This is probably a very basic question, but I've been going through all my course materials and looking through various books to find a simple way of figuring this out. But consider the following process: $e^- e^+ \rightarrow \mu^- \mu^+$. Why is the s-channel diagram the only one that contributes to the Matrix element (only considering 2 vertices)? I could imagine writing a u channel diagram with propagator momentum p_a (initial electron) - k_2 (final antimuon). Any help is much appreciated.