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.
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…