Over the past few days the results of the 2011 LHC run have been revealed at the EPS-HEP 2011 conference in Grenoble, where a press conference today marked the beginning of the next part of the conference, featuring summary talks. For some discussion of these results see for example here, here, here, here and here. The bottom line is much stronger results ruling out supersymmetry, extra dimensions, black holes and other exotica, restriction of the possible mass range of the Higgs to about 114-150 GeV, and a tantalizingly small and not yet statistically significant excess of possible Higgs events in the mass range 120-145 GeV.
The big surprise here is that the experiments have done a fantastic job of getting these analyses of the data done at record speed. Before the LHC turn-on, estimates based on experience at the Tevatron tended to be that it would be 2012 before we saw completed analyses of a significant amount of the 2011 data. A lot of people have been working long hours and going without a summer vacation… The bottom line though is not a surprise, but rather pretty much what many people (including myself) expected. The unconvincing popular theoretical models of the last few decades have finally been confronted with experiment, which is falsifying them, to the extent that they can be falsified. It’s an inspiring example of the scientific method working as it should. The remaining mass range for the Higgs is the expected one, and, as expected, this is the hardest place to separate the Higgs from the background. If it’s really there, the data collected during the rest of this year should be enough to give a statistically significant signal. So, within a few months we should finally have an answer to the question that has been plaguing the subject for decades: “Higgs or something else?”. This is very exciting.
For more than a quarter-century, supersymmetry has been advertised as the most significant prediction of string theory. Back in 1996 Gross and Witten responded to John Horgan’s skeptical take on string theory in The End of Science with an article in the Wall Street Journal where they claimed:
There is a high probability that supersymmetry, if it plays the role physicists suspect, will be confirmed in the next decade. The existing accelerators that have a chance of doing so are the proton collider at the Department of Energy’s Fermi Lab in Batavia, Ill., and the electron collider at the European Center for Nuclear Research (CERN) in Geneva. Last year’s final run at Fermi Lab, during which the top quark was discovered, gave tantalizing hints of supersymmetry. The situation should be clarified when this machine is upgraded in 1999. (A further upgrade, which would cost the Department of Energy about $300 million, should be seriously considered.) As for the CERN electron collider, its energy is being increased by 35% in the next few months. The results could be dramatic, since electron colliders, though their energy is generally much lower than that of proton colliders, are rather thorough and swift in exploring certain phenomena.
If supersymmetry is out of reach of these existing colliders then it is very likely to be discovered at the Large Hadron Collider, which will begin operation at CERN in about a decade…
Wherever it occurs, the confirmation of supersymmetry would open up one of the golden ages of experimental physics. It could provide us with essential insights about the unification of the four major forces; that is, a theory that would describe gravity, the strong nuclear force, the weak atomic force and the electromagnetic force as varying expressions of a single phenomenon. And it would give a big boost to the development of a remarkably rich new theoretical framework known as string theory. For supersymmetry is one of the basic predications of string theory.
The next year Physics Today published Gordon Kane’s String Theory is Testable, Even Supertestable, which included a plot showing gluinos and squarks as having expected masses in the range of 200-300 GeV (the latest results rule them out in typical SUSY models up to about 1000 GeV).
Today, the most prominent active string theory bloggers have blog entries reacting to the weekend’s news. Clifford Johnson has Living in Interesting Times, where he writes:
One of those hoped for stories is called Supersymmetry, which would imply the existence of several more particles besides just the Higgs. Now, the cool thing is that the simplest models of supersymmetry could be in danger as well if we do not see something in the coming several months. Wouldn’t it be interesting if both the Standard Model Higgs and the simplest models of Supersymmetry were ruled out? (I’m not saying that they are – it’s all to soon to tell – but it is a possible outcome.)
When the LHC turned on, Lubos Motl was blogging about Why supersymmetry should be seen at the Large Hadron Collider, giving the probability of the LHC seeing SUSY as “90% or higher”. After the results of the last few days, he’s done a 180 degree turn, with a new blog entry attacking phenomenologists and arguing that the LHC results just show that HEP theorists should be doing string theory, not phenomenology:
No hep-th theorist has ever claimed or boasted that the bulk of his work had too much in common with the data produced by the next-generation collider so of course, the hep-th work isn’t really affected by the “null” results from the LHC. Many theorists and many string theorists – but not all – would feel more excited if the LHC were generating totally new phenomena and their phenomenological friends would be really thrilled. However, it’s still true that the theorists don’t care as much as the phenomenologists do.
What I really want to say is that most of the phenomenological work has been a waste of human resources and time. Instead of producing 1,000 models that could be relevant for the sub-TeV observations, those people could have just waited for a few years and let Nature speak. And it seems that Nature has spoken – and it may still speak in an ever clearer language – and so far, the answer is that the right model of these phenomena is called the Standard Model…
So I hope that instead of shifting the energy scales from 200 GeV to 1,400 GeV and continuing in random guessing, many phenomenologists will buy some string theory textbooks and begin to think about the Universe at a slightly deeper and less sensationalist level.
Update: Lubos clarifies here: he’s only throwing some SUSY models under the bus, not all of them. It’s no longer above 90%, but he still thinks there’s a 50% chance that the LHC will see supersymmetry. And all the bogus claims for “tests of string theory” are my fault, since I created a hostile environment for string theorists where they felt they had to do this kind of thing.
Update: The MasterCode Project has moved up to higher masses its “best-fit” points for SUSY now that 2011 LHC results have ruled out previous “best-fit” points, see here. Now the “best-fit” for SUSY is not even a very good fit… Tommaso Dorigo explains and comments here.
Update: In his talk concluding the conference, David Gross throws just the CMSSM under the bus, saying it is now “on life support”. He argues though that this is just one possible SUSY model, and one can’t conclude much from the death of the CMSSM. Much of his talk was an advertisement for N=4 SSYM and AdS/CFT. He’s sticking to his prediction of last year that SUSY particles will appear within 10 years, no word on when he’ll give up if the LHC continues to see nothing. Near the bottom of his list of predictions was “string theory will start to be a THEORY, with predictions”, which drew laughter from the crowd. He acknowledged that it was next to last on a list ordered by plausibility, but insisted “Some day…”
Update: Pauline Gagnon reports on what theorists are up to in response to all this:
This summer, I had the opportunity to spend a week at a theory workshop. Being the only experimentalist there, I spent plenty of time discussing what was going on in their camp. Clearly, they are not sitting idle while we are frantically searching our recently collected data for signs of new physics or the Higgs boson. On the contrary, many of them were already hard at work trying to find excuses for supersymmetry and reasons why it has not shown up yet as anticipated.
At Cosmic Variance John Conway summarizes the situation, and draws flak from Matt Strassler, who explains more here, and has a new paper out about how to evade the LHC results:
This is a key job of particle theorists; make sure all the ground gets covered by the experimentalists before they give up and move on!
Given the huge number of possibilities and parameters for how to implement SUSY, insisting that all of it gets tested by experiment will ensure that SUSY phenomenology will be with us for a very long time. Ideas like SUSY can never be completely ruled out, they can just be made so unlikely that they’re not worth people’s time anymore, and the argument over how much more unlikely the LHC results make SUSY will continue…
“I suspect this year’s data may be enough to see the Higgs if it is there. ”
About when should the results for the end of 2011 Higgs/SUSY LHC results be available? Dec 2011 or Jan 2012?
I’ll mark it in my calendar.
The normal schedule would be to release results at one of the so-called “Winter Conferences” in early 2012. In this case though, if one or both of the experiments has a statistically significant Higgs signal, I find it hard to believe they’ll keep it under wraps waiting for an appropriate conference date. Maybe someone will post the news to a blog…
Hello Peter. Just to make sure I understand: if “somehow” we can prove that super-symmetry is false, does this rule out String Theory?
You can’t prove that SUSY is false, all you can do is show that certain versions of it that have been advertised as solving certain problems disagree with experiment. Similarly, you can’t rule out string theory, it’s compatible with just about anything. You can however, like SUSY, show that certain heavily advertised versions of string theory are falsified by experiment, and that’s what’s happening. Versions of string theory which have SUSY discoverable at LHC energies have always been the been string theorist’s best bet for a version that has some observable consequences.
It isn’t really appropriate to suggest that collider theorists are now backpedaling, and looking around for supersymmetric models that would evade current searches at the LHC, in some desperate effort to save supersymmetry. Many of us have been saying for a long time that the LHC experiments’ reliance on a missing-energy-based search strategy for supersymmetry (and supersymmetry-like models) would lead to large classes of very reasonable models being initially undetectable. (There are papers going back at least to the 1990s and probably earlier.) Many of us are also on record since the 1990s saying that it would take many years for LHC data to convincingly disfavor supersymmetry.
The current news from the LHC is neither a disaster for supersymmetry, nor a shock to those who like supersymmetry, nor forcing theorists to backpedal. In fact, rather than backpedaling, what many theorists are actually doing is pulling out old work they did years ago, knowing this situation was likely, and making their old results more precise.
Just as an example, look at a 1999 paper by collider theorists Konstantin Matchev and Scott Thomas, on gauge mediated supersymmetry with decays of the next-to-lightest superpartner to a Z or a Higgs plus a gravitino. This OLD class of models, which has a large parameter space, would not easily be observed with the strategies shown at the EPS conference last week — the missing energy signature is simply too small. And this is just one of many examples.
Thanks for the comments and interesting added details about this. To an outsider, it certainly seemed that pre-LHC, the kind of signals that are now ruled out were ones that were being promoted as typical expectations. The idea that it was going to take many years to see if SUSY was there as expected wasn’t something I remember seeing advertised. Instead, the typical kind of thing I remember hearing was Arkani-Hamed’s remarks at Strings 2005:
“If weak scale supersymmetry is right, then we will know by spring 2008 [e.g. a few months after expected start of LHC physics]. I think this is really a very sharp statement.”
From context, he was talking about roughly the sort of integrated luminosity now available. It’s true he was expecting an extra factor of two in energy, and 1-2 TeV mass particles, so he could be right if the gluino is 1.5 TeV. He definitely wasn’t saying anything though about it taking years to find a hard-to-see signal.
“The normal schedule would be to release results at one of the so-called “Winter Conferences” in early 2012. In this case though, if one or both of the experiments has a statistically significant Higgs signal, I find it hard to believe they’ll keep it under wraps waiting for an appropriate conference date. Maybe someone will post the news to a blog…”
Based on LHC current luminosity, would the data in the Jan 2012 LHC would be able to either provide evidence (i.e 2-sigma) or rule out in the more interesting “Higgs events in the mass range 120-145 GeV”?
One more comment: pre-LHC I remember often hearing that the Higgs would be hard, SUSY much easier. Now we’re almost at the point of seeing a Higgs or ruling it out…
No one is sure how well the LHC will do the rest of the year, but typical estimates are that it will produce a factor of 3-5 more data than analyzed at this latest conference (and there should be further refinements and improvements in the analysis). By some measures, the hint of a Higgs signal already seen is already getting to be around 2 sigma, it doesn’t seem unreasonable to expect a 3 sigma or more signal (depends on mass, much harder at lower masses).
The Higgs Hunting 2011 conference is now going on, see this presentation
for details on prospects for the near future and 2012. By my reading, if there’s a 140 GeV Higgs, both CMS and ATLAS expect to see a 5 sigma signal (if they get 5 inverse femtobarns) by the end of the year.
It might be sobering to recall some statements from Frank Wilczek’s Future Summary from a decade ago, http://arxiv.org/abs/hep-ph/0101187 :
5.1. Small Effects Among Known Particles
[…] The modern experimental limits on deviations from the Standard Model in each of these processes puts very significant pressure on the supersymmetric parameter space already [i.e. 10 years ago].
5.2. Proton Decay
[…] but it will not be easy to reconcile limits tau_proton > 10^34 years with straightforward models.
5.5. Produce the New Particles!
Of course, the ultimate test for low-energy supersymmetry will be to produce some of the predicted new R-odd particles. Even in the focus point scenario, there must be several accessible to the LHC.
I see that if the Higgs does NOT exist, then the 5 inverse femtobarns is enough data to exclude it even in the low-mass range. So apparently by Q1 2012, if the LHC collects 5 inverse femtobarns as expected, if the Higgs does NOT exist, the LHC 5 inverse femtobarns would be enough to exclude it over the 115-150 Gev range.
Is that a correct reading of page 9?
Do you think that the LHC with 5+ inverse femtobarns excluding the Higgs, (if it doesn’t exist) even in the low mass range, would be like the Michelson–Morley experiment null result on the luminous aether?
That’s also my reading of that document. If there is a 5 sigma exclusion of the SM Higgs late this year, that will have a huge impact on the field, maybe the Michelson-Morley null result would be a good analogy. I don’t know if this was the case with Michelson-Morley, but here I think most theorists are rooting for the null result, which would be a lot more interesting than an observation of a Higgs with SM behavior.
There’s no mention of technicolor in those slides. How does say 5 or 10 inverse femtobarns (at the end of 2012) data collection have to say about technicolor models?
I haven’t seen any results from the LHC yet specifically about technicolor.
“Given the huge number of possibilities and parameters for how to implement SUSY, insisting that all of it gets tested by experiment will ensure that SUSY phenomenology will be with us for a very long time. Ideas like SUSY can never be completely ruled out, they can just be made so unlikely that they’re not worth people’s time anymore, and the argument over how much more unlikely the LHC results make SUSY will continue…”
If LHC does NOT find evidence of higgs or technicolor, and no evidence of SUSY, would SUSY still be viable? If the LHC rules out the Higgs, would that also rule out SUSY?
The question of SUSY and the question of electroweak symmetry breaking (Higg or no Higgs? Technicolor?) are pretty much independent questions.
As far as electroweak symmetry breaking goes, SUSY and the SM aren’t very different. They both have electroweak breaking via a Higgs, only difference is that in the SUSY case you need at least two Higgs fields, unlike the SM, where you can get away with just one. If a Higgs is found, one big question will be whether it behaves like the SM Higgs, or like one of the two minimal supersymmetry Higgses.
Thank you, Peter, for taking the time to respond to my comment.
“As far as electroweak symmetry breaking goes, SUSY and the SM aren’t very different. They both have electroweak breaking via a Higgs, only difference is that in the SUSY case you need at least two Higgs fields, unlike the SM, where you can get away with just one. If a Higgs is found, one big question will be whether it behaves like the SM Higgs, or like one of the two minimal supersymmetry Higgses.”
But if come Jan 2012, the LHC rules out the higgs, it would also rule out two minimal supersymmetry Higgses. If it doesn’t even find a single Higgs, much less 2. Or to put it another way, if the LHC rules out electroweak breaking via a Higgs, wouldn’t it also falsify both the SM single higgs field, and the SUSY 2 Higgs fields at the same time?
Thanks DB, its good to see someone agrees with me. I absolutely discount these claims of a 3sig signal already been observed. This is very misleading because it suggest theres a 95% chance we have seen the Higgs. In reality there data more conclusively excludes the Higgs than vice versa. Why? Because these combined channle statistics make no sense in light of what the LHC has been seeing.
Take a look at the way the WW background behaves over the energy spectrum. Our SM expectation (this is loose becuase a whole pile of assumtpions are needed to related the SM predictions to what ATLAS actually SHOULD see) has about a 2sig deviation from what ATLAS actually is seeing over the entire 100-170GeV energy range. Thats not suprising, because of the W mass the branching ratio is very low there. Yet almost the entire of the present 3sig from the Higgs comes from these handful of events.
In this light we really have to look at the three main channels separately. Lets list the events as: expected background, expected background + SM Higgs, observed. I will sum events over the expected Higgs width in each channel (you can see the width in the ATLAS histograms). We have
In the WW channel a strong signal for a 150GeV Higgs: 43back/64 back+H/64 observed. So the data is spot on with a SM Higgs, at a 3sig CL. BUT caution, over the rest of the WW spectrum 20 events are expected versus 29 observed, a 2sig devitation, and also positive.
In the gamma_gamma channel for a 150GeV Higgs: 31oback/320back+H/380 expected. So we would have expected almost a 4sig Higgs signal, but got almost nothing. The Higgs is fairly narrow in this channel (only about 5 GeV, see ATLAS plot). So it makes sense to separate the spectrum. If we repeat the analysis at 130GeV, we have 310/340/370, that is 2sig expected vs 1 sig observed.
In the H-bb channel at 130 GeV: 700/700/820. So a 0sig deviation vs 4sig expected. (I neglect the bb via ZH since its muich smaller than via WH)
I think the data speak more clearly for no Higgs than anything else, I would be glad if someone can show where the above is wrong.
From the ATLAS presentation, almost all luminosity came in the last month. So in 3 months we will have 4 times the integrated luminosity, and so all the sigmas above will be multiplied by 2, and if the above trnd continues, this will exclude the Higgs at something like 5sig.
Im sorry for the spelling mistakes above. I should have checked it, I had too little time.
When you refer to “black holes” as “exotica”, are you suggesting that black holes in general don’t exist? I had thought black holes’ existence was accepted as deriving from general relativity. Is this incorrect?
He’s referring to hypothetical micro black holes that are supposedly produced in high energy particle collisions, not to the astrophysical black holes of general relativity.
So now it is official. Peter, you’re to blame for all these “string theory is testable” claims. You’ve been really mean to these guys! I hope your feeling sorry for poor Lubos and his pals.
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One of the arguments string theorists have made over the decades is that string theory is the “only game in town” when it comes to unification. Certainly it would seem to have been the most *interesting* game, given that other approaches have less mathematical interest and have also not had support from experimental data. Do you think these new findings (or lack thereof) change this situation at all? In other words, do the new findings provide a compelling reason for a would-be high energy theorist to work on anything *besides* string theory?