Simple calculation of Wigner rotations as functions of $s$ and $t$. Application to scattering and crossing properties

We present a simple inspeettonal method of calculating Wigner rotations for coplanar processes as functions of $s$ and $t$. The method is applied to two-body scattering and crossing, and in the latter case extends the results of Trueman and Wick to arbitrary frames. The method is not restricted to helicity quantization, but helteity quantization turns out to play a central role.