The State of SUSY

Results putting new limits on SUSY based on the entire first run of the LHC are starting to emerge (see for example this from CMS) with more likely at Moriond next week. Since one is dealing with a theory with a large number of parameters, these are hard to characterize in a simple way. One thing to focus on is the limits on gluinos, since just about every popular version of SUSY says these should be about the easiest thing for the LHC to see. Very roughly, the Tevatron was able to set typical limits of about 300 GeV on such things, and the LHC at 8 TeV (4 times the Tevatron) is now giving limits around 4 times higher, 1.2 TeV. This is not likely to change much for the next few years until after the LHC comes back at 13 TeV in 2015. One can with some confidence predict that the gluino mass limit will then go up to about (13/8)*1.2 TeV or around 2 TeV, maybe a bit more in years after that with a high-luminosity LHC. Farther out in time, the next machine under discussion that could raise the limit is the HE-LHC, at 32 TeV, giving limits around 5 TeV. The time scale for this though is something like 2030-40, even assuming such a project ever were to get funded. I suspect the right characterization of that project might be “not in my lifetime”.

There is a new paper out claiming to see evidence of a gluino in the data around 1000-1100 GeV. The same authors (see here), have been claiming to see such gluinos since the early LHC data, first at around 7-800 GeV, with a mass getting higher with each round of new data and higher mass limits.

Few are likely to pay attention to this, but what is getting taken much more seriously is the case that Nima Arkani-Hamed has been vigorously making recently (see for example his talk at the Higgs Symposium). Arkani-Hamed is now by far the most influential theorist in this area, with slides from his latest talks often appearing in many other people’s presentations, functioning as the embodiment of the conventional wisdom of the field. He also is the only phenomenologist with a $3 million Fundamental Physics prize, awarded for his work on models that have had great influence, although zero success experimentally.

One of these, split supersymmetry, is what he is now promoting as the explanation for the negative LHC results. In this model, which he developed with Savas Dimopoulos back in 2004, the main argument for SUSY, the hierarchy argument, gets abandoned in favor of anthropics. The Higgs mass and the electroweak scale are what they are not because of SUSY, but because otherwise physics would be different and we wouldn’t be here. Once one abandons the hierarchy argument, the remaining arguments for SUSY are extremely weak (I’ll try and explain these in more detail in a separate posting), but for some reason Arkani-Hamed still thinks the idea is worth promoting and that vindication for his $3 million may yet be had.

Split SUSY works by moving all scalar superpartners up to unobservably high energies, but a few particles including the gluino are supposed to be at potentially observable masses. Back in 2004, Arkani-Hamed and Dimopoulos were hopeful about the possibilities for the LHC seeing a split SUSY gluino, writing:

However, at peak luminosity of 30 fb-1 per year, the LHC may well be a gluino factory producing roughly a gluino per second(for m_g ∼ 300 GeV).

These hopes have now been dashed, and at the Higgs symposium talk, illustrative spectra show gluino masses at 2.1 and 2.3 TeV (this may just be because that’s about the limit of what the LHC could see). Arkani-Hamed and co-authors have a recent paper out discussing Simply Unnatural Supersymmetry, i.e. “the simplest picture of the the world arising from fine-tuned supersymmetric theories”. Here calculations are done for gluino masses ranging from 1.5 to 15 TeV, and the story is that we’ll have to be lucky to get any experimental evidence for this model. They end with:

If Nature has indeed chosen the path of un-natural simplicity, we will have to hope that she will be kind enough to let us discover this by giving us a spectrum with electroweak-inos lighter than ∼ 300 GeV or gluinos lighter than ∼ 3 TeV.

So, the current state of the conventional wisdom about SUSY from its most influential proponent is pretty much the following. It’s still the thing to try and sell to the public as the best bet for the future of physics, but the hierarchy argument is gone, and at a fundamental level it’s anthropics, the landscape and the multiverse. He’s pretty much given up hope of ever getting any experimental evidence for this, other than the outside possibility of maybe the gluino mass being just low enough to be visible in rare LHC events late in the decade.

The interesting question about all this I think is a sociological one: will this untestable and rather ugly theory based on anthropic reasoning become widely seen as the “best hope” for fundamental particle theory? In a post-LHC world where mankind has abandoned the high-energy frontier, will the conventional wisdom of the textbooks be that SUSY and those gluinos must be there, but unfortunately happen to be just out of reach?

Update: For a survey article that just came out this evening, which tries to show that the main argument for SUSY (the hierarchy problem) is not quite dead yet, see here.

Update: New Scientist has a special section this week about “Crunch time for physics” (unfortunately mostly behind a paywall). On SUSY, Frank Wilczek is still a believer, based on the renormalization group calculation he was a co-author of back in 1981. If no SUSY turns up at the next LHC run though, even he will throw in the towel:

I cannot believe this success is an accident. But in science faith is a means, not an end. Supersymmetry predicts new particles, with characteristic properties, that will come into view as the LHC operates at higher energy and intensity. The theory will soon undergo a trial by fire. It will yield gold – or go up in smoke.

He has a bet with Garrett Lisi that superparticles will be detected by July 8, 2015.

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65 Responses to The State of SUSY

  1. Peter Woit says:

    Bob,

    Pretty much anything. Arkani-Hamed himself is working on amplitudes. There’s a huge amount about the non-perturbative behavior of quantum field theories that we don’t understand.

    No, there isn’t some obviously better idea about BSM physics that everyone should work on instead of SUSY. But if people can’t think of something more worthwhile to spend their time on than a failed idea that has already received a huge amount of attention, maybe they should be in a different business.

  2. Toni says:

    Bob,

    the most promising would be an idea that keeps the standard model intact and just explains its parameters. Such an idea would have the advantage to agree with experiments. Alas, the idea itself is still hidden in the clouds.

  3. Cliff says:

    Peter: About F-SU(5), I don’t understand why it matters if they write a paper in 2016 saying they were wrong. Nobody has to bother reading what they see or smell in the data because they predict that new particles will be discovered by the LHC! Again I just fail to see what is so terrible about them describing that the best fit for some superpartner mass has increased. They’ve just released a new paper because there are new searches that will soon be published that they want to compare their model to. You seem certain that they are behaving dishonestly but you don’t seem to want to make that case based on any actual details of their claims.

    My bigger beef is that you’re conflating SUSY as a high-energy principle with SUSY as a feature of sub-TeV scale physics. Obviously it would have been nice if these two things went together, but there is no justification at all for saying the absence of SUSY at a scale of 1 TeV implies that its not a useful (or necessary) principle for high energies.

    The best ideas for what lies beyond the next bend haven’t actually changed much. Its obviously understandable why that fact is frustrating, but that seems like the inevitable implication of the logic here. Ingenius and totally unexpected insights are welcome of course, but until/unless they get here, not much has changed.

  4. Pingback: Desperately Seeking SUSY! | Whiskey…Tango…Foxtrot?

  5. Peter Woit says:

    Cliff,

    I don’t believe the F-SU(5) people are dishonest, but assume that they actually believe that SUSY will appear in the next release of data. However, if you keep publishing papers with such claims that don’t have much to back them up, and always turn out to be wrong, at some point your credibility goes to zero, which I think is the case for them right now.

    That SUSY was supposed to explain the hierarchy problem, and so appear before 1 TeV, was probably its most heavily advertised feature and best argument for paying attention to it. The failure of this seems to me worth noting and has major implications for the whole idea. Post that failure you can try and claim that SUSY is a wonderful idea, that adding huge numbers of unseen degrees of freedom with huge numbers of new parameters, in a way that explains nothing about observable physics, is something that people should take seriously as our best idea about how to move forward in fundamental physics. I don’t think though that many people are ever going to buy this.

  6. dark says:

    “No, there isn’t some obviously better idea about BSM physics that everyone should work on instead of SUSY. But if people can’t think of something more worthwhile to spend their time on than a failed idea that has already received a huge amount of attention, maybe they should be in a different business.”

    PW How would a discovery of dark matter in the coming years from various research groups affect your scientific judgment on the plausibility of SUSY?

    (and for SUSY advocates like Eric, how would the failure to detect DM in increasingly sensitive and sophistic experiments affect SUSY)

  7. Eric says:

    Dark,

    The bottom line is this: There is a good chance that the scalar quarks and leptons are too heavy to produce a signal at the LHC. However, even in this case supersymmetry can still solve the gauge hierarchy problem. Furthermore, it can do so without requiring much fine-tuning (the little hierarchy problem) for certain regions of the parameter space. Most of the superpartner spectra which fall into this category produce a relic neutralino density which is at or below the WMAP constraint. In addition, the proton-neutralino cross sections for direct detection are just at the point where they are within reach of experiment. Thus, it is quite possible that dark matter will be discovered in the near future and this would lend additional support for supersymmetry, even if no signal is seen at the LHC. On the other hand, if dark matter is not directly detected, there is always the possibility of R-parity violation or simply that the neutralino only provides a small fraction of the dark matter density.

  8. dark says:

    Eric
    thanks for the reply. I understand that not finding SUSY at LHC can be explained away, and “R-parity violation or simply that the neutralino only provides a small fraction of the dark matter density”

    what if they don’t find any dark matter – neutralino – or otherwise, at all?

  9. chris says:

    Thus, it is quite possible that [*next big experiment will measure something*] in the near future and this would lend additional support for supersymmetry, even if no signal is seen at [*last big experiment that failed to produce anything about SUSY*]. On the other hand, if [* next big experiment will give no signal*], there is always the possibility of [* next level of complication moves SUSY out of reach once more *].

  10. punter says:

    Most of that New Scientist section is accessible through free registration for the next ten days.

  11. Kavanna says:

    There is a strong, general argument for SUSY, based on the Coleman-Mandula theorem. It’s the one general type of symmetry that combines “internal” and “spacetime” symmetries allowing a natural combination of particle physics and gravity.

    But that’s a very general argument. It says nothing about implementation. The attraction for phenomenology has always been the power of SUSY’s opposite-sign cancellations across many areas (loops, cosmological constant). These arguments all rely on perturbation theory, a shaky starting point that assumes everything’s weakly coupled in the regimes we’re talking about.

  12. Kavanna says:

    Thus, the argument here is, in large part, not about supersymmetry, but about the validity and limitations of perturbation theory. If the crucial physics is strongly coupled, a different approach is needed.

  13. CLAUDIE says:

    Dear Peter Woit,

    I am a student of philosophy and will humbly admit I got half way through your article – and felt sad that I could understand so little. I do however understand the basic concept of SUSY and would adore an answer in ‘layman’s terms’ what you feel the future is for SUSY ?

    I would be so very grateful,

    (Ask me anything about Eastern Philosophy — but Physics, I struggle, but read on anyway because it fascinates me :-)

    Claudie

  14. Peter Woit says:

    Claudie,

    In layman’s terms, SUSY (as in superpartners for the standard model particles) is an idea that has gotten a huge amount of attention, despite not explaining much of anything. If the arguments for it were true, it should have been seen before the LHC. Now that it is being conclusively experimentally ruled out at LHC energies, the idea is basically dead, although it is going to take some people quite a while to admit this (they’re now saying “wait til 2015!”, and I doubt they’ll even give up then when results come in at the highest possible LHC energies).

  15. CLAUDIE says:

    Dear Peter Woit,

    Thank you kindly for your concise & considerate answer. I guess I have a vested interest in SUSY because it embodies some of what is predicted or at least theorised about by verdict philosophy – which is my filed of expertise. Also being a teacher and humanitarian – I had hoped for a theory of wholeness & unity — to open up a new discourse, whereby children can be taught to nurture the earth and nature in general, as a component of their own essence. I probably sound like a tree hugger (and undeniably I am :-)

    Kind Regards,

    Claudie