Moment-operator expansion for the two-meson, two-photon and fermion-antifermion states

A complete set of formulae in terms of the moment-operator expansion is presented for the two-meson, two-photon and fermion–antifermion states that can be used in the analysis of scalar (S) and pseudoscalar (P) meson production in p and γγ collisions: p → SS, PP, SP and γγ → SS, PP, SP. The method of generalization of the formulae for the amplitudes of multi-meson production in the two-stage processes: of the type (spin-j) resonance + meson → three mesons is also discussed.