Positivity Constraints on Spin Observables in Exclusive Pseudoscalar   Meson Photoproduction
  Positivity constraints have proved to be important for spin observables of exclusive reactions involving polarized initial and final particles. Attention is focused in this note on the photoproduction of pseudoscalar mesons from spin 1/2 baryons, more specifically gamma+ N -> K+ Lambda, gamma+ N -> K Sigma, for which new experimental data are becoming available. 
Xavier Artru, Jean-Marc Richard, Jacques Soffer