A simple and straightforward method to identify and remove the kinematic singularities of helicity amplitudes is constructed from the Trueman-Wick crossing relations. A set of amplitudes free of all kinematic singularities and zeros is obtained for two-particle {→} two-particle reactions of any spins and masses, except that for boson-fermion interactions with general mass assignments there is still a kinematic s1/2 singularity left in the amplitude.
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Ling-Lie Chau Wang (1966) studied this question.
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