What surprised me most about today’s Run 2 results (see here) was that CMS and ATLAS were able to already significantly push up limits on superpartner masses, especially the gluino mass. Limits on the gluino mass went from 1.3-1.4 TeV in Run 1 to something like 1.6-1.8 TeV in the new Run 2 data (this depends on exactly what channels one is looking at). This not only kills off Gordon Kane’s string theory prediction of a 1.5 TeV gluino, but it also removes a large chunk of the remaining possible mass region that the LHC will be able to access. And it wasn’t just the gluino: ATLAS quoted limits on sbottom masses moving up from 650 GeV in Run 1 to 850 GeV today. Whatever you thought the remaining probability was for SUSY after the negative Run 1 results, it’s significantly smaller today.
Almost all the news has been about the possible diphoton excess, ignoring the quite significant story about SUSY. Davide Castelvecchi at Nature though today talked to Michael Peskin, who has been one of the more consistent proponents of SUSY over the years, and this was part of his story:
Meanwhile, searches for particles predicted by supersymmetry, physicists’ favourite extension of the standard model, continue to come up empty-handed. To theoretical physicist Michael Peskin of the SLAC National Accelerator Laboratory in Menlo Park, California, the most relevant part of the talks concerned the failure to find a supersymmetric particle called the gluino in the range of possible masses up to 1,600 GeV (much farther than the 1,300-GeV limit of Run 1). This pushes supersymmetry closer to the point where many physicists might give up on it, Peskin says.
I had thought that the “physicists give up on SUSY” story wouldn’t get going until next year, but maybe it’s already started.
Update: In just a few hours after the announcement already 10 papers on hep-ph devoted to explaining the diphoton resonance. SUSY explanations not among the popular ones.
Update: Another eight or so papers explaining the diphotons. And the press has the obvious explanation: string theory:
The idea seems to be that since people were looking for Randall-Sundrum gravitons (which somehow counts as string theory) then if they find something in the diphoton spectrum it could be a graviton. I’m no expert, but none of the dozens of hep-th papers seem to discuss this possibility, and the papers about searches for Randall-Sundrum gravitons (like this one) set limits way above a TeV. On the other hand, I don’t doubt that some “string vacuum” can be found that will explain the diphotons, and that we’ll hear more about it in the press.
SUSY cannot be a (spontaneously broken) symmetry of the elmentary-particle physics. This proposition is a logical consequence of quantum Einstein gravity. It is quite surprising that there are still many people who do not want to give up SUSY!
Is there any chance at all that this 750 GeV bump could be the boson super partner to a SM fermion? I take it diphoton decay is not the expected SUSY signal?
Brandon,
I’m sure one could come up with some kind of SUSY model to explain this, but it’s not what’s expected in simpler such models.
Peter,
But isn’t electroweak SUSY production still not the last hope? I remember in these searches the gluino mass is assumed very high and “decouples” from the LHC phenomenology. The cross section for electroweak production is lower, so I would consider these scenarios to be still possible (not that I personally believe on them). I know ATLAS and CMS have produced limits there too, but should not be competitive with strong production.
Bernhard,
Sure, the easiest thing to see is strongly interacting superpartners, and it’s only those that the LHC is now putting new bounds on. Maybe all the strongly interacting ones are very massive, but there are electroweak ones you can detect with enough luminosity. Very much worth looking for, especially because you might find something else interesting. But, I don’t know of any serious motivation for the idea that strongly interacting superpartners are very massive and decouple, other than a desire to avoid conflict with what experiments are now telling us.
I suppose I should look at materials from people promoting the ILC, I think that looking for such things is one of the arguments for that kind of machine.
I believe that the SUSYers can “explain” the diphoton excess in the next few days. 🙂
David
Already done, see
http://arxiv.org/abs/1512.05333
Also, it seems that string theory can explain this, see
http://arstechnica.co.uk/science/2015/12/first-high-energy-lhc-results-supersymmetry-still-dead-watch-for-gravitons/
Brandon Enright: Is there any chance at all that this 750 GeV bump could be the boson super partner to a SM fermion?
But wouldn’t this violate the R-parity?
I’ve heard grumbling (Strassler alludes to this) that ATLAS didn’t combine with their Run 1 results (which would have lowered the significance considerably, since there seems to be no hump in Run 1 near 750 GeV). Any word on the reasoning here, since it’s presumably not naive cherry-picking?
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Douglas Natelson,
I’m no expert, but I’ve heard two problems with such a combination are
1. Backgrounds are different at 8 and 13 TeV.
2. Even if there is a new 750 GeV state, you don’t know what produced it, can imagine that production rates are quite different at 8 and 13 TeV. One way this could happen is if this new state is the result of a decay of something even heavier, heavy enough so that there’s a big difference in production cross-section.
The problem with RS gravitons is that they predict
sigma(pp->gamma gamma) = sigma(pp-> e+e- + mu+ mu-).
But no peak is observed in dileptons. See page 9 of
http://arxiv.org/pdf/1512.04933v1
AS,
Thanks for explaining that, and thanks for the useful reference.
@Douglas Natelson: I’m surprised that CMS did some combination, and apparently just for the backup slide in the presentation – I have no idea how they did it and they didn’t publish it. How to combine 8 and 13 TeV? The cross-section ratio depends on the production process, which is unknown.
The question of when to give up on weak scale supersymmetry is addressed in a recent paper arXiv:1509.02929 (PRD93 (2016) 035016 entitled
Upper bounds on sparticle masses from naturalness or how to disprove weak scale supersymmetry
Gluino masses can range up to about 4 TeV before much fine-tuning sets in.
In this case, LHC has explored only a fraction of the allowed SUSY parameter space.
The Higgsinos have the most direct connection to naturalness, but these particles are exceedingly difficult to see at LHC. They are better searched for at an e+e- collider such as the proposed ILC.
Howard Baer,
I find it hard to reconcile your present claims with the many colloquia about SUSY I’ve sat through during the past 20 years (Peskin and Arkani-Hamed are among the examples that come to mind), in which an explicit claim was made that if SUSY was the mechanism to explain fine-tuning, it would definitely be seen at the LHC. For another example of this, there’s your such 2009 talk
http://arxiv.org/abs/0908.2785
where you state
“On the theory side, quadratic divergences associated with the scalar sector require new physics at or around the electroweak scale. Also, we have no guidance as to the origin of the generations, the quark and lepton masses and mixings or the origin of the gauge symmetries. We also need a consistent merging of the SM with a quantum mechanical theory of gravity.
On the positive side, data from existing experiments are already pointing the way to a new, more elegant paradigm for the laws of physics as we know them. In addition, data from experiments soon to operate, especially from the CERN LHC, should clinch the deal.”
and
“These data, matched against SUSY theory, seem to point to the Minimal Supersymmetric Standard Model (MSSM) (or MSSM plus gauge singlets) as being the correct effective theory of nature between the weak and GUT scales. The truth will be revealed by experiment as the LHC era gets under way, since weak scale supersymmetry predicts a host of new matter states, many of which should be accessible to LHC searches.”
Given this history, I don’t see why one should take seriously claims that this idea isn’t dead because the LHC was never going to be able to resolve the issue.