Just a quick question regarding the tree level Feynman diagram(s) contributing to this process - I am wondering if I wanted to compute the unpolarised transition amplitude for the annihilation ##e^+ e^- \rightarrow \gamma \gamma##, are there two tree level diagrams that contribute or just one? I am thinking of an electron and positron as initial state, an electron/positron being the virtual particle and the two photons as the external state. (e.g photon a) tagged at vertex with the electron b) and another photon c) tagged at vertex with positron d)) That's one diagram. But I also thought, to take into account the bose symmetry, I would also need to consider an another diagram where photon a) is tagged at vertex with positron d) and another where photon c) is tagged at vertex with electron b)? Is it correct?