There’s a new paper out this evening from a large collaboration entitled Implications of Initial LHC Searches for Supersymmetry. Instead of just adding it to the bottom of my recent posting, I thought it would be a good idea to start a new one, and add a bit more explanation of what is going on.
For a good news story from today by Kate McAlpine, see this at Physics World. For excellent more technical explanations, see the latest blog postings at Tommaso Dorigo’s blog (today) and at Resonaances (yesterday). Physics World, Tommaso and the new arXiv preprint discuss published results from CMS and ATLAS, while Resonaances discusses even more stringent preliminary limits on SUSY from ATLAS made public last week at Aspen.
Tommaso also refers to a 2008 guest blog posting by Ben Allanach explaining how statistical predictions for SUSY masses were being made, adopting various simplifying assumptions (CMSSM) and assuming supersymmetry solves the problems it is advertised as solving (muon g-2 anomaly, dark matter, etc.). Allanach discusses the 2008 version of this kind of calculation by the same group that has just put out a new, 2/22/2011 version this evening.
The usual model for how science is done is that theorists make predictions before an experiment is done, then when the experimental results come in, they get compared to the predictions. That’s not quite what is going on here, where as far as I can tell, the new paper doesn’t directly compare the 2008 predictions to the new experimental results. Instead, the new experimental results are used to make new predictions. Since a large part of the parameter space favored in the 2008 predictions has now been ruled out, the new ones move the favored part of parameter space up to higher particle masses. The authors do make clear what is going on, showing on their plots a “snowflake” where the 2008 best-fit value was, and “stars” for where the new best-fit values are based on data from the two experiments. Note that the paper does not include the latest, stronger results from ATLAS announced last week, which presumably would move the “stars” up to even higher mass.
While the question this paper addresses about where supersymmetry might be given that it hasn’t been seen yet is interesting, it leaves unaddressed the more conventional question: do the LHC experimental results show that the theoretical predictions about supersymmetry made in 2008 before the machine was turned on were wrong? This is a statistical question, so should have a statistical answer. Assuming that the LHC continues to not see supersymmetry as it collects more data, I’m interested in the question of how the experimental data will falsify the theory. Will its proponents just keep calculating statistical predictions of higher and higher masses as lower ones get ruled out? Most will undoubtedly at some point throw in the towel, although there will be some who will never, never, never, never give up (see here):
SUSY may still be there even if it remains invisible to the LHC, indeed. And yes, I don’t hide that I will be convinced that SUSY is there even if the LHC doesn’t find it. The LHC will only confirm or exclude effects at particular regimes – usually low energy but it’s not quite accurate a description of the regime that may be excluded.
What I have been scared for several years is the pseudoscientific propaganda of your kind trying to claim – without any justification – that not seeing SUSY at the LHC should imply that physicists shouldn’t be allowed to work on SUSY or believe that it is a key feature of our Universe. There are many reasons to think it’s the case and theorists whom I consider any good will continue to treat SUSY as an essential feature whether or not it shows up at the LHC.
Update: See figure 1 of this evening’s What if the LHC does not find supersymmetry in the sqrt(s)=7 TeV run? to see how how much of the predicted region of superpartner masses was ruled out by initial LHC results, and how much of the rest is likely to be ruled out during by the 2011-2 7 TeV run.
Update: There’s a very new up-to-the-minute survey of LHC results concentrating on supersymmetry by John Ellis here. Unfortunately no figures that superimpose CMS/ATLAS exclusion regions on the statistically favored regions for supersymmetry that are discussed (based on assuming supersymmetry explains dark matter and the muon g-2 anomaly). It does look like this year’s data should be able to convincingly rule out the idea that supersymmetry explains both of these phenomena.
Update: The ATLAS results providing the strongest limits so far on SUSY are now out, see the paper here.
At this rate Tommaso will soon be able to lodge his bet as triple A rated security. It certainly looks a lot more solid than what most banks have in their vaults.
“The usual model for how science is done is that theorists make predictions before an experiment is done, then when the experimental results come in, they get compared to the predictions. ”
Although I agree things are getting ugly for SUSY you have to admit that the SM was completely tuned to data for each we have absolutely no idea from where parameters are coming from. So, whatever the beyond standard model will be, SUSY or not, it appears the same procedure will happen, so people tunning SUSY models to data are doing nothing new and if it was good for the SM I don’t see why this should be a problem if it’s BSM. I am not advocating anything for SUSY but your arguments this time sound less convincing.
My point: was the SM proven right or wrong? You can make a case for predicting W’s and Z (so in a sense it was proven right) but in the end it uses a “minimal particle content”. If any SUSY model will someday fit data better than the SM that would be an amazing achievement no matter how high the masses turn out to be, if that’s what Nature wants. If it indeed what it wants it’s THE question and I agree we should abandon SUSY (or regard it as unfavoured candidate as opposed to mainstream subject) if it does not show at the LHC at all.
Bernhard,
The SM case was quite different, with no free parameters available to significantly change the W and Z mass predictions made before the SPS started looking for them. If the W and Z hadn’t been found, you wouldn’t have had a group publishing a paper about how this changed the probability distribution for where the W and Z were. Instead, everyone would have just acknowledged that the SM was wrong and moved on to figuring out a better theory.
In this case, all we have are statistical predictions of superpartner masses. I’m curious to see some measure of how the new experimental results compare to those predictions (e.g., what fraction of the original probability distribution is now ruled out). There’s no way you can completely rule out supersymmetry the way the SM could have been ruled out, but you can quantify what new results say about predictions made before the machine was turned on.
Peter,
This isn’t a completely accurate historical account of the development of the SM. For example there was a developed history of topless models to account for the repeated non-observation of the top quark. After all, the top quark was a basic prediction of the Standard Model which wasn’t observed – and wasn’t observed again – and wasn’t observed again….
Sometimes the right response to an expected discovery that doesn’t show up is to go to higher energies, where it will. We don’t know whether the LHC will discover SUSY or not, but what is surely clear is that finally after many many years there is a machine capable of probing the TeV scale fully.
piscator,
The top (like the Higgs) is an example of something not tightly constrained by the model before the experiments that went looking for it. My point was just that the SM did make dramatic, tightly constrained predictions that could not be evaded (the W and Z masses). This is very different than supersymmetry.
I liked the first sentence in the collaboration paper.
“The results of experiments at the LHC will be make-or-break for supersymmetry.”
Something to quote in 2015.
Peter,
I agree the SM was way more robust than SUSY in providing evidence that could not be evaded. My point was that both models however share the fact that some aspects can adapted. The SM has a minimal particle content solution, which naturally makes the necessity of things not constrained by the model much lower. The price is to leave lots of things unexplained.
The prediction SUSY makes that should not be evaded, even if shared by other BSM scenarios are particles TeV scale accessible, reason I said one should abandon it the same way they would have done it if the W and the Z were not there, as you said. The question of model determination will be very hard but in the end this is the price for a hadron collider as opposed to a linear collider.
I would also like to see a probability distribution for spartner masses, guess the best way to get this is to bet with SUSY theorists they are not capable of doing that (joke).
It’s an interesting question – should we ditch SUSY if no hints of it are seen at the LHC? The problem is that supersymmetry appears to be a very important concept in quantum field theory – and the Standard Model is a QFT. To my mind, it would be perverse of nature not to be supersymmetric in some sense.
On the other hand, it is, to say the least, unconvincing to just dismiss lack of evidence for superpartners by saying “they must be at higher energy”. It’s a perfectly true statement, but where does this leave us?
I don’t think the null results so far are too much of a problem for susy though. But if nothing is found in 2-3 years, it will be very interesting indeed to see how the people working in the field react. It’s times like this I’m glad I work in QCD – because we know for sure that exists!
This is like saying that Newtonian mechanics is more robust than astrology. SM has always had huge amounts of quantitative agreement with experiment. SUSY makes no quantitative predictions, and no aspect of it has ever had any confirmation.
On-shell N=4 SYM: recursively solved to all orders:
http://motls.blogspot.com/2010/08/on-shell-n4-sym-recursively-solved-to.html
There’s no other 4d QFT that is understood so well analytically.
This alone will guarantee that SUSY remains one of the most important ideas in theoretical physics even if a Planck-energy collider fails to find it.
Roger: “SUSY makes no quantitative predictions”
What do you think the LHC results are being compared to? Maybe you are thinking of string theory.
Eh, Lubos is right on this one.
The hypothesis the LHC might disprove is that particle physics at the TeV scale is well-described by a supersymmetric theory. If LHC doesn’t find evidence for a supersymmetric theory, people will continue to wonder if physics at higher scales is supersymmetric. That’s their right, until someone actually does an experiment. Other people will no doubt speculate along other lines.
I doubt the idea of susy will ever go away entirely. If nothing else, it’s easy to imagine a QFT class 50 years from now starting with the professor saying, ‘Well, these theories aren’t realistic, but it’s useful to study them first, because they behave like the SubStandard Model in some respects but are easier to understand.’
They are being compared to the SM. And successfully too, as far as we know. SUSY does predict new particles, without saying much about their properties. That is a qualitative prediction, not a quantitative prediction.
Roger,
The only detailed property of the superpartners that SUSY-theories do not predict is the exact mass spectrum. This is because the mechanism of SUSY-breaking is not understood in detail. However, the masses are expected to be TeV-scale if supersymmetry is responsible for stabilizing the electroweak scale. If the superpartners are found, it will be considered a tour de force of theoretical reasoning. If not, then there is likely some other other new physics such as technicolor which will be found. The thing that must be kept in mind is that nothing will show up until the energy is raised high enough and/or enough data is collected. It is extremely foolish to draw any conclusions until these criteria have been met.
“If not, then there is likely some other other new physics”
the key word here is likely. likely as judged by arguments such as naturalness.
in the case of the W and Z that were mentioned above there was no “likely”. something needed to be there to make the S-matrix unitary. The SM however can provide for unitarity far beyond the Planck scale provided the Higgs mass is in the correct region.
i’d say it’s a really tough call. colliders of any currently imaginable size might just not give us any new particles. we’ll see.
Hi there,
two things:
1) The SM is already experimentally ruled out by the observation
(by several independent experiments) of Dark Matter – which on the other hand is automatically explained by SUSY (if R-parity is conserved).
2) SUSY, well, let’s better talk about the MSSM to be specific, does indeed not make any prediction for the SUSY mass scales. However,
2a) there are firm predictions for the mass of the lightest Higgs boson. If there is no Higgs below 135 GeV the MSSM can be considered as ruled out.
2b) there are predictions using the existing experimental data. Exactly these kind of predictions, as made in the paper that Peter discussed in the beginning of this blog (and of which I am co-author), tell us that we should expect relatively low SUSY mass scales. Consequently, the new bounds by CMS and ATLAS not only move up the best-fit points, but also in the case of ATLAS worsen the fit probabilities slightly. The chi^2/d.o.f. will be an interesting measure in the future for these theories.
However, one should also keep in mind that the GUT-based versions of the MSSM under investigation right now (CMSSM, NUHM1, VCMSSM and mSUGRA) are only one special subclass of the MSSM. Other realizations might look completely different. This has hardly been analyzed so far.
“Dark Matter – which on the other hand is automatically explained by SUSY (if R-parity is conserved).”
Strictly speaking even if R-parity conserving SUSY is discovered that would be strong circumstantial evidence of (one of the, maybe) the origin of dark matter, but not a proof.
Sven,
Thanks for the interesting comment. Given though that there’s no evidence that dark matter is actually a WIMP, that’s a pretty weak argument for adding a huge number of new particles and 120 or more new parameters to the SM.
Any evidence of a Higgs sector will definitely be the big news coming out of the LHC. If it contains no evidence for supersymmetry and the limits on strongly interacting superpartners continue to move up, covering almost all of the pre-LHC predicted region, I find it hard to believe that many people will continue to find the MSSM or its extensions very interesting.
AJ,
I don’t think Lubos’s concern that theorists won’t be allowed to think about supersymmetry anymore will be justified. While there are a lot of interesting aspects of supersymmetry, and supersymmetric qfts worth thinking about, the MSSM and its extensions don’t seem to me to qualify. The world might be better off in a future where students aren’t encouraged to spend a lot of time learning how to do calculations in this particular framework. It’s quite a bit harder than in the SM, not easier.
Sven,
I agree that there is a huge consensus that your arguemnt 1 is true, but it is a bit formal. strictly speaking the SM was disproven by neutrino oscilations. now for DM there is e.g. the axion possibility that carries no further implications for higher energies. if you want to make a case for the SM to be really disproven in the sense that at higher energies some new physics will show up i think the strongest point is matter-antimatter asymmetry and the associated need for B-L violation.
Bad theories never die. They just fade away to higher energies.
Bad theories indeed never die. They get ignored because alternative theories come along which can explain the facts better (and make successful testable/falsifiable predictions). Then they “die”. That is what happened with phlogiston, for example. People simply lost interest.
SUSY won’t die, at least not easily. Here is a theory that doubled the number of possible particles, without one of them detected at pre-LHC energies. Only one thing can make physicists consider such a theory–an enormous aesthetic appeal. That appeal will keep it in the race a while yet.
Anonymous:
“On-shell N=4 SYM: recursively solved to all orders:” is nice.
The question is “what theory describes the universe”, not “what theory can be solved to all orders”.
KD
I am an expert of 2+1 gravity, which is an exactly solvable model. But unfortunately we live in 3+1 gravity… I think the same is for the idea of supersymmetry, which enhances the integrability of the model. But it is not warranted that Nature is an integrable model, and I suspect that it isn’t.
To the readers of NEW. This guy, Peter Woit is an enemy of science.
He’s just looking for glory or money.
If you love Physics please:
ATTACK THIS BLOG!
YES WE CAN!
STOP PETER DON’T DELETE THIS COMMEN!
Very interesting what’s going on with this experimental data. Puts a lot of current grad students into a bind when deciding which area of mathematical physics to pursue…
Geometrick,
If graduate students have been making their career choices based on assuming low-energy supersymmetry would be seen at the LHC, they’ve been making a mistake. I don’t think this issue puts them much in a bind, they can pretty safely assume that the LHC won’t be seeing superpartners. There never was a good reason to believe this, and this should just become more and more clear as results come in.
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“…they can pretty safely assume that the LHC won’t be seeing superpartners. There never was a good reason to believe this, and this should just become more and more clear as results come in.”
I look forward to seeing you eat your words in a couple of years.
“I look forward to seeing you eat your words in a couple of years.”
Peter has a good track record of admitting when he’s wrong, something SUSY proponents seem to lack…
Dear Sakura-chan,
If SUSY is not discovered after the LHC has run for 1 or 2 years at maximum energy, I think that most proponents would admit that it is not relevant to the TeV-scale. However, this is far from the case at the present. There is a lot of parameter space available for the LHC to explore, and only very foolish people are willing to jump to conclusions. It is not possible to rule out anything yet, including ideas which are alternatives to SUSY for which data has apparently not shown up either, such as technicolor etc… For that matter, the last piece of the Standard Model has not been found yet either.
“If graduate students have been making their career choices based on assuming low-energy supersymmetry would be seen at the LHC, they’ve been making a mistake.”
If based on the least optimistic assumption that LHC would find nothing significant beyond the SM, graduate students in particle physics should seriously consider alternative options, because particle physics would become a dead field, more dead than now. At least you should be mentally prepared to switch research fields in case funding for particle physics drains in future. However, this mainly applies to the phenomenology. People working on the mathematically interesting aspects of SUSY and strings would be less affected. But keep in mind that mathematical physics is always a niche in the physics department.
Dear Peter,
concerning your last update, within the context of the MSSM it is no problem to accommodate neutralino dark matter and the muon g-2, and at the same time have a very heavy spectrum with TeV scale squarks and gluinos. I don’t think TeV scale supersymmetry will be ruled out anytime soon. More predictive models like Msugra or the CMSSM are easier to constrain and rule out however.
In general, the results of the LEP experiments and of the Tevatron have made it clear that new physics is most likely at or close to the TeV scale, with only certain weakly interacting particles (e.g. neutralinos, sleptons) still allowed to have smaller masses. This not only applies to the MSSM but also to other models that attempt to solve the hierarchy problem and have a dark matter candidate, like composite and little Higgs models, walking technicolor, 5D gauge Higgs unification and so on. For most of these models it is somewhat challenging to find regions of parameter space that are easily accessible at the LHC and not yet ruled out either by precision constraints or Tevatron searches.
P.
Thanks! My comment about muon g-2 was based on Figure 5. of the new Ellis paper that I linked to, where he shows as a pink band the region favored by the g-2 measurement. I really wish Ellis had superimposed on that figure the latest CMS/ATLAS results, but my attempt to eyeball it (and guess that the difference in tan(beta) isn’t important) indicated that much of the pink region is now excluded. It’s quite possible I’ve got this wrong. I really would love to see the pre-LHC plots of regions favored by assumptions about supersymmetry superimposed on the data.
Sven,
the Dark matter -> supersymmetry or vice-versa argument has been oversold. (WIMP miracle etc)
some facts :
if you assume dark matter is a non-thermal relic you can essentialy get any mass or cross-sections in order to be a dark matter candidate (axions to wimpzillas)
Also DM could be primordial black holes which also has almost nothing to do with particle physics.
Although this is not a watertightargument I claim that simple vanilla WIMP dark matter is almost close to been ruled out, given that DD experiments are reaching limits close to 10^{-43} cm^2 (which is precisly the cross-section for a particle to be weakly interacting)
Shantanu,
your arguments could be right if one would invoke SUSY just to explain DM.
However, SUSY was ‘invented’ for particle physics for completely different reasons, and it has many virtues. The fact that DM is easily explained, is just a free bonus.
Concerning the DD limits, also this has been analyzed in the context of the fits and those GUT based models, see, for instance, fig. 20 in http://arxiv.org/abs/0907.5568 or figs. 6,7 in http://arxiv.org/abs/1102.4585.
One can see that DD limits will be challenging in the near future, but are not very restrictive nowadays.
Another advantage of the models: they make clear and falsifiable 🙂 predictions.
Sven,
SUSY was invented in the framework of a theory that so far has nothing to do with reality (strings). The other big advantage it could have, i.e. to solve the hierarchy problem was killed by LEP, so the best one can say today is that it ameliorates it. This is an embarrassment for the theory, not an advantage.
The bonus of DM is very weak, as Peter pointed out.
I´m not sure about other advantages. I admit its falsifiable only if one takes the position that one has to find TeV scale particles in order to stabilize the electroweak scale. But there is no hard requirement on that too, reason why many proponents are already saying that will not give it up even if the LHC finds no clue of SUSY.
Did I forget something?
“The other big advantage it could have, i.e. to solve the hierarchy problem was killed by LEP..”
This is completely false statement.
Bernhard, I think you may be confusing the gauge hierarchy problem and the so-called little hierarchy problem. The LEP results in no way whatsoever killed the possibility of supersymmetry being responsible for stabilizing the electroweak scale, nor due the present results from LHC. However, the range of allowed Higgs mass from LEP is such that it creates a small amount of fine-tuning in the SUSY mass spectra.
I´m not confusing anything. The fact that LEP killed the possibility that s-particles could have the same mas as its SM cousins requires that SUSY must be broken. That introduced the little hierarchy problem which is the same thing as to say that the hierarchy problem cannot be solved, i.e., that although the electroweak scale can be stabilized (you misinterpreted me here, as I never said it could not, quite the contrary) it must do it by a little bit of fine tuning. So, that´s why I repeat. SUSY does not solve the hierarchy problem, it ameliorates it. You can call this the micro hierarchy problem if you will, but still a problem. If the theory were so great it should solve the problem by zero fine tuning not by a small amount.
Bernhard,
The hierarchy problem as I understand is equivalent to the problem of stabilizing the electroweak scale, in other words, preventing the Higgs mass from receiving large quantum corrections which push it to the Planck scale. The little hierarchy problem is due to the fact that LEP has constrained the mass of the Higgs to be > 114 GeV, whereas it has a more natural value of around 90 GeV. This causes there to be a small amount of fine-tuning in the SUSY mass spectra, but this amount of fine-tuning is not really a problem. In fact, if there ends up being a fourth-generation of fermions, the Higgs mass should be heavier.
Yes, I have a problem with this consensus that it is allowed to say you solve a problem by introducing a little one. I´m aware of this “understanding” people have, which for me it´s just cheating. But fine, this is polarizing into semantics not physics, since I agree with what you said. But contrary from you I believe that introducing a little bit of fine tuning for answering a problem of large fine tuning is far from great. Let´s forget how one calls it. My point: can SUSY stabilize the electroweak scale by no fine tuning and no fine tuning at all? If the answer for this is “yes”, than I would retreat and admit I was wrong as this is not the information I have. If however the answer for this is “no” or “kinda”, than sorry, stop calling this the little hierarchy problem and just admit the theory failed in solving the dam thing.
Bernhard,
The hierarchy between the TeV-scale and the Planck scale is sixteen order of magnitude. A model with TeV-scale SUSY cancels corrections to the Higgs mass of this order. You are complaining if the Higgs mass is roughly 25 GeV larger than the mass of the Z-boson. This seems extremely unreasonable to me. I don’t know about you, but I think that a mass difference of the order of the Plack scale is much larger than 25 GeV. Just sayin’….
Equally unreasonable is Peter’s claim that a theory should be able to predict the exact masses of unknown particle ahead of time, something which has almost never happened in the history of particle physics.
Eric,
That´s why I acknowledged SUSY ameliorates the problem (by a great deal, I can even add that if makes you happy), but if we are talking about a solution, than the 25 GeV difference is enough for complaining. You cannot expect someone to say a theory is great because it is off by only 25 GeV. What I would like to see is a real solution, something that solves the problem and afterwards you have the clear “aha” feeling that it was unavoidable. That´s not what´s happening here and everything you said, to my mind, reinforces that.
Of course there is still hope. If TeV scale SUSY is found than loop corrections to the Higgs mass will be saved almost naturally. Almost. But a real natural solution was discarded by LEP, my original point. Furthermore if on the other hand the LHC still does not find any signs of SUSY I really see not point on insisting with this theory.
Eric,
The problem with the motivation for SUSY that it “stabilizes the weak scale” is that the weak scale is around 100 GeV, and the limits on superpartner masses and thus the supersymmetry breaking scale are getting close to 1 TeV. So, there’s an order of magnitude problem here. This shows up in the Higgs mass as a requirement of of significant fine-tuning to get a mass above the LEP limit. I forget what the numbers are (ten percent, one percent?), but they’re much larger than just 25 GeV/Higgs mass.
Fine-tuning can easily be regarded as ill-defined, since there is no measure on the parameter space. Yes, precise measurements require precise values of input parameters. This is called ‘measurement’. 🙂
That the MSSM even in its most simple realizations (like the CMSSM) can easily produce a Higgs mass value high enough, was shown (again) in the paper discussed in the original post (and many others). To me it is really amazing that such simple models are in agreement with *all* experimental measurements, including (g-2)_mu and DM.
Another good point for SUSY: in its most simple realizations (GUT based models such as the CMSSM) it correctly predicted the top quark mass many years before its discovery. The requirement of correct electroweak symmetry breaking placed m_top between 150 and 200 GeV, which is exactly correct as we know now. 🙂
Sven,
“Fine-tuning can easily be regarded as ill-defined, since there is no measure on the parameter space.”
how right you are! and now please tell us again why it is so great that SUSY is supposed to solve the fine tuning problem of the Higgs mass?
Peter,
If you look in any elementary textbook, you will see that the electroweak scale is stabilized by SUSY so long as the splitting between the SM particles and the superpartners is of the order of a TeV or less. Thus, your above argument is completely ill-informed and spurious. I suggest you do a better job of educating yourself on the subject.
Best,
Eric
Eric,
One of many places I’ve learned about this is one that I went back to recently, Arkani-Hamed’s talk at Strings 2005 on “HEP Circa 2010”.
http://www.fields.utoronto.ca/audio/05-06/strings/arkani-hamed/
I suggest you listen to it, especially the part starting around 21 minutes in, where he discusses supersymmetry and the “little hierarchy problem”. He explicitly states that the problem is that one expects the superpartner spectrum to be at LEP energies, not TeV energies.
By the way, he was expecting first collisions summer 07- winter 08 and arguing that by the end of 2008 one would certainly know if supersymmetry had any relevance to the hierarchy problem, claiming that only weeks or months of data would be needed to see squarks and gluinos if they were there and relevant to the hierarchy issue. He was far too much of an optimist about the LHC initial energy and luminosity, but still.
I suggest you contact Nima and explain to him why he’s full of it and needs to better educate himself…