I am confused about the production of bosons in annihilation processes. If we have a positron and an electron coming together and annihilating, we can always find a frame in which the net momentum is zero, which would suggest that a single photon can never be produced in such an interaction (but a massive boson such as a Z could be). However, in a question asked by my prof. in lectures the other week, she said that future electron/positron colliders may operate at energies sufficient to produce top-antitop pairs. How would this occur? And also, how would Feynman diagram for this process look? I had imagined that the initial annihilation would simply produce a boson which would then decay into the top/antitop pair, however thinking about it, I feel as though the boson should be massive, and therefore not be a photon. Is this correct, or does the fact that the photon is virtual allow it to occur?