TrickMandelstam[expr, {s, t, u, m1^2 + m2^2 + m3^2 + m4^2}]
simplifies all sums in expr
so that one of the Mandelstam variables , or is eliminated by the relation . The trick is that the resulting sum has the most short number of terms.
ClearAll[s, t, u]
s + t - u) (2 SMP["m_W"]^2 - t - u)
(
[%, {s, t, u, 2 SMP["m_W"]^2}] // Factor2 TrickMandelstam
M^2 s - s^2 + M^2 t - s t + M^2 u - s u
[%, {s, t, u, 2 M^2}]
TrickMandelstam