Hello! I am a bit confused about the distinction between virtual and real particles. For example a Z boson, which has a very short lifetime, in all experiments will decay to some other stable particles (i.e. it is detected through its decay). This means that it will always appear as a propagator, and as far as I understand, the propagators are not, usually, on shell. The mass of the Z was obtained by looking for a resonance peak, and that is the quoted mass in PDG for example. But what does it mean for a Z particle to not be virtual i.e. do we even have a real Z? So my question is, especially for unstable particle, which always appear as propagators in some Feynman diagram, when is it real and when is it virtual? Thank you!