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The wrong turn of string theory: our world is SUSY at low energies 
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#19
Mar3111, 05:11 AM

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P: 5,464

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. 


#20
Mar3111, 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?) 


#21
Mar3111, 07:50 AM

PF Gold
P: 2,933

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. 


#22
Mar3111, 07:59 AM

PF Gold
P: 2,933

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] 


#23
Mar3111, 12:54 PM

PF Gold
P: 2,933

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 vectorlike 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). 


#24
Apr111, 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.



#25
Apr111, 02:02 PM

PF Gold
P: 2,933

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. 


#26
Apr211, 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. 


#27
Apr211, 06:37 AM

P: 757

I have identified a class of models which seem ripe for the inclusion of "leptohadronic supersymmetry" (LHS): singlesector gaugemediated 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 modelbuilding 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 modelbuilding choices which are standard in this literature. But it really looks like it could be done! 


#28
Apr211, 11:36 AM

PF Gold
P: 2,933

Also, it seems it is a hard job. Damn, give me another five years. 


#29
Apr611, 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 quarkgluino bound states too, and there ought to be leptonic resonances. How hard are those problems to fix, and are there other obvious problems?



#30
Apr711, 07:35 PM

PF Gold
P: 2,933




#31
Apr811, 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 unobserved 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.



#32
Apr811, 04:12 AM

P: 757

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 1213, Armoni actually mentions quarkdiquark 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 quarkdiquark 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. 


#33
Apr811, 05:56 AM

PF Gold
P: 2,933

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. 


#34
Apr811, 06:02 AM

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P: 5,464

OK, I'll try to get that



#35
Apr811, 06:04 AM

PF Gold
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).



#36
Apr1311, 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 weakscale superstrings exists, I think this is the least likely avenue (mentioned so far) towards a realization of verylowenergy 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 spacefilling orientifold plane, and then you also need 16 spacefilling Dbranes and their images reflected in the O9plane 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 Sdual heterotic SO(32) theory). I say "almost" because I'm used to a stack of n Dbranes giving rise to a SU(n) theory, not a SO(n) theory; I suppose the Oplane 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 "ChanPaton 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 ChanPaton 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 ChanPaton 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 spacefilling D9branes, the only stable branes in Type I string theory are D1branes and D5branes. 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 D5branes: open strings in Type I can end on the D1s and D5s as well as on the D9s, so there's a calculus of ChanPaton 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. 


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