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An electron and positron can scatter by annihilating to form either a virtual Z or virtual photon, either of which can then pair produce to give an electron/positron pair (amongst an infinity of other processes whose contributions need to be summed up, of course). Firstly, am I correct in thinking the process involving the virtual Z can still occur when the CM energy is less than the mass of the Z (uncertainty principle allows energy to be 'borrowed' for a sufficiently short time)? Second, if this is the case, then if the CM energy is indeed much less than the mass of the Z, I can take this to mean that the contribution of the process involving the Z is negligible (which I think can be understood from the 1/(P^2-m^2c^2) factor in the propagator)?

Thanks in advance, really new to a lot of these concepts so my understanding is a bit of a jumble at the moment.