Ok, today I’ve posted the article you’ve been waiting for: What does the LHC have to say, so far, about supersymmetry? [Here’s an article about supersymmetry and what it predicts, and another about standard ways to look for signs of it at the Large Hadron Collider, under certain assumptions; if you want to review the known particles and forces first, you can read about them here.]
If you’ve been reading the press, you may have seen statements such as “the air is getting thin for supersymmetry” or “we’re painting supersymmetry into a corner”. And recently on Cosmic Variance some broad statements about supersymmetry being in serious trouble were made by experimentalist John Conway (before being somewhat watered down after I raised an objection.)
[Update: in response to a comment posted below, I reread this post, and I see that indeed I did not, in the original version, give enough words attesting to the tremendous progress that has been made. So let me state clearly: what the LHC and the ATLAS and CMS experiments have achieved, in the supersymmetry-aimed searches made public so far, is fantastic. They have wiped not only many variants of supersymmetry, but also variants of many other speculative ideas, off the map. And I don’t mean in any way to downplay this… just to try to put it in proper perspective. To do so better, I also added a sentence after the list of assumptions below.]
A number of experimentalists seem to have (and, in their public statements, give) the impression that most supersymmetry theorists, faced with an apparent disaster, are rushing around in circles desperately trying to think of new ways that supersymmetry might have escaped notice. This view also seems to be showing up on various blogs. This is a profound misconception, one that history clearly contradicts. Rather than a picture of universally panicked theorists trying to figure out how to save their favorite theory, the image should be of a number of theorists sitting calmly in their chairs, saying, “we knew this situation was a reasonable possibility — and we’ve been trying to alert you to it for a very long time.” The statements have been in the scientific literature for years.
Many theorists, including myself [and let me emphasize that I am not a huge proponent of supersymmetry, though I have worked on it and do view it as a serious contender as a theory of nature] have been saying for years that the main search strategies planned at the LHC would be insufficient to cover large regions of the supersymmetric continent. Important papers on this subject go back into the 1990s and a good chunk of the last decade. Here is the point. The bulk of the search strategy used at the LHC to find supersymmetry rests upon three assumptions.
- in any process, the number of superpartners can only change by an even number;
- the lightest superpartner [which is stable, by assumption 1] is a superpartner of a particle we know (and therefore, to avoid conflict with other data, an undetectable neutralino or sneutrino);
- the superpartners that are affected by the strong nuclear force are significantly heavier than the other superpartners of known particles.
These assumptions are not unreasonable, and there are strong arguments in their favor, especially for assumption 1 and to a degree assumption 3 (described here). But we must keep in mind that if we relax any one of those assumptions, the limits on supersymmetry from current LHC data become much weaker. If you want to read more details about how this works, you can click here, though you will find it useful to read today’s article first unless you already know a lot about the subject. Or click on the figure below (taken from this post, which also explains why these assumptions are often made) to understand how these assumptions lead to the prediction of “jets and missing energy” on which the most powerful search strategies — including the ones we’ll hear about at this week’s conference in Mumbai — are all based.
So until you hear a consensus building, you should be cautious in making too much of statements that supersymmetry, as a part of nature and a solution to the hierarchy problem, is in serious trouble. The truth is that certain variants of supersymmetry — ones based on certain assumptions that might be wrong — are indeed strongly constrained by current data. Others, simply put, are not. In short, we have a long way to go.
8/19/11
9 Responses
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Dear Professor Strassler,
To be fair to Gordy Kane, Kane and his collaborators have been promoting the model based on the fluxless G2 compactifications of M-theory for the past 4 years, at least. Look at the title of Gordy’s 2008 talk here, for example: http://theory.caltech.edu/~seminar/old/2008-spring.html
I first heard Acharya’s talk on this topic way back in 2006 at KITP and I thought this stuff was a bit crazy and a bunch of crap, to be honest. However, after reading some of their later papers I now think that their model is actually quite elegant since it addresses multiple problems simultaneously and I’m sure that Gordy is quite delighted with the fact that the LHC has not found any light squarks 🙂 .
Naively, these vacua lead to a severe “little hierarchy problem” because of the very heavy scalars so this model has been rather unattractive to the bottom up phenomenologists but despite skepticism, Gordy has been sticking to his guns during the past 4 years and in fact, this recent paper http://arxiv.org/abs/1105.3765 demonstrated that there is a natural and large region in the parameters space of such models where the Higgs vev can be naturally below 1 TeV.
Dear Matt,
Nice post. I have a question. About 13 years ago, when Super-K announced evidence for non-0 neutrino mass, P . Ramond indicated that
the results provide evidence for low energy super-symmetry. See
http://focus.aps.org/stories/v2/st10/neutrinoquotes.html
Do you agree with this? and if yes, then based on nu-results can we predict more precisely when and in what channels low-energy SUSY will be seen?
Thanks
Shantanu
Circumstantial evidence, of course, and while I agree it adds a little to the body of evidence, it is not a particularly strong piece. It might be viewed as sronger evidence, perhaps, for physics near the Planck (gravitational) energy scale than for supersymmetry. Ramond is arguing for physics near the apparently slightly lower Grand Unification energy scale, which is traditionally thought to be different from the Planck scale, and which would fit with the popular picture of supersymmetry and grand unification that has many adherents. But consistency with a picture is not the same as direct evidence.
It is impossible to link any details of observable effects of supersymmetry with this indirect evidence. For instance, the three assumptions that I have listed are completely independent of any effects leading to neutrinos masses. So you can relax any one or more of those assumptions and keep neutrino mass physics unchanged.
Thanks for the response
Low energy SUSY is now less likely, and the most favorable models are ruled out. One has to turn to R-parity violating models etc to save it. That’s a profound experimental discovery. Its quite unfortunate to try to downplay these experimental results.
The general way in which the LHC experimentalists have reported their results has been transparent and fair, and consistent with the way all null experimental results have been reported in the past. When ADMX reported that they had ruled out a range for the axion mass, did axion theorists jump up and down and say “no, no, don’t say that publicly! We can invent more contrived axion models to evade the bounds!” No, theorists studied the data, accepted that it ruled out the most reasonable axion models in that mass range, and learnt from it. I think the same attitude should apply to SUSY.
I agree that the current results are fantastic, and that the results have been transparent. I don’t think I downplayed them either — as I said, those popular supersymmetric models are in serious trouble. [Actually, rereading it again, I’ve concluded your criticism that I didn’t make enough positive statements is warranted, and I’ve added some text in italics above.]
However, your physics statement isn’t correct. R-parity violation is definitely not needed to evade the current search techniques — I have been saying that for years, and so have others. [Look at my most recent paper on the subject for plots that demonstrate this explicitly.] These include models with a light top squark, or with an extra singlet or two in the Higgs sector, or with gauge mediation and decays of the next-to-lightest-superpartner mainly to Z and Higgs, and I don’t agree they are contrived. And I remind you these models were not invented just now, but in some cases a very long time ago.
As for the axion example — the way you phrase it is completely unfair. “No, no, don’t say that publicly! we can invent more contrived axion models to evade the bounds.” C’mon. If you want to have a serious discussion, I’m happy to have it. But if you set up a fake example and put words in my mouth that I don’t say, I don’t see how we can have that discussion.
And furthermore, the analogy is off-base. The public was not paying such close attention as they are here, and a 9 billion dollar experimental facility’s long-term future was not on the line. There are consequences to making incorrect or overoptimistic statements to the press. In my view, we need to be quite precise about what we say; the public, and the politicians that provide the money, want to know what is going on, and we will pay a price if we say one thing and then have to backtrack. And if we disagree with each other, that’s fine; it is best that the public and politicians know that there is no scientific consensus yet.
Dear Prof. Strassler,
I like Your blog very much; I will read and learn more from it whenever I find time.
In particular, it is pleasant to see emerging some more experts rejecting the forceful claims that SUSY is “stonedead” etc of a large number of other bloggers and the corresponding readers…
Keep up Your good work 🙂
Cheers