On empirical evidence for compositeness

[metroplex021]

Hi folks,

I just read a passage by the great Ashoke Sen in which he writes that, due to the possibility of particle production in collisions, "strictly speaking there is no experiment possible even in principle that can distinguish elementary from composite particles."

But how is this right? Is it not the case that the structure functions obtained from deep inelastic collisions showed unambiguously that protons were composite? Perhaps there is something buried in the 'strictly speaking' here, but if anyone has any thoughts to contribute on the matter I'd be most happy to hear them!

 

[Meir Achuz]

What Sen seems to mean is that if electrons fly out of the nucleus in beta decay, this does not mean there are electrons in the nucleus. You are right, that indirect tests can indicate sub-strucutre.

 

[mfb]

You can show that particles are composite particles.
The other direction is a bit tricky - it could always be a deeply bound state of two very heavy particles where (due to some weird coincidence) all particle properties agree with those you would expect from an elementary particle. Those models are the reason why LHC experiments look for compositeness.