Kinematic Structure of Helicity Amplitudes for Massless Particles

A study is made of the kinematic structure of helicity amplitudes describing scattering with massless particles of arbitrary helicities, using certain basic analyticity assumptions on the $M$ functions. The behaviors of the amplitudes at the thresholds and pseudothresholds are given explicitly, and, in the case of photons and gravitons, these give the suppression factors inherent in the various low-energy theorems. Parity conservation is not used, and the results are applicable to weak amplitudes.