Demystifier
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The same can be said for long-living (quasi-stable) particles, there is no sharp difference between resonances and long-living particles. Indeed, I think that's how top quark was discovered, as a short living resonance.vanhees71 said:Resonances are real as "bumps" in a cross section. What you meausure are cross sections describing the scattering of asymptotic free "in-states" to asymptotic free "out-states". It's described by a complex pole in the corresponding Green's function.