Implications of LHC Results

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
statement:

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.

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33 Responses to Implications of LHC Results

  1. articles says:

    The statements about the articles by Kane and Peskin are mean-spirited. They are not to be compared to Kane’s recent “prediction of the Higgs mass at 125 GeV” which rightly deserves the scorn heaped upon it. I recall reading the articles in Physics Today, and it was openly a contest to write science-fiction articles about “what might physics be 50 years hence.” (I quote from memory.) So the authors were explicitly encouraged to make up stuff. So they made updates and discoveries. So what? I do not recall if Daniel Kleppner wrote an article about physics 50 years hence. He did write an invited article on something of the kind (in Physics Today), although perhaps not in 1997 and not as part of a contest. If the statements in such articles do not pan out, as the years roll by, what of it? Many people (not all string/SUSY theorists) were anticipating that BSM physics would appear at TeV scales. Perhaps SUSY, perhaps not. To date, new physics has appeared at new decades of energy ~ there has been new physics at scales of eV. MeV, GeV … why not TeV? The actual energy scales do not follow a strict pattern (arithmetic or geometric), but why not new (BSM) physics at TeV scales?

    Mankind has been waiting for the Second Coming of Christ far longer than mankind has been waiting for the appearance of SUSY.

  2. Peter Woit says:

    articles,

    I don’t think my comments about the Kane/Peskin articles (which were projecting not 50 years into the future, but 13 and 18 respectively) were mean-spirited. Note that I just commented that that these provide “some perspective” on what they were arguing for back in 1997-8. Both of them thought superpartners were likely to appear at LEP or at the Tevatron, and if not there, then were a sure thing for the LHC. You can find many other locations where they made such predictions. Yes, they were less sure about extra dimensions, and that’s why they made these the topics of their speculative Physics Today pieces.

    The long history of arguments for SUSY that haven’t worked out is highly relevant to evaluating current arguments for SUSY by the same people. In this particular case, I think Peskin has devoted a lot of effort over the years to the SUSY argument for the ILC, with the laudable motivation that the ILC is the only viable idea for putting the US back at the energy frontier in HEP research any time soon. But this has always run up against the argument that one should wait and see what the LHC had to say about whether there really were SUSY particles and what their masses were. Peskin needs to acknowledge that LHC results are in and have blown a huge hole in his SUSY argument for the ILC.

    I think there’s probably a good case to be made for some kind of lower energy “Higgs factory” machine, maybe some version of the ILC, maybe something else. Unless something else turns up at the LHC though, this case needs to be made in terms of studying the Higgs. The failure of popular arguments for SUSY at LHC scales is a major implication of LHC results and can’t just be ignored. The “OK we didn’t see SUSY at the LHC, but we think it has to be there at lower energies at the ILC” argument is just not going to fly. Maybe superpartners will soon appear and save the day for him, but it’s much more likely that things will just get worse as SUSY exclusions get stronger. Making a big public case that the reason to build the ILC is SUSY is not going to end well in 2015-6 if that’s the time frame for a decision by some government to finance the project as well as when the design energy LHC results come in, finally completely closing the door on TeV-scale SUSY.

  3. articles says:

    They were asked to write speculative articles, and the postulated timelines and discoveries in those articles don’t mean anything. On the other hand, talks (Peskin at Lepton Photon or whatever) on SUSY at LHC and/or ILC energies are indeed to be rebutted.

  4. future 1 says:

    Lubos contemplates “ATLAS: a 2.5-sigma stop squark excess”.

  5. Peter Woit says:

    future 1,

    Besides the “stop excess”, in other recent blog posts Lubos has argued that CMS is suppressing evidence for SUSY because of Tommaso Dorigo, and that there is something fraudulent about Barack Obama’s Social Security number. I suggest you stick to more reliable sources of information, e.g. the slides of today’s talk that I linked to, which give the latest from the ATLAS collaboration about stops. Enough about Lubos, please.

  6. Thomas Larsson says:

    “Mankind has been waiting for the Second Coming of Christ far longer than mankind has been waiting for the appearance of SUSY.”

    Are you suggesting that these events are in the same category?

  7. Nex says:

    “Mankind has been waiting for the Second Coming of Christ far longer than mankind has been waiting for the appearance of SUSY.”

    Well, SCOC had a head start on SUSY, but the difference is bound to become insignificant.

  8. Jess Riedel says:

    Hi Peter,

    For what it’s worth, I agree with ‘articles’ that your main post does not make it clear enough that the essay contest was supposed to be fun and speculative sci-fi. (I certainly don’t find this mean-spirited, it just gives the wrong impression.) Maybe you could clarify by quoting the description given by Physics Today?

    If Peskin et al. have been arguing in a serious setting for the likelihood of SUSY at the Tevatron or the LHC, these talks shouldn’t be too hard to find and highlight. As you say, that would be very useful for people deciding whether to listen to them when justifying potential LHC successors.

  9. Peter Woit says:

    Jess,

    Here’s the description of the contest:

    “Physics Today, has announced an essay contest, “Can You Write Physics Tomorrow?” in celebration of the magazine’s 50th anniversary in 1998. This is an invitation to write an imagined “Search and Discovery” story about a future discovery, advance in physics, or new technology. ”

    They ran at the same time a separate request for short humorous pieces. The Kane and Peskin pieces are now quite humorous, but I’m pretty sure that was not the intention of the writers, or of Physics Today in c0mmissioning them.

    I’m not claiming that these pieces represent Kane and Peskin’s 1997 solid predictions for the future, what they represent is their hopes for the future (experimental study of extra dimensions). In both pieces SUSY is in the background as a given. They didn’t think it was worth promoting SUSY as a speculative discovery 15 years in the future, because they thought it was quite possible it would be discovered very soon (the LEP beam energy had just been increased).

    To find predictions about SUSY at LEP/Tevatron you have to go back to before they were turned on, late 1980s/early 90s, which takes some effort since this was before everything was easily available on-line. For LHC SUSY predictions, 15 seconds of Googling turns up for instance this from 2007:

    http://www.umich.edu/~mctp/SciPrgPgs/events/2007/kanefest/peskin.pdf

    which has, in red:

    “So the discovery of supersymmetry is not just the hope for this machine; it is the expectation.”

    I’ve heard many talks like this over the past decade, often making claims like this one, that the problem with the LHC will be such a complicated pattern of SM violations that it will be hard to disentangle them and figure out all the SUSY particles, their masses and couplings (remember the “LHC Olympics”?).

    For those willing to do some searching, I’d be curious if anyone can find any example of a SUSY proponent arguing pre-LHC that maybe the LHC wouldn’t see anything, but the ILC would. Personally I think trying to make such a case now doesn’t pass the laugh test, but maybe someone can point to such an argument.

  10. David Nataf says:

    I don’t really understand the arguing behind the arguments (metarguments) for the LHC successor.

    From the perspective of particle physics, an LHC successor should be built regardless of what the LHC finds. Different physical concepts will likely influence what kind of successor, but there should be a successor period. The standard model is not a complete description of nature: it doesn’t include dark matter, dark energy, and gravity among other proven concepts. I’m not sure if its consistent with matter’s domination over antimatter. It would be irrational, in my opinion, to stop building particle accelerators after the first accelerator that has less success than the prior accelerators, there should be a more objective criteria.

    There’s a 4-sigma significance for a dark matter annihilation line toward the Galactic center at E ~ 130 GeV, with some arguing its two lines. We will soon know if its real. If it holds up it will in and of itself justify several experiments — from a scientific perspective. I think you would want to be build an ILC just to know if it produces this particle.

    Now, with all of that said, it’s hard for me to imagine that government civil servants care about any of these arguments. I’m just having a hard time seeing a government bureaucrat turn down the money for an ILC on the basis that supersymmetry wasn’t found at the LHC. These scientific megaprojects are built to promote national prestige, like the space shuttle, or the olympics for that matter.

  11. Peter Woit says:

    David,

    I agree there should be a successor to the LHC, the question is going to be of what kind. Such arguments need to be scientifically solid though. I think you seriously underestimate the competence of “government bureaucrats” who will have to be sold on such a project and get it funded. Many of them are trained physicists who can easily tell the difference between a good argument and a bogus one.

    The possible dark matter signal you mention is a good example. First it has to hold up. Then it could be a good part of a justification for a linear collider, but you need some serious arguments about why it could not be seen in the LHC data, but would be visible at an LC. A crucial question about an LC that your arguments must address is what energy it should run at, which dramatically affects the cost.

  12. successor says:

    Indeed there should be an LHC successor regardless of what the LHC finds. That is to say, a search-and-discovery successor machine. Recall that after the b quark was found (at Fermilab in 1977), it was immediately postulated that there must be a partner top quark. The structure of the SM said so, and the SM by then was widely believed to be correct. But the SM did not give a mass for the top quark. So it was a case of go out and search. PEP, PETRA and TRISTAN were all built to find the top quark. Nature was unkind in that respect, the top quark was beyond the reach of those machines. But PETRA did produce good physics of 3 jet events and the discovery of gluons, also violations of scaling, vindicating predictions from QCD. An LHC successor will simply have to go “out there” and explore.

    A Higgs factory is a separate issue, although also a successor to the LHC. Clearly the immediate interest will be to study the properties of the Higgs, although eventually the machine will push to higher energies. In that respect, LEP operated initially as a Z factory, but it did eventually push on to sqrt{s} = 209 GeV.

  13. Peter Woit says:

    successor,

    Yes, but the question will be specifically whether the 250-500 GeV/beam ILC design makes sense as a search and discovery machine, after you’ve already operated the LHC at high luminosity and 7 TeV/beam. Is there really plausible physics that would be invisible at the LHC, but could be discovered at the ILC? Personally I don’t think SUSY models cooked up to fit this description qualify.

  14. anon. says:

    If the 130 GeV gamma ray line holds up you would absolutely have a case for a linear collider since it’s almost airtight that it implies the existence of new charged states and making charge states is what a linear collider does. You could imagine that at the LHC these states would be buried in QCD.

    Similarly, if the Higgs really does have an enhanced branching ratio to two photons, you have a no-lose case for a linear collider.

    Similarly, if an electron EDM is detected you have a case for a linear collider.

    But let’s be honest: right now, there is no compelling justification to spend the money on a linear collider. In a year or so that may change.

  15. successor says:

    SUSY has nothing to do with it. CERN had already produced the W and Z at the SppbarS, and for that matter CERN (or Rubbia anyway) had already discovered the top quark at 44 GeV/c^2 and monojets and supersymmetry, but CERN built LEP anyway. LEP produced good physics, but no BSM. In the 1970-80s, Fermilab was building the Tevatron, having already operated the Main Ring for years. Yet the NSF funded the construction of CESR at Cornell, even though CESR could not possibly match the physics reach of the Tevatron. (BTW note that CESR was not a DOE machine.) And then the SCOC (ok, the b quark) was discovered at Fermilab, at precisely the energy range of CESR, and the mission of CESR changed completely, and CESR became a dramatically successful Upsilon spectroscope.

    Factories are well and good (nay, excellent), but the concept of a factory implies that the object of study has already been discovered. Search and discovery is something else.

    The real problem is ultimately economic, that the machines are simply so expensive to build and operate. So the funding agencies (govts, ultimately) want to see some prospect of a return on investment. And it has to be a short term return. ( “right now, there is no compelling justification to spend the money on a linear collider.” Indeed.) It is the sheer cost that is the stumbling block. Pure search and discovery is just too difficult a concept to sell, at such a high price tag.

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  17. s n d says:

    None of this would be an issue of the successor machine cost 1% (say) of what the actual successor machine is likely to cost. Let us say (very approximately) $10B for a linear collider or muon collider. So suppose instead that the successor machine cost $100M. That is still a lot, but the US could fund such a machine all by itself, without outside contributions. There could be two such machines in the world (or even in the US), say an electron linear collider and a muon collider. (Gloss over the radiation problems, etc.) Credible SUSY models would not matter, although something of the kind would in practice be offered as a justification to build the machines. The machines would be built, and the search for new physics would be pursued. Powerful senators would of course have to be cultivated, but such individuals have existed, in California, Illinois, New York and Texas. They are not an extinct breed. And if BSM physics was found, it would not matter if it was SUSY or not. All that would matter is that it is BSM.

    Ultimately, the way forward probably resides in a breakthrough in materials science and technology. If 100T magnets can be mass produced, by industry, at a fraction of the cost of existing LHC magnets, then compact next-generation accelerators can be proposed at reasonable size and cost. (Even protons would emit synchrotron radiation at such energies and small bending radii, but let us say that the heat load in sc magnets can be dealt with. After all, this is all hypothetical …) The stumbling block is simply the cost, not the lack of plausible physics scenarios.

  18. strong field physicist says:

    “The real problem is ultimately economic, that the machines are simply so expensive to build and operate”

    The cost of the ILC is about that of 2-3 stealth bombers. Lets see… fundamental knowledge of benefit to all of humanity versus mass murder of villagers… hmm…

    or to put it another way… you could have 2 ILC’s for the price of the bonuses City of London bankers paid to each other last year from public bailouts

  19. particle bombs says:

    “The cost of the ILC is about that of 2-3 stealth bombers.” It doesn’t work that way. It never has. Money poured in to science in the years after WW2 very much because nuclear physicists made atomic bombs. There was a ticker tape parade for the nuclear physicists in New York city, they were celebrated as heroes. Bonuses to City of London bankers … I imagine the New York thieves will take umbrage that you chose London over NY as your example.

  20. Peter Woit says:

    All discussions of the cost of HEP research tend to quickly degenerate into the “they cost no more than socially undesirable X” argument, and from there into the idiocy of internet political discussion. Just stop right here.

    I will exercise my right as czar here to comment that the comparison to a few bombers makes me think “hey, that’s not so much money, it’s easily affordable”, whereas the comparison to half the City of London bonues makes me think “wow, that’s a lot of money..”

  21. martibal says:

    Off topic (Peter please apologize): I know you do not feel like to moderate a discussion on the possible dark matter annihilation line toward the Galactic center at E ~ 130 GeV, but could anybody indicate some links to read about this topic (a kind of “Not Even Wrong” devoted to astrophysics) ?

  22. David Nataf says:

    martibal,

    This is the most recent analysis:
    http://arxiv.org/abs/1206.1616

    This is the discovery paper:
    http://arxiv.org/abs/1204.2797

  23. martibal says:

    Thanks David.

  24. King Ray says:

    David, thanks also for the links.

  25. Thomas Larsson says:

    It might be sobering to recall what Wilczek wrote in his Future Summary from 2001:

    “Of course, the ultimate test for low-energy supersymmetry will be to produce some of the predicted new R-odd particles. Even in the focus point scenario, there must be several accessible to the LHC.”

  26. S. Molnar says:

    The two missing commas in the “three known multiple universes” quotation is faintly amusing, but not as good as the two extra commas in “eats, shoots, and leaves”. Still, it provides evidence that a comma conservation law holds in our universe(s).

  27. Peter Woit says:

    S. Molnar,

    Two extra commas would help, but I’m not convinced they can be placed so as to turn that particular word salad into something grammatically correct that makes sense.

  28. S. Molnar says:

    Surely the intent was to say “To fix them, alternatives have been proposed that involve a composite Higgs … that has extra spatial dimensions beyond our three known, multiple universes, and something called supersymmetry.” Awkward, I grant you, but not total rubbish. I actually thought you knew that and just found the error entertaining. Am I being naive?

  29. Peter Woit says:

    S. Molnar,

    It’s clear what input the theorists involved provided. I suspect though that the incoherence of the end-result in the article is not due just to typos, but expresses the writer’s failed attempt to make sense of what he was hearing (which, yes, is rather entertaining…).

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  31. Xezlec says:

    “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).”

    Wait… are you implying that it’s the center-of-mass energy alone that determines what can be discovered, and whether it’s a pure electron-positron collider like the ILC or a hadron collider like the LHC doesn’t matter much? I’m not a physicist, but that goes against everything I thought I knew about what a “parton distribution function” was and how it mattered.

    I’m actually not sure there’s a missing comma in that Berkeley quote. Reading your blog, Lubos’ blog, and Flip Tanedo’s blog, I feel like the three of you must surely live in different universes.

  32. Peter Woit says:

    Xezlec,

    You can crudely think of proton-proton colliders as colliding bags of partons together, and a proton as mostly 3 quarks. Then the LHC is providing parton-parton collisions with about 1.3 TeV/parton (although with a very wide energy spread). As you can see from Flip Tanedo’s latest posting, Z-primes are getting excluded up to about 2.5 TeV, so this crude estimate is reasonable. In 2015 the LHC energies will nearly double.

    The initial ILC design is for 250 GeV + 250 GeV, very much below the 1.3 TeV + 1.3 TeV rough estimate above of what the LHC is doing. The LHC partons are colored, which gives it much higher cross-sections for producing colored particles, but, as the Higgs discovery shows, it also can discover new color-less particles. The main advantage of the ILC is that the fact that the initial state is not strongly interacting makes for a much simpler environment and much lower backgrounds. A Higgs discovery at the ILC would have been much much easier, and for getting detailed info about the Higgs it certainly would be a huge improvement over the LHC.

    I’m sure one can come up with SUSY models with no new particle states observable at the LHC, although some that could be seen at the ILC at lower energy, but such things are rather contrived and I think few people will take them seriously. The argument for the ILC has always been that it would be the right tool not for discovery of particles at the energy frontier, but for the detailed study of things first seen much more crudely at the LHC.

    Personally I think I’m in the same universe as Flip, but you’re right, Lubos is evidence for the existence of another one.

  33. Xezlec says:

    Thanks for the explanation.

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