Analysis of a Scattering Process with Spin in the Infinite-Momentum Frame

A kinematical analysis is made of a two-particle scattering process with arbitrary spin in an infinite-momentum frame. It is shown how the description of the spin, in particular, can be simplified by going to this frame. The scattering amplitude is expressed as a function over an $E\left(2\right)\mathrm{}$ algebra in the infinite-momentum frame; this immediately leads to an impact-parameter representation of the amplitude for arbitrary spin. This representation is different from the fixed-energy impact-parameter representation; in particular, the unitarity relation, in the two-body approximation, is seen to have a simple form.