Lorentz invariant multiparticle amplitudes

Complete (equivalent) sets of kinematical constraints on Lorentz invariant multiparticle amplitudes, for a process involving massive particles with arbitrary spins, are established. The number of invariant amplitudes is the number of spin components. Explicit covariants expressing the invariant amplitudes in terms of spinor amplitudes are given. Covariant decompositions of spinor amplitudes are also written on the analyticity domain and on the physical region. (author)