Results putting new limits on SUSY based on the entire first run of the LHC are starting to emerge (see for example this from CMS) with more likely at Moriond next week. Since one is dealing with a theory with a large number of parameters, these are hard to characterize in a simple way. One thing to focus on is the limits on gluinos, since just about every popular version of SUSY says these should be about the easiest thing for the LHC to see. Very roughly, the Tevatron was able to set typical limits of about 300 GeV on such things, and the LHC at 8 TeV (4 times the Tevatron) is now giving limits around 4 times higher, 1.2 TeV. This is not likely to change much for the next few years until after the LHC comes back at 13 TeV in 2015. One can with some confidence predict that the gluino mass limit will then go up to about (13/8)*1.2 TeV or around 2 TeV, maybe a bit more in years after that with a high-luminosity LHC. Farther out in time, the next machine under discussion that could raise the limit is the HE-LHC, at 32 TeV, giving limits around 5 TeV. The time scale for this though is something like 2030-40, even assuming such a project ever were to get funded. I suspect the right characterization of that project might be “not in my lifetime”.
There is a new paper out claiming to see evidence of a gluino in the data around 1000-1100 GeV. The same authors (see here), have been claiming to see such gluinos since the early LHC data, first at around 7-800 GeV, with a mass getting higher with each round of new data and higher mass limits.
Few are likely to pay attention to this, but what is getting taken much more seriously is the case that Nima Arkani-Hamed has been vigorously making recently (see for example his talk at the Higgs Symposium). Arkani-Hamed is now by far the most influential theorist in this area, with slides from his latest talks often appearing in many other people’s presentations, functioning as the embodiment of the conventional wisdom of the field. He also is the only phenomenologist with a $3 million Fundamental Physics prize, awarded for his work on models that have had great influence, although zero success experimentally.
One of these, split supersymmetry, is what he is now promoting as the explanation for the negative LHC results. In this model, which he developed with Savas Dimopoulos back in 2004, the main argument for SUSY, the hierarchy argument, gets abandoned in favor of anthropics. The Higgs mass and the electroweak scale are what they are not because of SUSY, but because otherwise physics would be different and we wouldn’t be here. Once one abandons the hierarchy argument, the remaining arguments for SUSY are extremely weak (I’ll try and explain these in more detail in a separate posting), but for some reason Arkani-Hamed still thinks the idea is worth promoting and that vindication for his $3 million may yet be had.
Split SUSY works by moving all scalar superpartners up to unobservably high energies, but a few particles including the gluino are supposed to be at potentially observable masses. Back in 2004, Arkani-Hamed and Dimopoulos were hopeful about the possibilities for the LHC seeing a split SUSY gluino, writing:
However, at peak luminosity of 30 fb-1 per year, the LHC may well be a gluino factory producing roughly a gluino per second(for m_g ∼ 300 GeV).
These hopes have now been dashed, and at the Higgs symposium talk, illustrative spectra show gluino masses at 2.1 and 2.3 TeV (this may just be because that’s about the limit of what the LHC could see). Arkani-Hamed and co-authors have a recent paper out discussing Simply Unnatural Supersymmetry, i.e. “the simplest picture of the the world arising from fine-tuned supersymmetric theories”. Here calculations are done for gluino masses ranging from 1.5 to 15 TeV, and the story is that we’ll have to be lucky to get any experimental evidence for this model. They end with:
If Nature has indeed chosen the path of un-natural simplicity, we will have to hope that she will be kind enough to let us discover this by giving us a spectrum with electroweak-inos lighter than ∼ 300 GeV or gluinos lighter than ∼ 3 TeV.
So, the current state of the conventional wisdom about SUSY from its most influential proponent is pretty much the following. It’s still the thing to try and sell to the public as the best bet for the future of physics, but the hierarchy argument is gone, and at a fundamental level it’s anthropics, the landscape and the multiverse. He’s pretty much given up hope of ever getting any experimental evidence for this, other than the outside possibility of maybe the gluino mass being just low enough to be visible in rare LHC events late in the decade.
The interesting question about all this I think is a sociological one: will this untestable and rather ugly theory based on anthropic reasoning become widely seen as the “best hope” for fundamental particle theory? In a post-LHC world where mankind has abandoned the high-energy frontier, will the conventional wisdom of the textbooks be that SUSY and those gluinos must be there, but unfortunately happen to be just out of reach?
Update: For a survey article that just came out this evening, which tries to show that the main argument for SUSY (the hierarchy problem) is not quite dead yet, see here.
Update: New Scientist has a special section this week about “Crunch time for physics” (unfortunately mostly behind a paywall). On SUSY, Frank Wilczek is still a believer, based on the renormalization group calculation he was a co-author of back in 1981. If no SUSY turns up at the next LHC run though, even he will throw in the towel:
I cannot believe this success is an accident. But in science faith is a means, not an end. Supersymmetry predicts new particles, with characteristic properties, that will come into view as the LHC operates at higher energy and intensity. The theory will soon undergo a trial by fire. It will yield gold – or go up in smoke.
He has a bet with Garrett Lisi that superparticles will be detected by July 8, 2015.
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