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I've seen a bunch of articles talking about how new findings from the LHC seem to disprove (super)string theory and/or supersymmetry, or at least force physicists to reformulate them and change essential predictions.

Some examples:

So I'd like to know: has string theory really been hit that hard? Is it losing ground in the scientific community? Do you think it can recover from it? Are there any viable or promising alternative theories? (I've seen Michio Kaku in some clips saying string theory is "the only game in town".)

Note: a related question is What if the LHC doesn't see SUSY?, but I'm asking for more direct answers in light of the results found in the last 2 years.

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    $\begingroup$ Supersymmetry is not dead. Some of the most popular (and some would say naive) models have been ruled out, but supersymmetry can always be pushed up to a higher energy scale where we can't see it (yet). String theory is not dead either. In fact all of the popular press reporting on string theory is irresponsible and should be avoided! String theory made absolutely no predictions for the LHC except for some extremely contrived models. The natural home of string theory is the Planck scale. $\endgroup$
    – Michael
    Feb 22, 2013 at 9:21
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    $\begingroup$ In general, let me say that it is very difficult to link strings to LHC physics. Concretely, the absence of SUSY particles at the LHC doesn't falsify string theory, although the complete absence so far of non-standard physics is a disappointment to anyone working in the field. $\endgroup$
    – Vibert
    Feb 22, 2013 at 9:21
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    $\begingroup$ Popular media and magazines, such as Nature and the Scientific American write a lot of nonsense and exaggerations just to start controversies and sell more copies. Many real physicists do not take them seriously, often the mass media do not even bother to talk to real experts about such topics but prefer to quote the unqualified opinion of very vocal nonexperts. Prof. Strassler often has to correct blatantly wrong up to misleading or even dishonest things written in the media by careless journalists. $\endgroup$
    – Dilaton
    Feb 22, 2013 at 17:05
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    $\begingroup$ Well, that's why I'm asking here. But I would appreciate more specific replies than "these people/media are not credible". $\endgroup$
    – aditsu
    Feb 22, 2013 at 17:12
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    $\begingroup$ Suggestion to the question formulation (v3): Restrict the scope of the question to only ask about the status of SUSY rather than string theory (as the experimental status of string theory has already been adequately covered on this site in other posts). $\endgroup$
    – Qmechanic ♦
    Feb 23, 2013 at 20:30

3 Answers 3

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The idea which is being challenged, though certainly not disproved yet, is that there are new particles, other than the Higgs boson, that the LHC will be able to detect. It was very widely supposed that supersymmetric partners of some known particles would show up, because they could stabilize the mass of the Higgs boson.

The simplest framework for this is just to add supersymmetry to the standard model, and so most string models of the real world were built around this "minimal supersymmetric standard model" (MSSM). It's really the particle physicists who will decide whether the MSSM should lose its status as the leading idea for new physics. If they switch to some "new standard model", then the string theorists will switch too.

Whether they are aiming for the SM, the MSSM, or something else, the challenge for string theorists is, first, to find a shape for the extra dimensions which will make the strings behave roughly like the observed particles, and then second, use that model to predict something new. But as things stand, we still only have string models that qualitatively resemble reality.

Here is an example from a year ago - "Heterotic Line Bundle Standard Models". You'll see that the authors talk about constructing "standard models" within string theory. That means that the low-energy states in these string models resemble the particles of the standard model - with the same charges, symmetries, etc.

But that's still just the beginning. Then you have to check for finer details. In this paper they concern themselves with further properties like proton decay, the relative heaviness of the different particle generations, and neutrino masses. That already involves a lot of analysis. The ultimate test would be to calculate the exact masses and couplings predicted by a particular model, but that is still too hard for the current state of theory, and there's still work to do just in converging on a set of models which might be right.

So if supersymmetry doesn't show at the LHC, string theorists would change some of these intermediate criteria by which they judge the plausibility of a model, e.g. if particle physics opinion changed from expecting supersymmetry to show up at LHC energies, to expecting supersymmetry only to show up at the Planck scale. It would mean starting over on certain aspects of these model analyses, because now you have changed the details of your ultimate destination.

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Disclaimer: I am not a phenomenologist.

...

Having said that, I think there are two issues that are conflated here:

  • The first is that SUSY is more or less necessary for the mathematical consistency of string theory, yes.
  • The other is that if nature is supersymmetric at LHC-accessible energy scales, then we might have a solution to the Hierarchy problem, which is the question of why is the Higgs so light when we would a priori expect its mass to be close to the Planck mass, which is something like $10^{15}$ bigger.

I understand this second point historically has been one of the driving forces of SUSY research (the other is string theory of course) which is why many physicists find the prospect of no SUSY at LHC scales troubling. This is not really relevant to string theory itself however.

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  • $\begingroup$ "I am not a phenomenologist." Ah, but ... do you play one on the internet? $\endgroup$
    – dmckee --- ex-moderator kitten
    Feb 27, 2013 at 0:35
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    $\begingroup$ Insofar as it is understood that any harm that befalls you and/or your loved ones as a result of trusting anything I say is not my fault, then yes. $\endgroup$
    – user21299
    Feb 27, 2013 at 0:49
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Supersymmetry is not dead and cannot die because it is a mathematical construction, beautiful in its simplicity and power. What it may very well is not to be physical. Many string theory constructions are being postulated assuming some sort of unicity proved: types of compactifications, solutions to various no-go theorems, anomaly cancellations etc. In all cases the unicity is just desired and favoured by some physicists but not rigurously proven. What I think people should do is to start with the fundaments of these theories and re-check all the assumptions. My opinion is that string theory will be found non-existent (in the mathematical sense of being isomorphic with a far simpler theory) ans supersymmetry will be found not to be a symmetry of nature.

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