Mandelstam variables

These invariants are suited for 2→2 processes. s=(k_1+k_2)^2=(p_1+p_2)^2$$$$t=(k_1-p_1)^2=(k_2-p_2)^2$$$$u=(k_1-p_2)^2=(k_2-p_1)^2 They fulfill the following identity $s+t+u={\sum }_i{m}_i^2$ where the sum runs over the masses of all four external particles.