Origin of lepton/quark generations?
In the standard model (and in nature), Fermions appear in different generations, or flavors. Besides up and down quarks and electrons, there are strange, charm, muon, tau, etc.. I am wondering if there is any theoretical explanation for the existence of heavier generations.
What I'd like to know is especially: Why are there different, heavier copies of $e$, $u$, $d$? What makes these particles different from their lighter relatives besides their mass? What is flavour, where does it come from, and why is it approximately conserved (at least for charged leptons)?
Note: I'm posting this question as a follow up on my previous one "Origin of lepton/quark generations?", which didn't reallty get the type of answers I was expecting. Sorry if the old question caused some confusion. To be clear, I'm not asking:
- "Why must there be three generations (and not 2 or 4)?" -> e.g. CP Violation
- "How do we know that there are 3 generations?" -> Experiment
- "Why do we organize particles in this manner?" -> Quantum numbers
But rather: What possible theoretical explanation is there that our universe exibits flavor quantum numbers, and maybe how are they are related to particle masses?
Conceivable (although probably wrong) answers would be along the line of:
- "Because electrons and up quarks have substructure, and muons and charm quarks are the first excited states."
- "In XY theory, $e$, $\mu$ and $\tau$ are actually the same at first order, but the thingamajig symmetry is broken at low energies."
- "It is explained by string theory, if your vacuum has such-and-such properties it comes out naturally."
- "It's accidential in the SM and just experimentally observed, but in SU(...) GUTs it is a prediction."