Hi there, my question is the following. If an electron and positron annihilate, how can they result in ZZ? The issue i'm having is that due to charge conservation, the exhange particle can't be W- or W+. It also can't be a photon since the Z's don't have electrical charge to couple to. It also can't be a Z since the Z's don't have weak charge to couple to. I'm reading in my notes that a photon or a Z boson can mediate this interaction. How is that possible? Thanks for any replies.