Partial wave expansion for photoproduction of two pseudoscalars on a nucleon

The amplitudes for photoproduction of two pseudoscalars on a nucleon are expanded in the overall c.m. frame in a model independent way with respect to the contribution of the final state partial wave of total angular momentum $J$ and its projection on the normal to the plane spanned by the momenta of the final particles. The expansion coefficients which are analogues to the multipole amplitudes for single meson photoproduction contain the complete information about the reaction dynamics. Results of an explicit evaluation are presented for the moments $W_{jm}$ of the inclusive angular distribution of an incident photon beam with respect to the c.m.\ coordinate system defined by the final particles taking photoproduction of $\pi^0\pi^0$ and $\pi^0\eta$ as an example.