W and Z bosons are observed/discovered. But as force carrying bosons they should be virtual particles, unobservable? And also they require to have mass, but if they are virtual they may be off-shell, so are they virtual or not.
Answer
Seems to me there is a confusion between various concepts, let me try to clear it up:
Virtual particle is one that doesn't live forever, at some stage it gets converted to something else. As Jeff points out, none of us lives long enough to tell the difference, so the distinction between virtual and non-virtual is a matter of degree. Particles that live for a long time are declared "real", and particles that decay quickly are called "virtual". These are just names, there is no implication that "virtual" particles don't really exist, like white unicorns and other mythical creatures, those are all real measurable effects you can see with your own eyes...
Any particle can be either real or virtual, whether or not it is massive, whether or not it is bosonic force carrier, or fermionic matter. There is a sense in which massive particles tend to live shorter life (because they have more opportunities to decay), but this is just a rule of thumb.
Off-shell can be taken here to be synonymous with "virtual".
Hope that helps.
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