Three of the leading figures in HEP theory have today or recently spoken about their current view of SUSY in light of the negative LHC results, here’s a report:
- At the IAS recently, Nima Arkani-Hamed spoke on The Inevitability of Physical Laws: Why the Higgs Has to Exist. Yuri Milner was in the audience, and I gather that this (and Maldacena’s recent similar talk) was intended to fulfill the promise of giving a public talk that came with their receipt of $3 million Fundamental Physics Prizes.
Both Arkani-Hamed and Maldacena talked not about their own work, but about the Higgs, with Maldacena emphasizing the importance of gauge symmetry, Arkani-Hamed the constraints imposed by unitarity and Lorentz invariance. At the end of his talk, Arkani-Hamed gave a big advertisement for SUSY, with a new and somewhat bizarre argument I hadn’t heard before. He argued that since QM + special relativity imply that elementary particles must have spin 0,1/2,1,3/2,2, and until recently 0 and 3/2 were missing, the fact that 0 (the Higgs) has now been seen implies (by the “totalitarian principle” that everything that can happen must happen) that the next thing to be discovered will require a spin 3/2 particle and this needs SUSY. Of the various weak arguments put forward for SUSY, this seems to me to be the weakest yet.
- At CERN today there was a 70th birthday celebration for Chris LLewellyn Smith. I didn’t watch John Ellis’s talk, but his slides are here. Evidently he argued that SUSY is not dead yet, pointing to the latest paper from the MasterCode collaboration. Their most recent CMSSM SUSY “predictions” have gluinos at either 2000 GeV (hard for the LHC to see at full energy after 2014) or 4000 GeV (impossible for the LHC ever to see).
No mention was made of the similar pre-LHC predictions (see here and here) which had the Higgs at around 113 GeV, gluinos at 700 GeV or so, and squarks lighter than this, all of which have been shown to be radically mistaken. For some perspective on this on an even longer time scale, take a look at this 1984 survey article, Supersymmetry – spectroscopy of the future? : or of the present?. It gives much the same enthusiastic motivation for SUSY that we still get in all SUSY talks, with Ellis optimistic that the latest data had hints of SUSY with sparticle masses around 40 GeV (“nicely compatible” with the recent discovery of a 40 GeV top quark…).
- Also speaking today was David Gross, and I did get a chance to watch his talk. He commented on Ellis’s claim that SUSY was not yet dead by noting “we see no signs of life either”, then went on to lay out two “extreme scenarios”. The pessimistic one would be nothing but the SM at LHC energies and no detection of dark matter. In that case, about SUSY he commented that it “could be that Nature does not take advantage of this”, which I think is the first time I’ve ever heard him raise this possibility. The optimistic scenario was the usual picture sold pre-LHC: detection of SUSY and dark matter, non-SM Higgs. Gross said that he’s an optimist, but gave no argument for the optimistic scenario beyond the one that it’s a good idea in life for a scientist to be an optimist.
Update: More at Nature about SUSY’s problems and quotes from its defenders.
Update: Over at the Simons Foundation web-site, there’s an excellent new article about the SUSY debate by Natalie Wolchover.