Complete set of observables for photoproduction of two pseudoscalars on   a nucleon
  The problem of determining completely the spin amplitudes of photoproduction of two pseudoscalar mesons on a nucleon from observables is studied. The procedure of reconstruction of the scattering matrix elements from a complete set of observables is based on the expressions of all observables as quadratic hermitean forms in the reaction matrix elements which are derived explicitly. Their inversion allows one to find explicit solutions for the reaction matrix elements in terms of observables. Two methods for finding a complete set of observables are presented. In particular, one set was found, which does not contain a triple polarization observable. 
H.Arenhoevel and A. Fix
nucl-th