# Since second and third generation fermions are unstable and spontaeously decompose

1. Sep 18, 2006

### bananan

Are they really fundamental?

I am under the impression a fundamental particle would be "Stable", i.e first generation fermions.

Could second and third generation fermions be composite particles of first generation fermions?

Specifically,
since the 2nd gen lepton- muon decomposes rapidly into an electron and electron-antineutrino, and a muon-neutrino, then the muon is some kind of composite structure made of electron and electron-antineutrino, and a muon-neutrino momentarily bound together.

Last edited: Sep 18, 2006
2. Sep 19, 2006

### Severian

A muon isn't a bound state of an electron and antineutrino - it is a genuinely different particle. In fact, the muon doesn't even decay into an electron and neutrinos directly. It decays into a neutrino and a W-boson and the W-boson decays into an electron and antineutrino.

The only thing that determines which particle decays to which is the energy available in the mass. So the muon decays to the electron and neutrinos only because the electron is lighter. But being lighter doesn't make it more fundamental. Indeed, if you give an electron enough energy, it could very well 'decay' into a muon and neutrinos (which would of course decay right back again rather quickly).

To put in yet another way, no particle lives forever. If we observe an electron, we are interacting with it by hitting it with a photon. It isn't really the same particle afterwards, so the old electron has been turned into the new one. Therefore one should not use how long a particle lives to determine whether or not it is fundamental.

3. Sep 19, 2006

### arivero

If you keep looking, also one quark of the first generation in nuclean beta decay will "decompose" into a diferent quark plus an electron plus a neutrino.

4. Sep 19, 2006

### bananan

So string theory models these particle transformation as changes in tension?
How close is it to modelling MSSM?

I wonder if BT preon-ribon model works.

5. Sep 19, 2006

### bananan

With a preon model, i wonder if you can model the muon as a bond state of multiple preons, or charged preons, or excited preons, etc., in a sense a composite preon object that is a kind of multiple composite of the bundle that makes up the first generation.

6. Sep 20, 2006

### arivero

different topics

7. Sep 20, 2006

### CarlB

I used to think that the 2nd and 3rd generations were excitations off of the 1st generation. The problem is that there is no easy way of explaining why there is no 4th generation seen. The problem with "seen" is that there are ways of detecting the neutrinos of such a generation and the minimum limits on their masses are much much larger than any of the other neutrino masses. So if you do model the 2nd and 3rd generations as excitations, then you have to explain why the 4th generation has extremely heavy neutrinos.

Compared to the Planck mass, all these masses are zero, so I think it might be better to model them as if they all have the same energy state to first order, but are split. That way you won't have a potentially infinite sequence of excitations to have to explain away. The next level of excitations would all have energies around the Planck mass and would be outside the range of our ability to create.

Carl

8. Sep 20, 2006

### bananan

how does string theory explain the generations and lack of a 4th generation?

9. Sep 20, 2006

### Haelfix

It doesn't in general at this time... There are some compactifications of Calabi Yau manifolds that imply 3 generations and no more (so in that case you would say its a feature of the fundamental geometry that decides), but the situation is not generic and thus model dependant.

As far as making them excitations.. Huh! These things carry different quantum numbers!!

10. Sep 21, 2006

### CarlB

If I recall correctly, undergraduate quantum mechanics spends a good bit of time showing that the various excitations of the hydrogen atom carry different quantum numbers.

Carl

11. Sep 26, 2006

### Haelfix

Yes b/c the hydrogen atom is not fundamental.. We're talking about fundamental particles here, or aren't we?

12. Sep 26, 2006

### CarlB

That was the question posed in the original post. Assuming it is true in answering it would be a pretty obvious case of circular reasoning. Of course one can always take the approach that what is known in the standard model is true and everything else is lies and pointless speculation. That would remind me of human endeavors other than physics. Uh, that would eliminate string theory from the discussion.

13. Sep 26, 2006

### Severian

Bonuses all round then ;)

Just so!

Last edited by a moderator: Sep 26, 2006
14. Sep 26, 2006

### Haelfix

Well.. here is what I know of attempts to unify generations from phenomenology.

1) Early attempts at placing horizontal group structures amongst generations and then spontaneously breaking them (eg SU(3) horizontal, Froggatt et al). Somewhat contrived and has technical issues.

2) Preons. Hard to make them work, have strong anomaly matching constraints and issues with chirality.

3) Huge GUTs (like E8) that can presumably fit three generations. Chirality problems again, unless d != 4 (enter string theory phenomonlogy and extra dimensions)

15. Sep 27, 2006

### bananan

Would you include string theory under (3) huge GUT? How close to the SM or MSSM has string theory been able to reach, if you fine-tune the moduli vacua?

16. Sep 29, 2006

### Haelfix

I really dont know Bananan, im not a string theorist. I think they have some low energy limits where they can presumably output things like E8 GUTs (or some copies thereof) + extra fields, and the fact that ST doesnt naturally live in d = 4 makes it attracting from a phenomonological standpoint.

17. Oct 2, 2006

### bananan

By why couldn't you model the muon as a bound state of a muon and neutrino, with the electron as the ground state? The same for other fermions.

18. Oct 2, 2006

### CarlB

Well, you'd have to include two neutrinos, one an antiparticle. That is, muons don't decay into just electrons and "muon neutrinos", but also you get a "electron anti-neutrino". Here's a link showing the decay of a $$\mu^+$$, the antiparticle of the $$\mu^-$$:
http://cmms.triumf.ca/intro/ppt/intro/img9.html

The worst part of this is that the "electron neutrino" is not a true particle, if you define the true particles as the things that are eigenstates of mass. The electron neutrino is a combination of three neutrinos, the $$\nu_1, \nu_2, \nu_3$$, as is the muon neutrino. Quite a complicated bound state.

If on the other hand you don't define the true particles as the things that are eigenstates of mass, then you've lost the ability to distinguish between the electron, muon and tau. You could replace them with various linear combinations and call them the elementary particles.

And then there's the problem with modeling bound states. You'll have to specify a force. Hmmmm.

The real problem with this sort of speculation is that the standard model is very tightly knit together. You can't modify one small part of it without having repercussions all over the place.

Let me quote Feynman. From the book "Genius, the life and science of Richard Feynman", paperback edition page 368-9:

My feeling is that the standard model is a "perfect thing", and making small modifications of it (well, other than neutrino masses or, for that matter, making changes to the masses or couplings of any of the various elementary particles) is not possible. Especially in the area of eliminating muons as fundamental particles. But I should also admit that I don't think that the muons are fundamental. I think all the quarks and leptons are composite.

Carl

Last edited: Oct 2, 2006
19. Oct 3, 2006

### bananan

What would happen to the SM if 2nd and 3rd generation fermions are "excited" states of the first gen, with the first gen as a ground state?

Last edited: Oct 3, 2006
20. Oct 3, 2006

### bananan

A muon could decay into an electron and photon. Is there a reason this process doesn't happen?