A Line on String Theory

According to the Harvard Gazette, it seems that string theory predicts a very distinctive experimental signature that should be easily observable at the LHC. The claim is that string theory predicts that the LHC should produce stau particles, with a lifetime of a minute or so. I’m no experimentalist, but I’d think a charged particle with no strong interactions, a mass of many hundreds of GeV, and long-lived enough to go all the way through the detector, should stick out like a sore thumb. This might be the kind of thing you only need one of to claim discovery of a new particle, and could even be expected to show up very early after the LHC is turned on.

So, at least if you believe the Harvard Gazette, we may be only a few weeks away from having an experimental result that will settle the string theory question once and for all. Either Vafa and collaborators will be getting the 2010 (or 2011 at the worst) Nobel prize, or string theory’s prediction will have been wrong and we can say goodbye to the theory for good. Next year should be exciting…

Update: Some commenters were pessimistic that the first year LHC would produce these supposed staus at an observable rate. If I read this presentation correctly (page 54), only 40 inverse pb are needed to produce 3 events of a 200Gev stau. Maybe this model will get verified or killed during 2010. From the same conference, see Michael Peskin’s summary talk for more about what the LHC might see in 2010.

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27 Responses to A Line on String Theory

  1. Eric says:

    The long-lived stau occurs in gauge-mediated supersymmetry breaking with a high scale of SUSY-breaking where the gravitino is the LSP and the stau is the NLSP. This is the type of scenario that is characteristic of the F-theory compactifications that Vafa has been studying with Heckman.

  2. Hi Peter,

    “Next year should be exciting” –> if you are gullible enough 😉

    There are indeed searches for decay signals of long-lived particles designed in CMS and ATLAS. The idea being that you get signals when the beam is off! You trap these particles in the detector material, and then they decay at their leisure.

    I doubt anybody would stake a decent meal, let alone their scientific reputation and line of research, on such a prediction. It is just fun to know that we’ll be able to knock such a thing off the board with little early-beam data, but not overly exciting.


  3. Peter Woit says:

    Hi Tommaso,

    This thing is not just long-lived, but it’s also charged, so you should see it go through your detector, not have to wait for it to decay, right?. How hard would it be to recognize a, say 200 GeV stable charged particle? And how much luminosity at 7 TeV would be needed to produce a few such particles (assuming MSSM or something like it)?

  4. Javier says:

    It is important to clarify tht if that signature isn’t actually found it is not string theory what would be falsified but only the F-theory approach of Vafa.

    Still, whatever we would see (or not see9 is quite interesting because this F-theoretical approachsare a whole area of string phenomenology and, nowadays, the best developed one.

    I would add that this F-theory approachs also have concrete implications in cosmology, concerning drk matter and what can find FERNI. In particular it is expected that the ATIC and PAMELA aparent signatures of WIMMPS would be false.

    By the way, this conclusions were presented by Vafa in the slides of the last string theory 2009 in a very assequible way.

  5. piscator says:

    look. the chain of reasoning that says that metastable staus are a real prediction of F-theory GUTs (let alone of string theory) is euphemistically described as dubious. i think this is clear to anyone who has studied them.

    btw, there seems to be no text in the linked article?

  6. Peter Woit says:


    I see, so the headline of the story is wrong, this isn’t a prediction of string theory. Any idea why someone would put an incorrect headline on a story like this?


    Maybe you have an idea about how to reconcile the headline with your “euphemistically described as dubious”? Link works for me, giving a web-page with text.

  7. Eric says:

    I think the point is that this signature is characteristic of a specific mechanism for breaking supersymmetry. If this is observed at LHC, it would be strong evidence for gauge mediation and could suggest that the specific type of F-theory compactifications considered by Vafa and his collaborators is the right direction in which to pursue phenomenology.

  8. Toma Susi says:

    I’m not an expert, but aren’t these pretty big assumptions?

    “Vafa and Heckman devised two constraints that greatly narrowed the possible string universes. First, they assumed that gravity does not have to play a role in the unification of the other three forces. And second, they assumed that one property of string theory, called supersymmetry, is present at the energy levels generated by the LHC.”

  9. piscator says:

    >>Maybe you have an idea about how to reconcile the headline with your “euphemistically described as dubious”?

    I think the author or sub-editor has a good future writing articles for New Scientist.

    The link just gives me a large picture of Vafa with no text, although have just discovered the article by looking at the html source.

  10. Thomas R Love says:

    Like piscator, when I followed the link I did not get the article, but the entire mag is at:


  11. Peter Woit says:


    Low-energy supersymmetry has always been advertised as a positive feature of superstring unification. Decoupling from gravity is essential to get anything. Without this, your particle physics predictions depend on the details of your Calabi-Yau, fluxes, etc, and you basically can’t predict anything (this is the landscape problem).

    You can certainly give up one or both of these things, but if you do, it seems likely that you’ll never predict anything that can be observed.

  12. You are right, Peter. If it is charged, it can also be seen with other -more mundane- techniques, such as the large ionization it leaves in the tracker. All in all, the limitations in the detection of such things is pretty much driven by their production rate, and so the model details.


  13. anon. says:

    “It would be the smoking gun for our stringy models,” Vafa said.

    I hope that’s a misquote. It’s a signature of a wide range of gauge-mediated models. “Smoking gun” usually means a direct implication, not a hint. It’s also not at all clear that F-theory GUTs necessarily imply gauge mediation, and there are many issues (like moduli stabilization) that are glossed over in extracting something resembling an effective low-energy theory from their string constructions. They end up with something that is very close to minimal gauge mediation (and signatures that have been studied for more than a decade), but it’s not at all clear that this is the only thing one could get from similar constructions.

    The one hint of high-scale physics that one could hope for in this scenario would be a measurement of the stau lifetime (which would require the sort of trapped particle Tommaso mentions), which would be an indication of the gravitino mass and hence of the fundamental scale of SUSY breaking. For Vafa and company, this is expected to be just below the intermediate scale, roughly as high as gauge mediation can possibly be pushed.

  14. Shantanu says:

    Peter , there have been some interesting conferences at PI

    anything interesing in this ?

  15. Theodor says:

    Something to smile about:

    A well-known physicist writes: (standard) “particle theory predicts supersymmetry”. (This statement is found on http://insti.physics.sunysb.edu/~siegel/vs.html )

    It shows how working 30 years in the same field distorts your perception of reality…

  16. Martin says:

    Hi Peter,

    the article states that “Supersymmetry, which was first discovered in the context of string theory,…”.
    A question for the history of string theory buff:
    Did string theory really give birth to supersymmetry?


  17. Peter Woit says:


    No, that statement is not really true. 4d space-time supersymmetry of the kind relevant to the LHC was first discovered by Golfand-Likhtman (in 1970, published in 1971) and Akulov-Volkov (1971/72).

    Independently, in 1971 Gervais and Sakita recognized a version of 2d world-sheet supersymmetry in the new fermionic string theory. This led to Wess and Zumino rediscovering 4d supersymmetry in 1973.

    For more details, see


  18. Tom Rizzo says:


    To directly produce an essentially stable 200 GeV stau would require the 14 TeV LHC to run full out, i.e., 100 fb^-1. The production cross section for the pair production of heavy states by only electroweak processes is quite small. It is more likely that such an object might be found in the cascade decay of a strongly produced SUSY state. At 10 TeV, 100 pb^-1 barely lets you pass the LEP limit for such an object.

  19. Peter Woit says:

    Thanks Tom,

    Looking at the Vafa-Heckman papers more carefully, I see that they are claiming a mass range of about 50-300 GeV, less tha 172 GeV if it’s going to be the NLSP. I don’t see a cross-section, but for the case where a neutralino is the NLSP, they quote 3 x 10^2 fb (presumably at 14TeV) for the production of the kind of strongly produced SUSY state you mention, with branching ratios “relatively large” to the neutralino.

    So, OK, maybe not in the first few months…

  20. anonymous says:

    Look, it’s obviously extremely irresponsible of anyone to claim that
    string theory or specifically F theory predicts something so
    specific at this point. That signal could suggest
    gauge mediated SUSY breaking, consistent
    with many UV completions even within
    string theory. We’ll be lucky if
    SUSY itself is cleanly diagnosed very soon (as opposed to
    other kinds of new physics). Gravity-mediated
    SUSY breaking is a bit more UV sensitive, as is high
    scale Gauge mediation. CMB data has a certain
    amount of UV sensitivity. People should try to be responsible
    about making accurate, measured (so to speak)
    statements about these directions. It’s not all or nothing;
    the fact that there are not known smoking gun predictions
    of F theory or string theory phenomenology does not mean
    that we can’t test specific scenarios and learn as
    much as we can from the data
    as well as from the structure of the theory.

  21. Andrew says:

    I’m sure if the LHC produces pink bunnies, string theory can be modified to explain it.

  22. Javier says:

    Hi Peter, some clarifications.

    First of all, this F-theory is “landscape free”. By this I mean that the requirement of gravity decoupling is enough to allow a very restricted number of choices that resemble the standard model, with concretes values of couplings and masses, unifying to SU(5). This is contrary to the landscape idea that, even after that decoupling (i.e. working in”local models” that intend to engnier the known physic) there are a very large number of different string vacua that result in a low energy effective lagrangian implementing the standard model, but with very different values of the coupling constants (and possibly other quantities) for each of them, which implies that string theory doesn’t predict any value for them. This kind of landscape is different from that implied in the solution of the tiny cosmological constant. I must admit that I still don’t understand all the details, and in particular I am not sure how the F-theory GUT’s are expected to face the cc problem.

    The second questions is that the claim of Vafa is that the peculiarities of his F-theory GUT’s are so specific that it could be possible to distinguish them for the other similar SUSY models with gauge mediation SUSY breaking, precisely the contrary to what many commenters have said here. The actual paper (I am not sure if that was the one you were referring to in one of your comments) is :http://arxiv.org/abs/0903.3609

    I am not the most qualified person to give a authoritative opinion about this topic,but as far as I understand it if the LHC fits the predictions of Vafa there is not too much window for other theories that string theory, in the F-theory in this specific limit to account for the known physic. If it gets falsified one would have to rely on other type II-B brane constructions, that have many restrictions to be unified theories so one would possibly have an standard model tht doesn’t unify, or, more probably, natures is not described by type II -b string theory in weak or strong coupling. That is too much to say. Possibly mirror symmetry would also almost invalidate type IIa constructions. As a result possibly if this F-theory GUT is not verified that would mean that nature would be expected to realize some heterotic (M theory heterotic) string vacua. Said this It would be really fine if most informed people as Motl or Distler (or whoever) would comment on what I say.

  23. Gphillip says:

    I’ll give any and all takers 1000 to 1 odds that these particles will never be found. The reasoning is that if these massive charged particles take 1 minute or so to decay, we would be swimming in them from cosmic ray collisions with our atmosphere. We wouldn’t even need the LHC. Just the detector pointed at the sky should work fine.

  24. Peter Woit says:


    The number of cosmic rays with energy sufficient to produce interactions of higher energy than that studied at the Tevatron is non-zero but quite small. Since the probability of producing such a new particle is also very small, you’re not going to see these things in cosmic rays.

    This problem is very general, and it’s why you need accelerators like the LHC: you need to produce not just high energy collisions, but a lot of them, and you can’t get this from cosmic rays.

  25. Gphillip says:

    Perhaps I’ve misunderstood something here. At 10 ^13eV cosmic rays strike the atmosphere at a rate of a few per sq. meter per year. Many more at lower energies. There’s about 197 million square miles on the surface of the earth. That’s a very big non-zero number.

    I was just joking about pointing a detector at the sky, but if that many heavy charged particles are being generated, and take 1 minute to decay, I believe we would have (or at least could have) detected them by other means. But as I said, perhaps I’ve misunderstood something.

  26. Peter Woit says:

    To get LHC energies in the center of mass you need cosmic rays of something like 10^17 ev. They’re rare. And it’s not like you can afford to instrument the entire earth’s surface.

    Once you have one of these, the probability that it will produce this kind of new heavy particle is extremely low. Remember, at the LHC, you’re talking about a few events/year out of a collision rate of 10^9/sec or some such.

    You can do the math, but there’s no way you’re going to get a non-negligible answer…

  27. Gphillip says:

    Of course you are right. At or above 10^17 eV only about 10 cosmic rays per sq. kM strike the atmosphere per year. It’s enough to eliminate “the LHC will devour the Earth” conjecture, but not enough to generate many particles if only a few are generated per year from 10 ^ 9 collisions per second. I suppose this eliminates the possibility of finding some stuck to my refigerator magnet. I stand corrected.

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