarrow \gamma \gamma n$ reaction near threshold from both the atomic $l=0$ and $l=1$ orbits.

The following sections are devoted to a brief survey of these theoretical works on double radiative capture in pionic hydrogen.