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The analyticity properties of helicity amplitudes for binary reactions of particles with arbitrary spins are studied using the following three properties: (i) the analyticity properties of scattering amplitudes, assuming these are correctly predicted by perturbation theory; (ii) the crossing relations of helicity amplitudes near s=0; (iii) the threshold behavior of partial-wave amplitudes. Making use of these properties, kinematical singularity-free amplitudes for any spin are constructed by modifying helicity amplitudes. MacDowell reciprocity is generalized to arbitrary spin. Helicity amplitudes are proved to satisfy the Froissart limit at the high-energy limit.
Yasuo Hara (Mon,) studied this question.