[Koshkin]

Both real and virtual "versions" of a type of particles are described as excitations of the same quantum field; the difference is that only the real particles can be detected experimentally, whereas the virtual ones are used to model interactions between (real) particles and thus only appear in calculations.

[iainmerrick]

Thanks, that helps, I think...!

Would it be roughly correct to say that virtual particles by definition always vanish (mutually annihilate, etc, whatever) before the measured outcome of an experiment, therefore they definitely cannot be directly observed? But they're an integral part of the model, therefore if the model is correct, they really do have a real physical existence (in some complex quantum sense)?

[Koshkin]

I do not believe so. Generally speaking, mathematical models often include what I call "scaffolding," which is something that, while being part of the model, does not represent anything real. (Different models are likely have different scaffolding, and some future theory might do away with virtual particles as a mechanism of interaction.)