https://particle.phys.uvic.ca/twiki/pub/Main/AlisonFaulkner/Drell-Yan.jpg [Broken] The Feynman diagram for Drell Yan has a gamma-star or a Z as the mediator. Does gamma-star simply mean a photon with a sizeable amount of energy? Is there a well-defined point in the energy of the mediator when it's a Z instead of a gamma-star? The peak of the Z resonance on an invariant mass plot is 91 GeV, but as a wide resonance it could also have less energy than that. If a W acts as the mediator, is it still classified as Drell Yan?