It is shown that the photoproduction of charged pseudoscalar mesons at threshold computed to lowest order in the meson-nucleon mass ratio but to arbitrary order in the meson-nucleon coupling constant is essentially equivalent to the weak-coupling result obtained from second-order perturbation theory. The $\frac{\mu }{M}$ corrections can be estimated from the ratio of the positive to the negative production cross section. It is therefore suggested that the measurement of the $S$-wave photoproduction of charged mesons may be one of the more reliable methods presently available for determining the meson-nucleon coupling constant. As the threshold production proceeds entirely through the mediation of negative energy states the connection of this result with the possible suppression of nucleon anti-nucleon pairs is discussed.

• Received 20 August 1953

DOI:https://doi.org/10.1103/PhysRev.93.233