What is the mechanism for proton decay in the Standard Model?

[zefram_c]

I recently saw posts indicating that the SM does not have baryon and lepton number as exact symmetries, and in fact allows protons to decay, but with much smaller probabilities than GUT's. What is the mechanism by which such decays can occur (is it non-perturbative?), and what order of magnitude are the amplitudes for such processes?

 

[Haelfix]

Yea it is nonperturbative, actually a topological effect in the sdm.

Its a vanishingly small contribution, but still in principle detectable.. Cosmologists were very excited about this back in the early 90s for leptogenesis models.

I can't find T'Hooft's original 1976 paper online, but here is an experimental papers on the matter, which should include some discussion suitable for a novice.

http://arxiv.org/PS_cache/hep-ph/pdf/0307/0307034.pdf

as well as a review article (I actually found a few errors in this one, but whatever its ok)

http://arxiv.org/PS_cache/hep-ph/pdf/0209/0209227.pdf