If SUSY is found, is String Theory the expected why?

[Jimster41]

As I understand it the proposed SUSY model is based on an "expectation of symmetry", which also happens to nicely solve specific problems with the SM. If the LHC does find evidence of SUSY structures - is string theory a candidate for a fundamental theory that would "explain the harmony" of the SUSY model? Is it the only candidate?

 

[mfb]

String theory requires SUSY, but the other direction is not true. SUSY could exist without string theory. And string theory is still far away from making actual predictions about particles. The SM would get extended to include the SUSY model(s) that fits best to observed particles.

 

[rootone]

Nevertheless, if SUSY becomes seen to be true, is there anything else on the table other than string theories which could explain it?

 

[Roy_1981]

Well to be precise, String theory requires supersymmetry at the fundamental scale or "high-scale supersymmetry". As you probably know, if theory X has a symmetry in the lagrangian or say the UV fixed point, it need not be that the lowest state(s) have that symmetry - a phenomenon dubbed "spontaneous symmetry breaking". So discovering SUSY in nature does not prove string theory and neither does not detecting susy rule out string theory. But discovering SUSY could/would indicate that we must be inhabiting a solution/vacuum of string theory that has supersymmetry unbroken (spontaneously) if at all the theory of nature is string theory. That's all, I don't know why the popular press is so to hell-bent on associating SUSY-detection with string theory as far as LHC is concerned. Low-scale (TeV scale) SUSY is not what String theory needs nor does it predict low scale susy for ALL string theory vacua/ground states.

 

[Haelfix]

String theory requires SUSY, but the other direction is not true. SUSY could exist without string theory. And string theory is still far away from making actual predictions about particles. The SM would get extended to include the SUSY model(s) that fits best to observed particles.

Technically, the existence of supergravity is really what would 'imply' string theory. There are various rather powerful arguments that suggest that you cannot decouple supergravity from string theory. The weak link in the reasoning is whether or not the existence of low energy supersymmetry then leads to supergravity (one could say imagine modifying gravity from the canonical form for instance).

This isn't actually as strong a statement as one might think. There are various people that believe that any consistent theory of quantum gravity has to be a a string theory (or more precisely dual to a string theory), but again this really isn't different in information content at this time than saying that there exists a large space of theories of matter that couple to gravity in some consistent way so as to make sense of some boundary observable. It doesn't necessarily tell you which theory corresponds to the real world.

 

[Jimster41]

Thanks for those replies. Answered my question and improved my cartoon of how the progression toward higher energies in collision experiments relates to "rebuilding broken fundamental symmetries".

Which is to say the ontologies of configuration, symmetry, invariance, entropy, energy, time, and... reversibility now support excessive rumination... in an even more completely un-healthy way.

And I think I have a more tantalizing sense of why practitioners consider the "gravity bosons?" an area of potential connection between SM, String theory, and the new angle of approach, LQG.

 

[Demystifier]

There are various rather powerful arguments that suggest that you cannot decouple supergravity from string theory.

Interesting! Can you give some examples of those arguments?

 

[Haelfix]

Yes, for N=8 supergravity see the following paper, which has the virtue of absolute simplicity:
arXiv:0704.0777

I've seen similar games done for N=4 Sugra as well.

For N=1, the above style of argument is not tractable in the same manner, but nevertheless one kind of goes at it in a around about way, arguing that you need to extend the SuSY to soften the perturbative divergences of the theory, but that you can't spontaneously break maximal SuSY to N=1 in 4 dimensions, therefore you are stuck. In fact you *almost* have a no-go statement at that point. The only known solution is to run string theory in reverse, and undo the compactification procedure for M theory, which recovers N=1 in the appropriate limit.

 

[arivero]

I have an alternative link from susy to open strings, from my writtings on "composite squarks and sleptons". For newcomers, remember that this compositeness speculates that pairs of light quarks, in SU(5) flavour -this is, all except the top- peculiarly form the same number of combinations that the required susy scalar partners. The question should be which "hypercolor" at TeV scale can link the quarks in a useful way -this is, cancelling the fermionic loops and then controling Higgs quadratic divergence, etc-. A string theory where the five quark flavours are in the terminations of an open string would be then a nice object to study.