The past few days at CERN there has been a workshop on Implications of LHC results for TeV-scale physics. This is the third in a series of these workshops, which have a goal of evaluating the implications of LHC results for choosing what new HEP facilities to design and fund.
One can argue about the implications, but the LHC results so far are in some sense very simple
- the Higgs has been discovered, with properties consistent with SM predictions, more detailed tests of this consistency to come.
- no evidence has been found for non-SM phenomena. The LHC has produced stringent bounds on extra dimensions and strongly interacting superpartners. The only remaining hope for a strongly interacting superpartner in the current data is for the stop, but evidence against that is accumulating, see for instance here.
The hope that the LHC would see extra dimensions was always quite a stretch, but the idea that it would see strongly interacting superpartners has been conventional wisdom for a very long time. It seems to me that many theorists who have spent the majority of their careers arguing for this conventional wisdom are having trouble admitting what has happened.
For some perspective on this, I recently ran across a 1997 Physics Today essay contest, which asked for submissions that would reflect what a “Search and Discovery” piece from the future might look like. The winner was Gordy Kane’s Experimental Evidence for More Dimensions Reported. It’s supposed to be from May 2011, and assumes that GUTs and supersymmetry were discovered long ago, even
fully accepted in 2000 after the discovery at Fermilab of the needed supersymmetric partners.
According to Kane, 2011 would see discovery of extra dimensions at the LHC, through observation of a 950 GeV KK state.
Michael Peskin also submitted something similar to the Physics Today contest, purporting to be an October 2016 Search and Discovery column entitled Do Squark Generations Show Geometry? In Peskin’s account, the first superpartner was found at LEP in 1999 when it got up to a center of mass energy of 200 GeV, By 2008 a large number of superpartners had been discovered, with ATLAS reporting precise values for four squark masses. Like Kane, he not only conjectures that by now we’d have a huge, well-tested SUSY phenomenology, but that our decade will see the discovery of extra dimensions, of a sort predicted by string theory. For Peskin, the discovery of extra dimensions comes about in 2016 from an electron-positron linear collider operating at a center of mass energy of 1.7 TeV.
Today Peskin gave a talk entitled Will there be Supersymmetry at the ILC?. He starts off by explaining his motivation as follows:
One often hears:
“If SUSY is not found at the LHC before the shutdown, then we will know that SUSY will not be found at the ILC.”
People attending this workshop know that this is incorrect. I hope that this will be explained clearly in the report to the European Strategy Study.
Despite the negative LHC results, Peskin is still trying to argue that one can expect to find supersymmetry at the ILC (which operates at a much lower center of mass energy than the LHC). He asks the question “Are light SUSY particles excluded at the LHC?” and answers it with:
I will first give some sociological evidence against this
1. No theorist who believed in SUSY before 2009 has renounced SUSY in the light of the LHC exclusions. (*)
2. Model builders are still building models with 200 GeV charginos.
(* Gordy Kane might be considered an exception. )
I’m assuming the remark about Kane is a joke…
He goes on to argue that surely at least one strongly interacting superpartner (the gluino) will be found after the long shutdown, when the LHC operates at or near design energy:
So, when we eventually reach the gluino at LHC 14 TeV, the
generic jet+MET observables will begin to work and SUSY will be discovered unambiguously.
The light SUSY sector will still be hard to explore at the LHC. We
will feel lucky that we are already constructing the ILC !
After the long history of LHC SUSY predictions that haven’t worked out, I’m not sure how seriously this will be taken as an argument for funding the ILC. There’s a draft of a section of the report on the implications of the negative LHC results here.
I suspect that arguments about whether to build the ILC over the next few years will revolve around its capabilities in terms of doing a much better job than the LHC to study the properties of the Higgs. Attempts like Peskin’s to argue that it should be built in order to look for supersymmetry are not likely to be taken very seriously by anyone outside the community of those who have been devoting the last few decades to thinking that SUSY is right around the corner, and still are unwilling to give up on this.
Bonus Higgs section: The last couple weeks have seen about a hundred Higgs-related things I could have linked to. For a random sampling, see this interview with Higgs, this from the Daily News and this survey of atrocities.
Bonus culture section: From last night’s first episode of the new season of Breaking Bad:
“We’re living in a time of string theories and God particles. Feasible, doable, why not?”
Update: Geoff Brumfiel at Nature has some quotes from various theorists, including
- From Joe Lykken:
Under the weight of the LHC’s hard evidence, SUSY and other beloved theories are feeling the strain. “There’s going to be a huge massacre of theoretical ideas in the next couple of years,” predicts Joe Lykken, a theoretical physicist at Fermilab in Batavia, Illinois….
And hopes of finding extra dimensions that would mysteriously swallow up energy from collisions in the LHC are evaporating faster than the postulated microscopic black holes that also failed to make an appearance. “I was one of the people who pushed the idea of extra dimensions that we could see in our lifetime,” says Lykken. “Now that we have data, I’m becoming much more conservative.”
- Frank Wilczek is hanging in there:
It is too soon to write off SUSY, agrees Frank Wilczek, a physicist at the Massachusetts Institute of Technology in Cambridge who was awarded the Nobel Prize in Physics in 2004 for his work on the standard model. “The last man standing, as far as ambitious ideas beyond the standard model go, is supersymmetry.”
- Last year Gordon Kane was predicting SUSY discovery (gluinos) this summer. Now:
It will take years’ more data to test some of the most promising ideas, says Gordon Kane, a theorist at the University of Michigan, Ann Arbor, and a longtime SUSY champion.
Update: At Berkeley they had an event to explain the implications of the Higgs to the public, which learned that we need to go beyond the “three known multiple universes”:
…the Standard Model Higgs has problems. To fix them, alternatives have been proposed that involve a composite Higgs – one composed of other matter particles – that has extra spatial dimensions beyond our three known multiple universes and something called supersymmetry.