The May issue of Scientific American has a very good cover story by Joe Lykken and Maria Spiropulu, entitled Supersymmetry and the Crisis in Physics (the article is now behind their subscriber paywall, but for those with access to Nature, it will soon be here).
Here are some excerpts:
It is not an exaggeration to say that most of the world’s particle physicists believe that supersymmetry must be true—the theory is that compelling. These physicists’ long-term hope has been that the LHC would finally discover these superpartners, providing hard evidence that supersymmetry is a real description of the universe…
Indeed, results from the first run of the LHC have ruled out almost all the best-studied versions of supersymmetry. The negative results are beginning to produce if not a full-blown crisis in particle physics, then at least a widespread panic. The LHC will be starting its next run in early 2015, at the highest energies it was designed for, allowing researchers at the ATLAS and CMS experiments to uncover (or rule out) even more massive superpartners. If at the end of that run nothing new shows up, fundamental physics will face a crossroads: either abandon the work of a generation for want of evidence that nature plays by our rules, or press on and hope that an even larger collider will someday, somewhere, find evidence that we were right all along…
During a talk at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, Nima Arkani-Hamed, a physicist at the Institute for Advanced Study in Princeton, N.J., paced to and fro in front of the blackboard, addressing a packed room about the future of supersymmetry. What if supersymmetry is not found at the LHC, he asked, before answering his own question: then we will make new supersymmetry models that put the superpartners just beyond the reach of the experiments. But wouldn’t that mean that we would be changing our story? That’s okay; theorists don’t need to be consistent—only their theories do.
This unshakable fidelity to supersymmetry is widely shared. Particle theorists do admit, however, that the idea of natural supersymmetry is already in trouble and is headed for the dustbin of history unless superpartners are discovered soon…
The authors go on to describe possible responses to this crisis. One is the multiverse, which they contrast to supersymmetry as not providing an answer to why the SM parameters are what they are, although this isn’t something that supersymmetry ever was able to do. Another is large extra dimensions as in Randall-Sundrum, but that’s also something the LHC is not finding, with few ever thinking it would. Finally there’s the “dimensional transmutation” idea about the Higgs, which I wrote about here last year. About this, the authors write:
If this approach is to keep the useful virtual particle effects while avoiding the disastrous ones—a role otherwise played by supersymmetry—we will have to abandon popular speculations about how the laws of physics may become unified at superhigh energies. It also makes the long-sought connection between quantum mechanics and general relativity even more mysterious. Yet the approach has other advantages. Such models can generate mass for dark matter particles. They also predict that dark matter interacts with ordinary matter via a force mediated by the Higgs boson. This dramatic prediction will be tested over the next few years both at the LHC and in underground dark matter detection experiments.
It’s great to see such a high-profile public discussion of the implications of the collapse of the paradigm long-dominant in some circles which sees SUSY extensions of the Standard Model as the way forward for the field. One place where I disagree with Lykken and Spiropulu is their claim that “It is not an exaggeration to say that most of the world’s particle physicists believe that supersymmetry must be true.” Actually I think that is an exaggeration, with a large group of theorists always skeptical about SUSY models. For some evidence of this, take a look at this document from 2000, which shows a majority skeptical about SUSY at the LHC. By the way, I hear those on the right side of that bet haven’t yet gotten their cognac, with the bet renegotiated to wait for results from the next LHC run.
Update: I hear that the 2000 bet was revised in 2011, with a copy displayed publicly at the Niels Bohr Institute. The new bet is about whether a superpartner will be found by June 16, 2016, and the losers must come up with a bottle of good cognac. There are 22 on the yes side (including Arkani-Hamed and Quigg), and 22 on the no side (including ‘t Hooft, Komargodski, Bern). Also, 3 abstentions. It explicitly is an addendum to the 2000 wager, with those who lost the last one given the option of signing again, forfeiting two bottles of cognac, or accepting that “they have suffered ignominious defeat.”
Update: This report from the APS spring meeting includes the following about Spiropulu’s talk there:
Supersymmetry and dark matter have become so important to particle physicists that “we have cornered ourselves experimentally,” said Spiropulu. If neither is detected in the next few years, radical new ideas will be required. Spiropulu compared the situation to the era before 1905, when the concept of ether as the medium for all electromagnetic waves could not be verified.
You can watch the talk and see for yourself here.