If preons are real, and make up the SM, and SUSY is real and unbroken

[bananan]

Would you still have a doubling of the SM, with all its SUSY-Partners, or would unbroken SUSY be at the preon level only, so that you have a preon and its SUSY-partner preon with 1/2 difference in spin, to compose the SM? (maybe with dark matter candidate)

 

[Haelfix]

How you break SuSy is model dependent, there is a considerable wealth of ways to do it (and many have associated problems). I am sure people have worked out models where its broken at the Preon scale, wherever that may be, but note that much of the purpose of introducing SUSY in the first place is to help alleviate the hierarchy problem. Pushing it up to arbitrarily high energy scales where new physics enters leaves that can of worms unsolved. Indeed a large portion of the initial justification for SuSy goes out the window.

 

[bananan]

How you break SuSy is model dependent, there is a considerable wealth of ways to do it (and many have associated problems). I am sure people have worked out models where its broken at the Preon scale, wherever that may be, but note that much of the purpose of introducing SUSY in the first place is to help alleviate the hierarchy problem. Pushing it up to arbitrarily high energy scales where new physics enters leaves that can of worms unsolved. Indeed a large portion of the initial justification for SuSy goes out the window.

Well thanks. But if Preons are fundamental, would SUSY have to be broken? If SUSY is unbroken, and preons are fundamental, would you still have SM particles with equal-mass SUSY-partners?

 

[Haelfix]

SUSY has to be broken, or else its instantly falsified (we have not observed a single superpartner ever) and yes SuSY implies a corresponding fermion for every boson in your theory and vice versa.

Preons are quite a distinct concept from SuSy, and have much less justification and formal interest (indeed most of the simplest preon models are falsified). They show up once in awhile when people are dealing with deeper theories, but other than that most of the physics community has lost interest in them.

I personally find preon models exceedingly ugly, and really don't understand why people are so interested in them on this board. They are in many ways just a massive increase in degrees of freedom, without much output (eg you put a lot in *by hand* into your theory, you don't get much out)

 

[bananan]

I think a lot of the interest is generated by

1- publication of Woit's Not Even Wrong and Smolin's The Trouble with Physics, both imply the string theory project is a failure and
2- Bilson's ribbon model, and embedding it in LQG

Isn't the idea of a string generating the SM a kind of preon like idea?

In light of the failure of SU(5) GUT, which is the most conservative extension of SM, SUSY-breaking theories seem far more speculative than SU(5) -- positing various unseen particles and forces.

I'm curious as to whether you regard string theory hegemony a disaster for particle physics, given that it is also a huge increase in the degrees of freedom (4D to 11D, SUSY, and 10^500 vacua) without any output.

SUSY has to be broken, or else its instantly falsified (we have not observed a single superpartner ever) and yes SuSY implies a corresponding fermion for every boson in your theory and vice versa.

Preons are quite a distinct concept from SuSy, and have much less justification and formal interest (indeed most of the simplest preon models are falsified). They show up once in awhile when people are dealing with deeper theories, but other than that most of the physics community has lost interest in them.

I personally find preon models exceedingly ugly, and really don't understand why people are so interested in them on this board. They are in many ways just a massive increase in degrees of freedom, without much output (eg you put a lot in *by hand* into your theory, you don't get much out)

 

[Severian]

Wyou have a preon and its SUSY-partner preon with 1/2 difference in spin

Would that be a spreon or a preonino?

 

[bananan]

Would that be a spreon or a preonino?

well in string theory, there's just one string despite supersymmetry.
there isn't a string and a stringino.

 

[Severian]

I was being tongue in cheek.

More seriously, the reason string theory doesn't distinguish is because they are different manifestations of the same thing. The fermion is the projection of the superfield onto 'normal' bosonic space-time dimensions, and the boson is the projection onto the new fermionic space-time dimensions. They are (literally) different sides of the same object.

However, when SUSY is broken you should distinguish, since one will gain an extra mass. And it needs to be broken, as Healfix said, since otherwise it would be ruled out (eg. you would have a selectron with the same mass as an electron). In order to have a different selectron mass coming from the prons you would have to break the susy at the preon level.

 

[bananan]

I was being tongue in cheek.

More seriously, the reason string theory doesn't distinguish is because they are different manifestations of the same thing. The fermion is the projection of the superfield onto 'normal' bosonic space-time dimensions, and the boson is the projection onto the new fermionic space-time dimensions. They are (literally) different sides of the same object.

However, when SUSY is broken you should distinguish, since one will gain an extra mass. And it needs to be broken, as Healfix said, since otherwise it would be ruled out (eg. you would have a selectron with the same mass as an electron). In order to have a different selectron mass coming from the prons you would have to break the susy at the preon level.

While anyone with enough skill and imagination can concoct a preon theory as they wish, is there a reason that, say, LQG-spinfoam cannot incorporate SUSY, break it based on spin foam dynamics, (as opposed to highly speculative messenger particle method) and posit Bilson like preon ribbons, that


"projection of the superfield onto 'normal' bosonic space-time dimensions, and the boson is the projection onto the new fermionic space-time dimensions. They are (literally) different sides of the same object."

It would be a "preon" theory in 4D with stringlike approach.

 

[Haelfix]

"While anyone with enough skill and imagination can concoct a preon theory as they wish,"

No! You really can't. Preon theories have issues with chirality, which is why many of their models are ruled out. The T'Hooft anomaly matching constraints are exceedingly strict and inescapable and drastically cut the space of models into something pretty small and contrived.

Some of the toy model proposals floating around out there are strictly speaking, not field theoretic implementations of what high energy physicists normally are talking about when they say preons. They are instead semi classical algebraic/combinatorial constructs, but be sure they will have problems with quantum mechanics the second you try to measure something in an accelerator, or indeed the theory will need to be updated into something more familiar.

 

[bananan]

"While anyone with enough skill and imagination can concoct a preon theory as they wish,"

No! You really can't. Preon theories have issues with chirality, which is why many of their models are ruled out. The T'Hooft anomaly matching constraints are exceedingly strict and inescapable and drastically cut the space of models into something pretty small and contrived.

Some of the toy model proposals floating around out there are strictly speaking, not field theoretic implementations of what high energy physicists normally are talking about when they say preons. They are instead semi classical algebraic/combinatorial constructs, but be sure they will have problems with quantum mechanics the second you try to measure something in an accelerator, or indeed the theory will need to be updated into something more familiar.

Dear Haelfix

How does string theory get around issues of chirality and "The T'Hooft anomaly matching constraints"?

I infer you think Yershov and Bilson's fail on these accounts?

Thanks

 

[Haelfix]

I think you are a little confused about a main point, judging from this post and several others. At this time string theory really doesn't deal with low energy physics.. Almost at all. It doesn't have much to say about which phenomonological theory is the right one or not, all it can do is sometimes have a limit that might look like one (but not always). I don't know if string theory can incorporate preons or not, I'd imagine it can as well as a whole host of other possibilities we haven't even thought off yet. The restrictions on the matter content at very low energies (read the scales we are dealing with now) I don't think are very tight, or at least its an open question. Its only when you get close to the Planck scale that it has very fixed predictions.

Now having said that, there are some things that seem more natural than others in string theory. For instance, GUTS seem natural, as does the existence of supersymmetry. The falsification of either at accessible energies is not a direct falsification of the theory perse, but it starts becoming 'unnatural'.

Yershov and Bilson's preons are different than the ones high energy physicists usually refer too. Apples and Oranges.