I want to calculate the Mandelstam variables of a 2 --> 2 collision.
The Mandelstam variables u and t are defined as: \begin{equation}t = (p_1 - p_3)^2 = (p_4 - p_2)^2\end{equation} \begin{equation}u = (p_1-p_4)^2 = (p_3-p_2)^2\end{equation}
Consider the collision $p + p \to p' + \Delta^+$
How do I decide how to label the four momentum of the proton and delta-baryon or more concrete, should I assign $p_3$ to the delta baryon and $p_4$ to $p'$ or $p_3$ to p' and $p_4$ to delta?