Classical sum rules and spin correlations in photoabsorption and photoproduction processes

In this paper we study the possibility of generalizing the classical photoabsorption (γa→bc) sum rules to processes bc→γa and crossed helicity amplitudes. In the first case, using detailed balance, the sum rule is written as ∫νth∞(dν/ν)KΔσBorn(ν)=0 where K is a kinematical constant which depends only on the mass of the particles and the center of mass energy. For other crossed helicity amplitudes, we show that there is a range of values of s and t for which the differential cross section for the process γb→ac or ac→γb, in which the helicities of the photon and particle a have specific values, is equal to the differential cross section for the process in which one of these two helicities is reversed (parallel-antiparallel spin correlation)