Abstract

In this paper we study the possibility of generalizing the classical photoabsorption (?a?bc) sum rules to processes bc??a and crossed helicity amplitudes. In the first case, using detailed balance, the sum rule is written as ?vth? (dv/v) K??Born(v)=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 ?b?ac or ac??b, 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). © 2002 The American Physical Society.