The kinematics of elastic two-particle collisions may be summarized by the convenient four-vector equation q = Rθp, where p and q are four-momenta of the same particle before and after collision in the laboratory frame, and Rθ is the rotation induced in the laboratory frame by a two-dimensional rotation of angle θ in the center-of-mass frame. It is shown that this form has the advantage of being conceptually clear and computationally convenient.

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