Hi everybody! I'm a new Physics Forums user and hope someone could help me out with my minor dilemma. I'm a PhD student in mathematical/theoretical physics and I' working on the Boltzmann equation in QFT. Up to now, there was no major emphasis on Feynamn diagrams - the approach was rather more mathematical with the fundamentals of QFT (definition of Wick products, Wick expansion, Wick reordering, Bogoliubov's formula for interacting fields and such). But now I have to make sense of some expression that emerges in my computations. To make a long story short, I'm working with \phi^4 theory and need to consider a 3 -> 3 scattering process in perturbation theory. Also, I'm using a (-,+,+,+) signature for the metric - opposite of most textbooks so some signs might appear odd. Now, this can only happen to second order in the coupling constant, and at that order, there are no loop diagrams. With the possible permutations, the diagrams I have to consider are the following Code (Text): q_1 q_2 q_3 \ | / \ | / \ | / \|/ | | | /|\ / | \ / | \ / | \ p_1 p_2 p_3 and Code (Text): q_1 q_2 q_3 \ \ / \ \/ \ /\ \/k \ /\ \ / \ \ p_1 p_2 p_3 I have no problems with the first one, but the second one is causing me some confusion. If I write down the amplitude associated to it, I get (neglecting symmetry factors) (-i g)^2/(k^2 +m^2 -i \epsilon) = (-i g)^2/[(p_1 + p_2 - q_1)^2 +m^2 -i \epsilon], and g is my coupling constant. The physical interpretation seems clear: two particles scatter and, subsequently one of them scatters off the third one. And this is one way to see the origin my confusion. The momentum k is associated to a real particle, even though it's not an "external" particle. In other words, k is now on-shell, i.e. k^2 = -m^2, giving me a divergent amplitude! I've never heard of divergent tree level amplitudes... What am I missing? Most probably the reason why k is not on-shell but why is it not? This is the physical amplitude for the above process and all the momenta are on-shell. What's going on here? Thanks for your time!