Back now from vacation, and found that there have been quite a few interesting talks at the KITP in Santa Barbara this week which are now available on-line:
Since the EPS-HEP conference last month, the “First Year of the LHC” program has some interesting results to discuss. Yesterday Matt Reece gave a talk on Assessing SUSY after 1 fb -1, on the hot topic of how worried SUSY proponents should be that no sign of SUSY has been found at the LHC so far. He takes the point of view that the failure of direct collider searches to see anything is much less of a problem than the pre-LHC failure of SUSY to show up indirectly in flavor physics or in cosmology. While it’s true that SUSY was in trouble pre-LHC, there’s psychologically a big difference between indirect effects not showing up, and directly looking for something and finding it’s just not there. The discussion with the audience is quite interesting, with some audience members a lot more worried about SUSY. One of them reminded people that SUSY is supposed to solve naturality problems, so relatively light squarks were expected, but now “those models are being screwed.” Someone else (Lisa Randall, I think) reacted to Reece’s mentioning R-parity violating models as one way to evade the LHC limits with “Is there any good reason to think about R-parity violation?” All in all, the discussion gives a good indication of what prominent theorists are thinking now that the initial results from the LHC are in.
About a year ago on this blog, I had the following exchange with a well-informed phenomenologist on this blog:
If there’s no sign of supersymmetry in this year’s LHC data, how discouraging will this be for those who expect to see supersymmetry at this energy scale?
In 2010 data? Not discouraging at all. In 2011 data? Fairly discouraging. In 2014 data? Enormously depressing.
The LHC has now gathered as much data as expected for all of 2011, so I think that with the negative results, “fairly discouraged” is where SUSY proponents would have expected to be and are now. “Enormously depressed” is on the agenda for late 2014, early 2015, after the LHC reaches design energy.
Adam Falkowski, the Jester of Resonaances fame, also gave an interesting talk this week, on Higgsless theories.
On the mathematical end of things, Ed Frenkel gave a very nice expository “Blackboard Lunch” talk on What do Fermat’s Last Theorem and Electro-magnetic Duality Have in Common?, explaining to physicists a bit about the Langlands program and the connection between geometric Langlands and QFT pioneered by Witten and developed by him and others over the past few years. For something more technical with newer ideas about the relationships between TQFT, gauge theory and representation, see David Ben-Zvi’s talk on Geometric Character Theory.
Regarding R-parity violation, I think the important thing that must be kept in mind that this is only introduced to solve the problem of rapid proton decay through dimension-4 operators in the MSSM. However, if the MSSM is extended such that baryon and lepton number conservation result from local gauge symmetries, then R-parity really isn’t necessary. So, in such a scenario, there is no problem with R-parity violation, which makes detecting superpartners much more difficult.
Peter, are people worried about finding no evidence for extra dimensions (thinks like Randall-Sundrum models etc)?
Shantanu,
I don’t think anyone seriously thought the LHC would find evidence for extra dimensions. This was always just a talking point for the media, and a class of models that some people found interesting to study for various reasons, but not because there was any serious reason to believe they would actually show up at the LHC.
Supersymmetry is different. In that case, quite a few people did expect it to be seen at the LHC.
Peter,
what do you think will happen to string theory if supersymmetry does not show up at the LHC at all? I know what string theorists are going to say, i.e., string theory never predicted it would, etc, but I wonder what will happen in terms of their funding, in terms of presence in the media, respect from the rest of the community. I cannot believe (or I don´t want to) that the strange situation we have now, that a theory with no predictions will go on receiving so much attention. Do you agree?
Bernhard,
I think string theory in the last few years has already been seriously affected along the lines you ask about, partly because of its failure to make any real LHC predictions. It has been clear for a while to most people in HEP that string theory is a failure at saying anything at all about TeV scale physics, and that has had an impact. Finding supersymmetry was kind of the last hope along those lines, and no supersymmetry will just continue current trends.
One thing I’ve learned is that it’s just about impossible to get people to give up on an idea once they have a certain amount invested in it. Many string theorists will go to their graves insisting string unification is a great idea, not a failure, and they’ll be prominent in the media, as well as having influence on funding. Their colleagues will just take them less and less seriously. Instead of being “cutting edge” they will be seen as has-beens.
The situation of supersymmetry itself may be a more dramatic story. There’s a large community of people who have invested 20-30 year careers in TeV-scale supersymmetry phenomenology, and by 2015 or so, that may be a really dead subject. How they will deal with this will be interesting, it’s already kind of fascinating to see how this is starting to play out.
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Peter,
is there any recent news on what Ed Witten is thinking, now that Susy looks so bad?
I have a general (factual) question, coming from a math student who doesn’t know modern physics, but is trying to follow. When you and others say string theory makes no predictions, does this mean more precisely that, “it does predict things like supersymmetry, but leaves open a wide (perhaps infinite?) range of energies at which the first SUSY phenomenon would be detectable, which in particular means it can never be absolutely invalidated by a (finite) number of experiments?”
,
Thanks! I’ve always been confused about this point. And another similarly fundamental and ignorant question: what do people mean by the “landscape”? Is it related to the above point about an infinite range of predictions?
Clara,
I don’t know what Witten thinks about the latest discouraging news for SUSY. In the past he has given talks about SUSY that acknowledged that SUSY effects should have already showed up, at least indirectly, so it was not at all guaranteed to show up at the LHC (this is somewhat the same at what Matt Reece starts off by pointing out).
gradstudent,
The problem with supersymmetry has always been that it has to be a broken symmetry, and the scale of the breaking is undetermined. There have been arguments (the “hierarchy problem”) made that the scale should be around the electroweak breaking scale (100 GeV), and that is what is now being ruled out by experiment. In string theory, normally you need supersymmetry, but it predicts nothing about its breaking (or about much of anything else..). By the way, I wrote a book about this, where things are explained carefully, in a way that should be accessible to math students…
So with string theory being thrown under the bus, what is the new theory of everything? I’ve heard LQG has even more problems and is also not a leading contender.
Dave,
It’s SUSY that string theorists are throwing under the bus. It may be a while before string theorists throw themselves (i.e. string theory) under the bus, although that might happen. LQG is not something that has ever had a way of giving a unified theory.
As far as unification goes, I think the situation is that it’s pretty clear right now that no one has a good idea. Not an unusual circumstance in face of a hard problem.
It’s worth pointing out that TeV-scale SUSY has not *yet* been ruled out. It has not appeared in the most obvious places, and for sure my degree of belief is rather less now than it was 6 months ago, but let’s not react prematurely.
Peter, if it is discovered, which I consider possible but not probable, you’re going to have some back-tracking to do…
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