As particle physicists eagerly await results from the LHC, many theorists are already promoting interpretations of what they hope it will find. This week’s Chronicle of Higher Education has a cover story on the LHC entitled The Machine at the End of the Universe (see associated articles here and here). In it, Gordon Kane enthusiastically describes the LHC as “It is certainly the most important experiment of any kind in the past century, without qualification” and “the most important thing ever in our quest to understand the fundamental laws of nature and the universe.”
The question that the LHC will actually address, that of electroweak symmetry breaking, doesn’t get much attention. Instead, the focus is on supersymmetry, extra dimensions and string theory. While noting that there’s no evidence for string theory, the article reports:
The new collider could change that, says Joseph D. Lykken, a physicist at Fermi National Accelerator Laboratory. “Either the discovery of supersymmetry or extra dimensions is a triumph of string theory.” While such a finding would not conclusively show that string theory is correct, it would provide a first crucial experimental test, he says.
At New Scientist this week, there’s another article about the LHC, entitled Awaiting a messenger from the multiverse. In it, Savas Dimopoulos explains that there’s already quite a lot of experimental evidence against weak-scale supersymmetry:
“After a lot of experiments there has not been any hint of SUSY,” says Dimopoulos. For each individual one of those predictions, he says, you can find plausible explanations for why they are not seen. “But by the time you look at the whole package of ‘things that should have happened but didn’t’, you start getting a somewhat baroque structure.”
Dimopoulos instead promotes his joint work with Nima Arkani-Hamed on the idea of “split-supersymmetry”, where the supersymmetry breaking scale is very high, so can’t explain the hierarchy problem. One possible experimental signature of such models would be a long-lived gluino. They promote the idea that such a thing would be a “Messenger From the Multiverse”, the idea being that if supersymmetry doesn’t explain the hierarchy problem, the explanation must be the anthropic landscape:
That powerful piece of evidence would have dizzying implications. “It would be a strong indication that there is a string landscape or a multiverse,” says Dimopoulos. “I think the majority of opinion would come around to that point of view.”
One aspect of this argument is that it also works if no gluino is seen. If no superpartners at all are found at the LHC, and thus supersymmetry can’t explain the hierarchy problem, by the Arkani-Hamed/Dimopoulos logic this is strong evidence for the anthropic string theory landscape. Putting this together with Lykken’s argument, the LHC is guaranteed to provide evidence for string theory no matter what, since it will either see or not see weak-scale supersymmetry.
The New Scientist article does explain that this kind of argument for anthropics has its critics:
However, anthropic arguments remain controversial, and despite the authors’ heavyweight reputations, split supersymmetry is no exception. “I’m not a big fan of it,” says John Ellis, a theoretical particle physicist at the CERN laboratory, where the LHC is being prepared for its first run later this year. His criticism is that the anthropic approach gives you too much freedom to answer any troublesome question in physics. “At some point you might just as well say ‘let’s fine-tune everything’ and go home,” Ellis says.
Theorist Frank Wilczek at the Massachusetts Institute of Technology is also unhappy with the idea of split SUSY. “I think it’s a logical possibility, but if we really have to appeal to anthropic considerations I think that’s a big retreat. It would mean the explanatory power of theoretical physics would be limited.”
And even if the LHC does find Dimopoulos’s stopped gluinos, not everyone will be persuaded that arguments based on the multiverse are good science. “My opinion of anthropic reasoning is likely to remain unprintable,” says Ellis.
Update: Forgot to add one more piece of LHC-related news. France is now joining the US with its own national LHC web-site: LHC-France. Because of the Gallic fondness for cartoons, it includes a section Le LHC en BD.