Are Second and Third Generation Fermions Truly Fundamental?

[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.

 

[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.

 

[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.

 

[bananan]

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.

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.

 

[bananan]

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.

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.

 

[arivero]

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.

different topics

 

[CarlB]

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.

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

 

[bananan]

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

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

 

[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!

 

[CarlB]

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

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

 

[Haelfix]

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

 

[CarlB]

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

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.

 

[Severian]

Uh, that would eliminate string theory from the discussion.

Bonuses all round then ;)

That would remind me of human endeavors other than physics.

Just so!

 

[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)

 

[bananan]

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)

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?

 

[Haelfix]

I really don't know Bananan, I am 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 doesn't naturally live in d = 4 makes it attracting from a phenomonological standpoint.

 

[bananan]

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.

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.

 

[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:

The path-ingeral formulation of quantum mechanics might be empirically equivalent to other formulations and yet--given less-than-omniscient human physicists--find more natural-seeming applications to realms of science not yet explored. Different theories tended to give a physicist "different ideas for guessing." Feynman said. And the century's history had shown that when even so elegant and pure a theory as Newton's had to be replaced, slight modifications could not suffice.

"To get something that would produce a slightly different result it had to be completely different. In stating a new law you cannot make imperfections on a perfect thing, you have to have another perfect thing."

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

 

[bananan]

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

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?

 

[bananan]

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

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.

 

[bananan]

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

However difficult it may be to exclude a 4th generation of fermions, I still think this idea might be worthwhitle esp regarding Bilson's preon ribbon model. Just coming up with a formula to describe 2nd/3rd generation as quantized excitations of 1st generation would be very impressive, as well as predictions with particle half-lives and masses. As for the neutrino problem, well maybe neutrinos play by a different set of rules. Or maybe the excited state of an electron isn't a muon but a W-boson (which decays into an electron and neutrino), and the excited state of the W-boson is the muon (i.e spin changes with excitation).

Such a formula could show a 4th generation is
1- not energetically or entropically favored
2- decays too quickly
3- results in a structure that is unbounded, much like an electron-proton system, where n=infinity, the electron is effectively removed from the proton.

 

[CarlB]

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?

The usual excited states of things decay by releasing a photon or two. This is forbidden for flavor decays, if I recall correctly. You'd have to come up with a new definition of "excited", and it wouldn't be easy. Like Severian says, you'd also end up with trouble regarding the W and Z particles.

I think that one has to sit through a couple of particle physics classes to appreciate how "perfect" the standard model is. It might appear to be a conglomeration of arbitrary assumptions that are easy to make small modifications to, but this is not the case. The whole thing is frighteningly consistent.

Making changes in the standard model of the sort you're talking about would be similar to making changes to the more familiar chemical theories by supposing that the uranium atom is composed of a combination of a lead atom and a helium atom. Or maybe not quite so bad, but still, the theory as it stands is a perfect theory and one cannot easily make small changes to it, other than changes to its parameters.

Carl