Register to reply

The wrong turn of string theory: our world is SUSY at low energies

by arivero
Tags: energies, string, susy, theory, turn, world
Share this thread:
tom.stoer
#19
Mar31-11, 05:11 AM
Sci Advisor
P: 5,464
Quote Quote by arivero View Post
Or do you refer to the whole idea? I am very surprised that you all are not impressed. ... But if someone is into particles, to notice that the exact number of scalars of the SSM can be produced from this simple combination game -and with the right charges for almost all
The problem is that up now it's only algebra w/o any fundamental dynamics. It looks like a bottom-up approach, but I can't see if this will produce something like a dynamical theory - or perhaps it may - but this will then be some sort of string theory again.

Regarding gravity: w/o gravity there is no need for string theory as far as I can see; string theory requires SUSY + additional dimensions - which we do not see in nature. String theory seems to be kind of framework to "produce theories as something sitting on top of vacuum states". OK, this is nice but afaik there's no additional benefit. w/o gravity it seems that string theory is nothing else but a very complicated "dual reformulation" of a huge class of (SUSY) gauge theories.

That's the reason why I am not very much impressed.
S.Daedalus
#20
Mar31-11, 06:54 AM
P: 211
Also, leaving the top out is like letting the fat kid not play ball, which conjures up unpleasant childhood memories for me.

(More seriously, though, this is interesting speculation that I unfortunately probably won't have the time to fully familiarize myself with. There seems to be just enough fuzziness such that things might end up being merely a coincidence after all, if a suggestive one. Also, I'm unclear about the mass scales -- do the proposed superpartners have the same masses?)
arivero
#21
Mar31-11, 07:50 AM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by mitchell porter View Post
, so it may not even be necessary to regard the two approaches to hadronic supersymmetry as mutually exclusive.
And in fact one can go directly with susy without arguing about string theory, but still my feeling is that the algebra will have its origins from strings, at the end.

Quote Quote by mitchell porter View Post
The extension to leptons is a lot more problematic,
One could say that the coincidence for charged leptons is trivial, as it follows from the coincidence for quarks: given p,r quarks, the number of charged mesons of a kind is also p*r.

My hopes for leptons are based in some hints: One, that for neutral mesons it works only for the p=2 r=3 solution, ie the total of neutrals in SU(p+r) is (p+r)^2 - 2p*r -1, and it is a very happy think that oscillations have doubled the number of neutrinos from Weyl to Dirac. Two, that we have the muon at the same scale that QCD and particularly very near of the pion. And three, that the theory of Koide predicts the charged leptons from a quantity amazingly close to QCD "current quark mass". Of course there is some tension between this and the second point.
arivero
#22
Mar31-11, 07:59 AM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by S.Daedalus View Post
Also, leaving the top out is like letting the fat kid not play ball, which conjures up unpleasant childhood memories for me.
It is more as, the fat kid will be the goal keeper. And indeed unpleasant.

Hey, I think you have explained why topcolor and ETC (extended technicolor) theories are dismissed in favour of Higgs mechanism... the Higgs mechanism does not have any particular role for the top. But then it fail to explain why [itex]y_t=1[/itex]

Quote Quote by S.Daedalus View Post
Also, I'm unclear about the mass scales -- do the proposed superpartners have the same masses?)
What happens is that it is broken, but only mildly broken. And then all of the phenomenological work we have is not useful to us, because it is done on the assumption that the break is huge enough to hide the scalars up in the TeV scale.
arivero
#23
Mar31-11, 12:54 PM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by tom.stoer View Post
The problem is that up now it's only algebra w/o any fundamental dynamics. It looks like a bottom-up approach, but I can't see if this will produce something like a dynamical theory - or perhaps it may - but this will then be some sort of string theory again
No problem about it been a string theory, we were not telling that the wrong turn was to do strings, the wrong turn was to do "elementary Planck scale strings". And yes it is only algebra and we could do it without referring to strings nor dual models. But it does not seem to me an fuzzy or ad-hoc algebra. Let me review all the steps and what do we get in each.
  1. We postulate that some number [itex]r[/itex] and [itex]p[/itex] of two kinds of particles, call them A and B, must form equal number of combinations of kind AA and kind AB.

    There is no loss of generality. The number of combinations AA is [itex] r (r+1) / 2 [/itex] and the number of AB is [itex]r p[/itex], so equality implies that
    [tex]r= 2 p -1[/tex]
  2. For the abelian U(1) charge, there are two posibilities: either A=0 and then AB has equal charge than B; or A not zero and then AA must be -B and AB=-A. In the first case the charged leptons have charge B, in the second case they have charge 3A. Note that the first solution is not really valid because AB should be an antiparticle, but I mention it because it is very similar to the exotic quark assignment found by anomaly arguments. With the second solution, we conclude that the A quarks are down quarks, and the B quarks are up quarks. We conclude that there are [itex]p[/itex] quarks of type "up" and [itex]r= 2 p -1[/itex] quarks of type "down".
  3. Note that for the total of combinations to be an even number -to do pairs of scalars- then [itex]p[/itex] itself must be even (because [itex]r[/itex] is always odd). So the minimal solution is [itex]p=2[/itex] [itex]r=3[/itex] and they produce six equal scalars and thus three generations of them.
  4. The number of charged "sleptons" of a given charge is [itex]p r[/itex] too, so always equal to the number of "down" type "squarks". No surprises here
  5. The number of neutral "scalarleptons" is, from SU(p+r) and substracting the charged ones, [itex](p+r)^2-1-2pr[/itex]. Asking it to be not two but four times the number of generations, we have
    [tex] (p+r)^2-1-2pr = 2pr [/tex]
    so that [tex](p-r)^2 = 1 [/tex] and the only solution compatible with the quark sector is the minimal solution.

What about the combination BB? Where, there are [itex]p (p+1) /2[/itex] of them. For the above solution, it means three of each BB type. On other hand, the gauge part of the Supersymmetric Standard Model has six scalars in the W and Z supermultiplets. My guess is that the combination BB can not partner to three generations of Dirac particles and because of this it is somehow blind to vector-like charges, ie blind both to colour and electromagnetism, while it can see the chiral charges (hypercharge and perhaps SU(2)). Not seeing color, there is only three BB (uu, uc, and cc) and three antiBB, and then they would match with the above six scalars. This impression of mine comes with some extra support from the p=16 example I spoiled in #15 before, where the "BB" combination also had a role related to symmetry breaking (between SO(32) and SO(16) or between E8 and E7).
mitchell porter
#24
Apr1-11, 07:20 AM
P: 757
Let's look at this from another angle. Is the essential idea that all the sfermions (squarks and sleptons) are actually diquarks and mesons? Please correct me if that overlooks something - I have really struggled to get the idea straight in my head - but I think that is the qualitative essence of the proposal.
arivero
#25
Apr1-11, 02:02 PM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by mitchell porter View Post
Let's look at this from another angle. Is the essential idea that all the sfermions (squarks and sleptons) are actually diquarks and mesons? Please correct me if that overlooks something - I have really struggled to get the idea straight in my head - but I think that is the qualitative essence of the proposal.
The strong version of the idea is as you describe. There is also a weak version, that the Supersymmetric Standard Model has a hidden global SU(5) symmetry, but this weak version is irrelevant here.

And there is a stronger version: that all the scalars of the Supersymmetric Standard Model are actually diquarks and mesons. This version needs more handwaving, because it involves some play with chirality, Dirac vs Weyl, etc. But in this version, also the scalars that give mass to the W and Z should be a peculiar kind of diquarks, build from the uu uc cc combinations.
mitchell porter
#26
Apr2-11, 12:04 AM
P: 757
So far I have three ideas for how to realize this:

1. Look for a supersymmetric preon model in which this is a residual trace of supersymmetry among composite particles.

2. Look for a version of the supersymmetric standard model in which there is an emergent extended supersymmetry. It has to be an extra supersymmetry because in the SSM, by definition, the superpartners of the fundamental fermions are fundamental sfermions, not QCD composites, so if certain QCD composites are also superpartners of the fundamental fermions, it has to be a different supersymmetry.

3. Look for a "less than minimal" supersymmetric standard model, in which the only supersymmetry is the postulated relation between fundamental fermions and QCD composites. This is the fuzziest idea. It could involve looking for a hidden supersymmetry in the standard model itself, or for a hidden trace of supersymmetry in a supersymmetric theory broken to the standard model.
mitchell porter
#27
Apr2-11, 06:37 AM
P: 757
I have identified a class of models which seem ripe for the inclusion of "lepto-hadronic supersymmetry" (LHS): single-sector gauge-mediated supersymmetry breaking models, especially when approached holographically. There's too much to sum up now, but see talk by Kachru, papers 1 2 by Kachru and others, talk by Gherghetta, paper by Gherghetta (especially part 7).

These papers are all written under the usual assumption that the superpartners of the known particles exist at high energies, and the model-building choices reflect the interaction of that assumption with various other conventional assumptions about how the world works. So implementing LHS in this framework will necessarily subvert some of the model-building choices which are standard in this literature. But it really looks like it could be done!
arivero
#28
Apr2-11, 11:36 AM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by mitchell porter View Post
Hey, really I am enyoing these talks. With things as gauge bosons involved in susy breaking, and compositeness for al the quarks except the top, it sounds very much as if they were following research lines near to the conjecture here, even to the "strongest version".

Also, it seems it is a hard job. Damn, give me another five years.
mitchell porter
#29
Apr6-11, 07:10 AM
P: 757
Let's look at this from a simple angle again. One version of what we're looking for would be a theory where hadrons are quarks bound by gluons, and where leptons are gluinos bound by squarks. Two immediate problems: there ought to be quark-gluino bound states too, and there ought to be leptonic resonances. How hard are those problems to fix, and are there other obvious problems?
arivero
#30
Apr7-11, 07:35 PM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by mitchell porter View Post
Let's look at this from a simple angle again. One version of what we're looking for would be a theory where hadrons are quarks bound by gluons, and where leptons are gluinos bound by squarks. Two immediate problems: there ought to be quark-gluino bound states too, and there ought to be leptonic resonances. How hard are those problems to fix, and are there other obvious problems?
I am not sure if it is the same theory, because I am not sure about how to describe leptons and quarks. I am intrigued that the QCD strings is inherently a bosonic string; can we build an extended string out of a fermionic field? I'd say no. Can we find a superpartner to the QCD string?
tom.stoer
#31
Apr8-11, 12:44 AM
Sci Advisor
P: 5,464
Sorry to say that, but I still don't understand why the SUSY explanation shall provide any benefit. It adds new and un-observed particles (and perhaps resonances / bound states). It adds new questions and nearly no answers. It seems to be a solution hunting for a problem b/c there is no problem in QCD, we perfectly understand its structure.
mitchell porter
#32
Apr8-11, 04:12 AM
P: 757
Quote Quote by tom.stoer View Post
Sorry to say that, but I still don't understand why the SUSY explanation shall provide any benefit.
As far as I'm concerned, the question is: Is there a supersymmetric theory which gives us the standard model, and in which the numerical coincidences which Alejandro has noted, actually arise from supersymmetry? Alternatively, can we prove that no such theory exists? If it can't be done, it would be good to understand why.
Quote Quote by arivero View Post
Can we find a superpartner to the QCD string?
This is a basic question about how supersymmetry works in theories like super-QCD, to which I do not know the answer. Supersymmetric theories are diverse and very complex. For example, my earlier remark about "gluinos bound by squarks" was rather naive; it looks like the most important interactions of gluinos are with gluons. In discussions of MSSM, you will find people saying that the superparticles will in any case decay to ordinary particles, so composites would not be stable, but that is under the usual assumption that they must be too heavy to have been seen already. So among other things, one should probably look at the behavior of massless super-QCD first - a theory which already comes in many forms: "pure SQCD" with no quarks; SQCD with adjoint quarks, SQCD with quarks in the fundamental representation; SQCD with various numbers of flavors and colors. The 1990s results of Seiberg on electric-magnetic duality look to be of basic importance in understanding these theories.

In all these theories, massless and massive, the elementary fields can be arranged into superfields. But what about composite objects like mesons and baryons - are they generically part of supermultiplets as well? This is what I don't understand. By the way, Seiberg's duality involves the appearance of an extra meson superfield on one side.

Back in comment #18, I mentioned a minor research program from string theory - "orientifold planar equivalence" - in which meson strings have baryon strings as superpartners. In the baryon string, the third quark is smeared along the length of the string. See these lectures by Armoni (they also appeared in Nucl Phys B, but not on the arxiv). In the third lecture, pages 12-13, Armoni actually mentions quark-diquark supersymmetry (Lichtenberg's hadronic supersymmetry), and says this is an alternative explanation (he explicitly says that a certain fermion in N=1 SYM becomes superpartner of the meson). Though I wonder if this picture, with the third quark smeared along the string, might arise from a symmetrized version of the quark-diquark string.

Anyway, obviously we need to look at this and see if it can be extended to include your extension of hadronic supersymmetry to leptons. The framework is unfamiliar to me ("type 0' string theory") so I don't know what pitfalls lie ahead.
arivero
#33
Apr8-11, 05:56 AM
PF Gold
arivero's Avatar
P: 2,933
Quote Quote by tom.stoer View Post
Sorry to say that, but I still don't understand why the SUSY explanation shall provide any benefit. It adds new and un-observed particles (and perhaps resonances / bound states). It adds new questions and nearly no answers. It seems to be a solution hunting for a problem b/c there is no problem in QCD, we perfectly understand its structure.
The whole "program" is to have less un-observed particles that in standard susy (as in the MSSM). On one side, you know that the the minimal SSM is smaller that the MSSM by two scalars, at the cost of not having a higgs mechanism. On other side, you can arrange all the scalars of this SSM using a SU(5) based flavour symmetry with seems very much as QCD with five flavours. The goal is not to understand QCD, the goal is to understand this flavour and see if it allows a formulation in terms of gluon-like composites, so that the total of particles in the "sBootstrapped SM" is still less than in the SSM.

Even if we understand QCD, we dont understand yet ETC, ie the multiple reincarnations of topcolor and extendedtechnicolor that could be still around the corner in CERN (and Fermilab!). So there is a benefit even if you dont buy the whole program.
tom.stoer
#34
Apr8-11, 06:02 AM
Sci Advisor
P: 5,464
OK, I'll try to get that
arivero
#35
Apr8-11, 06:04 AM
PF Gold
arivero's Avatar
P: 2,933
A problem I still dont get about relating gluons to strings is that the QCD string is not a single gluon state... the QCD string appears at long distances and intuitively it seems as a "classic field". But a classic field is a collective of elementary excitations; it is because of it that force fields are always bosons, is it not? It is not easy to build a collective field out of fermions (note that a fermionic field, at least in 3d, is proportional to hbar: it dissapears in the classical limit).

Quote Quote by mitchell porter View Post
So among other things, one should probably look at the behavior of massless super-QCD first - a theory which already comes in many forms: "pure SQCD" with no quarks; SQCD with adjoint quarks, SQCD with quarks in the fundamental representation; SQCD with various numbers of flavors and colors. The 1990s results of Seiberg on electric-magnetic duality look to be of basic importance in understanding these theories.

(...) By the way, Seiberg's duality involves the appearance of an extra meson superfield on one side.

Back in comment #18, I mentioned a minor research program from string theory - "orientifold planar equivalence" - in which meson strings have baryon strings as superpartners. In the baryon string, the third quark is smeared along the length of the string. See these lectures by Armoni (they also appeared in Nucl Phys B, but not on the arxiv). In the third lecture, pages 12-13, Armoni actually mentions quark-diquark supersymmetry (Lichtenberg's hadronic supersymmetry), and says this is an alternative explanation (he explicitly says that a certain fermion in N=1 SYM becomes superpartner of the meson). Though I wonder if this picture, with the third quark smeared along the string, might arise from a symmetrized version of the quark-diquark string.
I am downloading them for the weekend!
mitchell porter
#36
Apr13-11, 12:37 AM
P: 757
I want to tackle your idea, alluded to in comment #7, that the QCD string could be the Type I fundamental string; an idea arising from the 1987 paper by Marcus and Sagnotti, in which the SO(32) group of the Type I theory is derived by placing 5 "quarks" (and their antiquarks) at the endpoints of the string. For anyone just tuning in, the logic of the idea, explained in the first link above, is that the top quark decays too quickly to form hadrons, so all the mesons we know about are only formed from the first five flavors. Although the idea of weak-scale superstrings exists, I think this is the least likely avenue (mentioned so far) towards a realization of very-low-energy supersymmetry. But it might be instructive to walk a short distance down this road, and see what there is to discover.

So, to begin, here's a modern exposition of Type I string theory by Lubos. The Type I theory is just Type IIB with a space-filling orientifold plane, and then you also need 16 space-filling D-branes and their images reflected in the O9-plane in order to cancel anomalies. 16+16=32 and you can almost see where SO(32) comes from (and a similar relation holds in the S-dual heterotic SO(32) theory). I say "almost" because I'm used to a stack of n D-branes giving rise to a SU(n) theory, not a SO(n) theory; I suppose the O-plane has something to do with the latter.

So what's going on in Marcus and Sagnotti's paper? I have put together an explanation, a crucial part of which came from section 3.3 of the thesis of Sven Rinke (which was turned into a book). The fundamental issue is how to obtain the "Chan-Paton factors" which contribute to the amplitude when you have strings ending on branes. When M&S wrote their paper, it wasn't even understood that there are branes in the Type I theory, so they came by their construction another way. But in Rinke I read that, normally, the Chan-Paton factor is obtained from a Wilson line in the worldsheet theory of the brane(s) to which the string is attached, a Wilson line which follows the path of the string endpoint. The method of M&S is an alternative, in which you have fields living on the endpoints and the Chan-Paton factor comes from including them in the path integral. They are called boundary fermions and they have had a revival in recent years, including an application in Berkovits's pure spinor formalism.

Along with the space-filling D9-branes, the only stable branes in Type I string theory are D1-branes and D5-branes. I had thought that maybe I could find a braney explanation of why M&S needed five pairs of boundary fermions (quark, antiquark being one pair) in the D5-branes: open strings in Type I can end on the D1s and D5s as well as on the D9s, so there's a calculus of Chan-Paton bookkeeping which extends to those lower branes as well. But I haven't done the work to understand it yet. You can read about some of it in section 14.3 of Polchinski volume 2.


Register to reply

Related Discussions
LHC and SUSY Aug 2010 results, implications for string theory Beyond the Standard Model 0
Would string theory research papers have value if string theory is physically wrong? Beyond the Standard Model 8
Can string theory/gravitons/broken SUSY reproduce GR time dilation? Special & General Relativity 0
Susy and string theory Beyond the Standard Model 1
A positive cosmological constant * SUSY/string theory Beyond the Standard Model 2