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What theoretical explanations exist for the fact that there are three generations of leptons and quarks?

I'm not so much asking why there are exactly 3 generations, but rather what makes electron, muon and tau differ. Also, since the three families of quarks don't have to be a priori related to the three families of leptons, I'm interested in answers for either quarks, leptons, or both.

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Last I heard "Who ordered that?" was still the last word on the matter, but I will defer to our theorists... –  dmckee Dec 4 '12 at 2:59
    
I often hear that there are a lot of ideas, but nobody ever goes into any detail. A couple of catchwords I've heard that may or may not have to do with this: Triality, SU(5) grand unification (and other GUTs), Preons (yuck), Koide formula, geometrical interpretations (extra dimensions, strings), ... –  jdm Dec 4 '12 at 13:58
    
@jdm: You've changed your question enough that you might as well start over. Perhaps you could return this one to its pre-edit state, so that the answers match the question, and then ask the question you intended to ask in a new question? –  user1504 Dec 6 '12 at 13:29
    
Well, I explicity asked for current theories explaining the "generation" phenomenon, and I explicitly said I wasn't interested where the number "3" comes from. I'm surprized this got so misinterpreted... but you're probably right –  jdm Dec 6 '12 at 13:34
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@user1504 for future reference, jdm did the right thing in editing his post to clarify why the existing answers were not addressing it. It's important to note that the edit didn't change the meaning of the question, it clarified it. In general, in a case like this, you shouldn't recommend returning it to the pre-edit state, and you shouldn't recommend reposting a new question on the exact same topic. (We might make an exception this once, but I'm commenting just so everyone knows for future reference.) –  David Z Jan 1 '13 at 11:55

3 Answers 3

here is another argument (disclaimer: I'm an experimentalist):

e.g. Wikipedia states that:

"Direct" CP violation is allowed in the Standard Model if a complex phase appears in the CKM matrix describing quark mixing, or the PMNS matrix describing neutrino mixing. In such a scheme, a necessary condition for the appearance of the complex phase, and thus for CP violation, is the presence of at least three generations of quarks.

the same article also says further down:

The universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter as might be expected. It can be demonstrated that, to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter–antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.

The Big Bang should have produced equal amounts of matter and antimatter if CP-symmetry was preserved; as such, there should have been total cancellation of both—protons should have cancelled with antiprotons, electrons with positrons, neutrons with antineutrons, and so on. This would have resulted in a sea of radiation in the universe with no matter.

In other words, if there were less than three generations of quarks (unless the initial state of the universe had a matter/anti-matter asymmetry), there would be no matter and we ultimately wouldn't be able to discuss this topic here...

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We don't have a good explanation for why the quarks and leptons fall into generations. But we have some very strong arguments that it has to be this way, because of the way the weak interactions behave.

First, the weak interactions tell us that each lepton should be paired with a neutrino, and that each charge 2/3 quark should be paired with a charge -1/3 quark. This pairing is necessary just to write down the Lagrangian for the weak interactions.

The second bit is even weirder. The weak interactions are chiral; they don't treat left-handed particles in the same way that they treat right-handed particles. Quantum chiral gauge theories, like the SU(2) x U(1) gauge theory describing the electroweak interactions, are somewhat delicate beasts. Most classical chiral gauge theories can not be quantized; quantum mechanical effects give rise to anomalous gauge symmetry breaking, which ruin the consistency of the theory.

In the case of the Glashow-Weinberg-Salam model, there's a consistency condition for avoiding anomalies: 3 times the sum of the charges in a quark doublet + the sum of the charges in a lepton doublet must equal zero. This condition is satisfied by the Standard Model particles: 3(2/3 - 1/3) + (0 - 1) = 0. Which tells us that the quark and lepton doublets in a generation really are paired in a non-trivial way. If they weren't paired up, the theory would most likely be inconsistent.

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Am I missing somethin, but this says nothing about the number of generation? –  Anixx Dec 6 '12 at 13:56
    
I was addressing the (implicit, I thought) question of what a generation is. –  user1504 Dec 6 '12 at 14:24
    
@Anixx: I didn't mean to ask about the number of generations. I know that answer, besides, it was already handled before here: physics.stackexchange.com/q/2051/825 . I opened a new, clarified question here: physics.stackexchange.com/q/46097/825 –  jdm Dec 6 '12 at 15:12

Here is an experimentalist's answer:

The Standard Model of particle physics is not a theoretical invention, it is a laboriously built up compilation of the quantum number behavior in interactions of elementary particles. So it is an experimental fact.

It first started with SU(2) groups and one found that the symmetries fitted the baryon nomenclature for proton and neutron in nuclear physics to start with, allowing for theoretical potential models to be built up for the nuclear force.

Then came the high energy experiments in accelerators that gave a plethora of particles with well recorded quantum numbers organized by theorists in assuming SU(3)xSU(2)xU(1) symmetry for the groups describing all the symmetries of the particles and the way they interacted with each other. Again, these are data measured, describing Nature.

These group symmetries do not have separate masses for each particle. In the structure they could all have zero mass. So theories came up which proposed that there is symmetry breaking down in the low energies and if one goes to high enough energies everything is massless. Theories evolved to describe the symmetries and explain the data.

An example are Grand Unified Theories:

A GUT model basically consists of a gauge group which is a compact Lie group, a connection form for that Lie group, a Yang-Mills action for that connection given by an invariant symmetric bilinear form over its Lie algebra (which is specified by a coupling constant for each factor), a Higgs sector consisting of a number of scalar fields taking on values within real/complex representations of the Lie group and chiral Weyl fermions taking on values within a complex rep of the Lie group. The Lie group contains the Standard Model group and the Higgs fields acquire VEVs leading to a spontaneous symmetry breaking to the Standard Model. The Weyl fermions represent matter.

Recent theories that embed the Standard Model and the workings of GUT are String Theories.

So the three generations come from quantum number classifications of data and the theories explaining the observations have spontaneous symmetry breaking at low energies, and the the masses differentiate between generations by the mediation of the Higgs field.

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First thanks for your comprehensive post! I'm not sure however it answers my question. Of course the standard model has 3 generations because we see them in experiments. What I'm looking for is a deeper explanation of this structure. Why do some leptons have muon-ness, while others don't? Also, the symmetry breaking don't explain the masses at all (much less the muon-ness). Thanks to the Higgs, we don't have to put in explicit (non-renormalizable) mass terms, but can convieniently use the Yukawa couplings to store the masses. But it's still a mystery where they come from. –  jdm Dec 4 '12 at 13:49
    
The deeper explanation is that "thats the way the cookie crumbled in our reality". The muonness or quarkness of a particle depends on the interaction observing it and it follows the interaction laws of the Standard Model. There fore there is conservation of muonnes, conservation of strangeness etc, until an interaction that changes these quantum numbers happens and it follows the SM. It is a bit like asking : why this sun Sol and this Earth. There could have been different groups and a different standard model, but this is what we are discovering. –  anna v Dec 4 '12 at 15:25

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