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> It is now common to describe the SM as an “effective” theory that should be derived from some more fundamental theory at higher energies. I'm curious why the

> It is now common to describe the SM as an “effective” theory that should be derived from some more fundamental theory at higher energies. I'm curious why the more fundamental theory should be at higher energies. In some sense I know part of the answer is 'because we haven't found it at low energies yet', another being 'because it takes sufficiently high energy to produce additional particles (e.g., Higgs)'. Still, it is backwards from my intuition.

Koshkin 5y ago
Well, it’s not that you need higher energies to invent a new theory (even though you do, on a personal level), the problem is that it takes a discovery of a new particle to see which of these theories may be correct (and which of them aren’t), and new particles are only expected to be found as products of an interaction between particles at higher and higher energies.
andreareina 5y ago
Interactions at lower energies are very well predicted by the Standard Model, so deviations are either so small they're hard to find (but see the muon g-2 experiments), or at energies higher than what we've probed. This isn't like the discovery of quantum mechanics where we knew there were things in the "everyday" regime we couldn't predict (e.g. blackbody spectra), more like general relativity where we have an otherwise extremely successful theory that breaks down in an extremal regime. Except we don't even have an example like the precession of Mercury's orbit to point at and say, "this is where it breaks down". "We haven't found it at low energies yet" understates how much they've looked and how much hasn't been found.
galcerte 5y ago
We believe we should have new physics at higher energies because treating the SM as an effective field theory introduces additional terms in the Lagrangian which are suppressed (~ divided) by some energy scale. These additional terms are the ones allowing lepton flavor violation (so a muon decaying to an electron and a photon, for example) but we know from experiments that these events are incredibly rare. This tells us that the energy scale of new physics is very high. However, we could also have new physics at lower energy scales, as is the case with axions.

Koshkin 5y ago

Well, it’s not that you need higher energies to invent a new theory (even though you do, on a personal level), the problem is that it takes a discovery of a new particle to see which of these theories may be correct (and which of them aren’t), and new particles are only expected to be found as products of an interaction between particles at higher and higher energies.

andreareina 5y ago

Interactions at lower energies are very well predicted by the Standard Model, so deviations are either so small they're hard to find (but see the muon g-2 experiments), or at energies higher than what we've probed. This isn't like the discovery of quantum mechanics where we knew there were things in the "everyday" regime we couldn't predict (e.g. blackbody spectra), more like general relativity where we have an otherwise extremely successful theory that breaks down in an extremal regime. Except we don't even have an example like the precession of Mercury's orbit to point at and say, "this is where it breaks down". "We haven't found it at low energies yet" understates how much they've looked and how much hasn't been found.

galcerte 5y ago

We believe we should have new physics at higher energies because treating the SM as an effective field theory introduces additional terms in the Lagrangian which are suppressed (~ divided) by some energy scale. These additional terms are the ones allowing lepton flavor violation (so a muon decaying to an electron and a photon, for example) but we know from experiments that these events are incredibly rare. This tells us that the energy scale of new physics is very high. However, we could also have new physics at lower energy scales, as is the case with axions.