Kinematic Singularities and Constraints of Helicity Amplitudes

The kinematic singularities and constraints of helicity amplitudes are studied in a unified way. Relatively simple and compact formulas are obtained which enable one to read off the singularities and constraints for any particular two-body process. The points studied are the thresholds and pseudothresholds, $s=0$, and the boundary of the physical region. The fundamental assumption which underlies the method used here is that all kinematic singularities result from singular definitions of the helecity states.