(* ::Package:: *)

(* Mandelstam definitions for massless
   psi(p1) psi(p2) -> psi(p3) psi(p4). *)

ClearAll[s, t, u, p1, p2, p3, p4, mandelstamRules, masslessConstraint];

masslessConstraint = s + t + u == 0;

mandelstamRules = {
   SP[p1, p1] -> 0,
   SP[p2, p2] -> 0,
   SP[p3, p3] -> 0,
   SP[p4, p4] -> 0,
   SP[p1, p2] -> s/2,
   SP[p2, p1] -> s/2,
   SP[p3, p4] -> s/2,
   SP[p4, p3] -> s/2,
   SP[p1, p3] -> -t/2,
   SP[p3, p1] -> -t/2,
   SP[p2, p4] -> -t/2,
   SP[p4, p2] -> -t/2,
   SP[p1, p4] -> -u/2,
   SP[p4, p1] -> -u/2,
   SP[p2, p3] -> -u/2,
   SP[p3, p2] -> -u/2
};