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A particle by definition is an asymptotic free state. In your case you start with a nucleus ("asymptotic free state") which decays due to ##\alpha## decay. All you can describe and so far also all that has ever been observed is that after the decay you have another nucleus and an ##\alpha## particle. This ##\alpha## particle interacts with the material in the detector and with some probability this leads to a signal from the detector.WernerQH said:Thanks. We all know the "simple facts".
And what happens physically at that instant when the particle is detected? There is no wave function collapse in your view, is there? You didn't answer my question, and it is easy to dismiss it as meaningless. But if you insist on calling QFT a local theory, you should be able to say how the quantum field of the alpha-particle (continuously) evolves from point to point. Otherwise I'd call this use of the word "locality" empty jargon. It would apply only to some mathematical fiction. Perhaps we can agree to call the quantum field a useful book-keeping device that has no direct correspondence to the real world.
The description of interactions is local in a very specific sense, indicated in my original posting. Locality is indeed a property of the mathematical description that is sufficient to guarantee consistency of this description with relativistic causality (no causal effects between space-like separated events) and Poincare invariance of observables as well as unitarity of the time evolution and thus also for the S-matrix.