Space-time, Quantum Mechanics and the Large Hardon Collider

The title of the posting is that of Nima Arkani-Hamed’s public lecture last week at the IAS, with the spelling that of the title at the beginning of the video (available here, lower resolution version here).

The bulk of the talk is devoted to expounding the idea that the central problems of fundamental physics are two hierarchy problems, that of the CC (why isn’t it at the Planck scale?) and that of the Higgs mass (why isn’t it also at the Planck scale?). Given that we don’t understand quantum gravity, and don’t know that the Higgs phenomenon is due to an elementary scalar, it’s not clear to me that these are yet real problems. In any case, Arkani-Hamed gives the anthropic multiverse argument for the CC problem, and claims that if the LHC doesn’t see supersymmetry or large extra dimensions, then we’re stuck with the anthropic multiverse argument also for the electroweak scale.

The LHC only puts in an appearance in the last fifteen minutes of an hour and a half talk. Back in 2005 (see his talk at Strings 2005) Arkani-Hamed claimed that we would know whether supersymmetry solves the hierarchy problem within a year or so of first collisions at the LHC (then scheduled for summer 2007). Now that initial results from the LHC are in, showing no evidence of supersymmetry, his estimate is:

We’re going to have answers one way or another to this question on the time scale of 2020.

One of his slides estimates production of 1 squark/minute given 1 billion collisions/sec, which would mean about 50 squarks already produced in each detector. While it’s true that the LHC won’t be running at full energy until 2014, no explanation is given for why we need to wait until 2020 to find out about supersymmetry. Back in 2005, before the machine was turned on, enthusiastic predictions of quick results were being made. Now that the data is coming in, the story seems to have changed.

Update: Nature News has a new article up by Geoff Brumfiel: Beautiful theory collides with smashing particle data (also available here). While Arkani-Hamed is arguing that one will have to wait until 2020 (the sLHC perhaps?) before knowing whether supersymmetry is at LHC energies, John Ellis appears willing to give up much earlier, maybe the end of next year:

“I’m wouldn’t say I’m concerned,” says John Ellis, a theorist at CERN, Europe’s particle-physics lab near Geneva, who has worked on supersymmetry for decades. He says that he will wait until the end of 2012–once more runs at high energy have been completed–before abandoning SUSY. Falkowski, a long-time critic of the theory, thinks that the lack of detections already suggest that SUSY is dead.

“Privately, a lot of people think that the situation is not good for SUSY,” says Alessandro Strumia, a theorist at the University of Pisa in Italy, who recently produced a paper about the impact of the LHC’s latest results on the fine-tuning problem. “This is a big political issue in our field,” he adds. “For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track.” Ellis agrees: “I’ve been working on it for almost 30 years now, and I can imagine that some people might get a little bit nervous.”

The article ends with a very sensible quote from experimentalist Chris Lester, who evidently doesn’t share Arkani-Hamed’s view that it’s SUSY or the Multiverse:

“Plenty of things will change if we fail to discover SUSY,” says Lester. Theoretical physicists will have to go back to the drawing board and find an alternative way to solve the problems with the standard model. That’s not necessarily a bad thing, he adds: “For particle physics as a whole it will be really exciting.”

Update: It seems that the video files have been temporarily removed, presumably for editing. I fear that some poor tech person is having a bad morning…

Update: New video files with typo fixed are now available.

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69 Responses to Space-time, Quantum Mechanics and the Large Hardon Collider

  1. Bernhard says:

    “If we don’t find that scalar field, the SM is wrong.”


    That´s not true. Would mean only that we got wrong this specific sector of the model. It ´s like having U(1)-q X SU(WRONG). If SU(WRONG) is wrong does it mean that QED is wrong? Of course not. So of course it is conceivable to have the SM with another origin for electroweak symmetry breaking.

  2. Emile says:


    I guess we have a different definition of what is commonly referred to as the Standard Model. Take most (any?) text books and what constitutes the SM is clearly defined: SU(3)xSU(2)LxU(1)Y, has 3 generations of fermions, a Higgs boson etc. If you want to extend this definition to the point where we can get the the gauge groups wrong, and still call it the SM then I guess it is up to you. Not finding the Higgs would mean that many text books would need to be revised… I really like the fact that the SM (according to my definition of what the SM is) has a scalar field in it and that I can go and look for it and possibly exclude it. That you could still say that the SM is alive and well even if we don’t find the Higgs is precisely what I don’t like about certain theories. You can’t kill them…

  3. Emile says:

    Peter: I guess I was trying to make the point that there are theoretical issues with the SM even without the need to speculate about new physics or other energy scales. I thought what you wrote sounded as though we only ran into problems with the SM if we assumed physics beyond the SM.

  4. Bernhard says:


    I was really hoping here that you would not come with this “argument”.. If you want to discuss the label SM fine. Then you are saying that if I come up if a model that has exactly the same fermion, quark, group sector, everything and has a different mechanism of electroweak symmetry breaking that actually INVALIDATES, i.e., makes it wrong (if you want to be picky about wording here it goes too) that other model identical to the first one in every single aspect but this sector? OK, then with this twisted way of seeing things and with a precious caution about how you name it and nothing else in mind but this, then I guess you´re right. My bad.

  5. Eric says:

    Dear Bernhard,

    It might help if you actually specified the alternative to the Higgs mechanism that you are thinking about and said something about whether or not this can be fit into the minimal SM as is. Also, please enlighten us as to whether or not your alternative mechanism can do completely the same job as a fundamental scalar and still satisfy experimental constraints.

  6. Giotis says:

    Peter really, what do you mean when you say “speculative new physics at a speculative new energy scale”?

    I don’t suppose you imply that maybe the SM is all there is? Do you?Regularization is not a trick, is the admission of the SM that it holds up to a certain energy scale. Without this admission it wouldn’t make any sense.

  7. Peter Woit says:


    My problem is with arguments like: “we must explain why the electroweak breaking scale is so much smaller than the GUT scale!” or “we must explain why the electroweak breaking scale is so much smaller than the Planck scale”. We have no evidence at all for what physics is like at those scales, just speculation about their significance. Claiming that you’ve identified one of the most important problems in fundamental physics, but basing it on some purely speculative sector of the theory that you have no evidence for seems to me unconvincing.

    The question of how high in energy we can extrapolate the SM before running into various problems is certainly interesting, but seems to me different than the usual “hierarchy problem”.

  8. Bernhard says:


    The SM is a model composed of parts, parts that fit together but even the anomaly cancellation is “trivial” (in the sense that they are applied separately for each family) and the Higss sector appears as an almost plug-in accessory that sophisticated and clever as it is, might be incorrect. I really don´t think that it is correct to affirm that not finding the Higgs makes all the success in explaining every single experiment so far a failure, because statements like “if we don´t find the Higgs the SM is automatically wrong” pass this idea. I think it is conceivable that this specific mechanism could be clever but wrong, and my original discussion with you was the need for SUSY, a theory with problems of its own, to correct it. So the SM Higgs create something already begging for physics BSM, and I´m just saying that this might be actually a longer shot away from the SM that is actually needed, but of course I don´t know that, SUSY, Higgs and cia might all be there and and if that happens to be confirmed by experimental evidence I will happily acknowledge it. I however I´m not so much without imagination to think that an alternative is impossible and even if it happens to be a solution less simpler than the Higgs I will not have a sudden epiphany that the SM is completely wrong since its precious Higgs was not found.

  9. Eric says:

    Hi Bernhard,

    I certainly agree with you that there are alternatives to the simple Higgs mechanism which don’t involve a fundamental scalar. The problem with these, generally, is they also introduce a lot of BSM physics which introduces problems of their own. For the most part, these problems are much more severe than a SUSY + Fundamental Higgs scalar solution. However, it is not at all impossible that LHC will find technifermions or something similar which will point directly to something besides SUSY. Personally, my opinion is that if the simplest ideas with a three generation SM + fundamental Higgs + SUSY doesn’t pan out, then the most likely possibility is that in addition to the above there is a fourth generation. Because of the large Yukawa couplings required for the fourth generation, the fourth generation quarks become strongly coupled and can play a role in EWSB. In any case, what we know is that it’s extremely unlikely that the SM is all there is, unless things are just extremely fine-tuned.

  10. Bernhard says:


    “In any case, what we know is that it’s extremely unlikely that the SM is all there is”

    Agreed 100%!!! 🙂

  11. Roger says:

    Seesaw says that what I call mystical beliefs, other people call beauty and theoretical ingenuity. Maybe so, but I don’t see any good reason to doubt that the SM is all there is.

  12. Anonymouse says:

    The Standard Model includes the Higgs. Like it or not, that is the definition. No Higgs means no Standard Model.

    The hierarchy problem is nothing more than fine tuning. As Peter correctly has stated, it is not a problem unless there is some kind of high scale physics beyond the Standard Model. I personally think it is likely something like that exists, but there is no evidence as of yet.

    The Standard Model includes exactly one dimensionful parameter, which is the Higgs mass parameter (or alternately, the VEV which determines the Z and W masses). There are no large quantum corrections to it. What you call large quantum corrections are just the renormalization required to move from an unphysical number in a Lagrangian to a physically measurable quantity (such as any of the masses listed above).

    Even perturbative gravity is soft, and does not correct the Higgs mass parameter by a positive power of M_Planck. (Gravitational couplings are negative powers of M_Planck). Of course, there may be non-perturbative gravitational phenomena which upset this, so the best we can do is to say that there is no obvious contribution to the Higgs mass from quantum gravity. Without a verified quantum theory of gravity at small distance scales, there isn’t much more one can say.

    But once there is new physics with massive particles, such as super-particles, stringy states, GUT bosons, right-handed neutrinos for a seesaw, or anything else with a relevant interaction with the Higgs, the hierarchy problem exists. So it is fair to say that SUSY must first create the problem before it solves it.

    If we see some clear sign of heavy physics, any at all, this situation will change. Neutrino masses don’t work because there need not be any heavy new degrees of freedom.

  13. Bernhard says:


    I agree with your point. I was imprecise to exclude the Higgs in the framework of the SM so I take it back. Emile, I acknowledge you also had a point. I guess I just wanted to say I didn´t want to have a pointless discussion about naming, since if want really to be picky neutrinos oscillations, even if they don´t include new degrees of freedom are enough to say we have physics BSM and that neither this nor an absence of a Higss sudden turn the

  14. Bernhard says:

    *(sorry, sent too early)*

    SM wrong. I agree however that the absence of the Higgs would require solving a much harder puzzle to maintain the SM minimally affected. That is, a new mechanism of electroweak symmetry breaking that does not destroy the current structure is a a tough problem.

  15. Bernhard says:

    …and a just extending a little bit the last thing I want to say about this, neutrino oscillations for example already require a significant change in the SM since something that was supposed to be conserved (lepton family number) was violated. One can require total family number still to be conserved but that´s characteristic already of many BSM models, like the 331 model.

  16. OhDear says:

    “something that was supposed to be conserved (lepton family number) was violated”

    Sure, but what do you mean by “supposed to be” ? Before we found neutrino oscillations it just happened to be true that lepton family number was conserved. Then it turned out it wasn’t true, and I’m tempted to say “big deal”. The essence of the standard model is the gauge group, the specification of the matter particles, and the higgs mechanism. All else is just detail. Neutrino masses can be accomodated in the same way as quark masses. Mind you, nobody really believes that the SM way of giving fermions masses is correct.

    And to Roger, who sees no reason why the SM cannot be all there is – what about gravity? There is no gravity in the SM.

  17. Bernhard says:


    you should have followed the discussion, not my isolated comment before trying to be clever…

    Sure I also don´t give a dime for lepton family conservation, but if you want to be precise about how the SM was DEFINED, then it was defined with a massless neutrino and a massive neutrino represents a violation of this definition. If it is to be precise and consistent I can agree, but then let´s be consistent to what the model defined not matter what.

  18. Bernhard says:

    and by the way, not giving a dime to lepton family number conservation can be very significant depending on the how you see it. If for example the decay “mu–> e + gamma” were to be observed you are forced to say you STILL haven´t seen any violation of the SM since you suddenly changed your mind that this principle can easily be thrown out of the window. I think most experimentalists would disagree with you on that.

  19. OhDear says:

    Bernhard, my point was that the SM is *not* defined to include lepton family number conservation. But you make a good point about “mu–> e + gamma”. That would be a big result, but wouldn’t violate my (perhaps too narrow) definition of the SM.

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