Of Particular Significance

About the Future Circular Collider (FCC)

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/06/2024

A number of people have asked me my opinion concerning CERN‘s proposal for a new, larger and more powerful particle physics collider… or rather, two completely different colliders that would operate in the same tunnel:

  • Phase 1 (two to three decades from now): an electron-positron collider targeted at the detailed physics of Higgs bosons, Z bosons, W bosons and top quarks, using them to search for subtle high-energy phenomena and for rare but dramatic low-energy phenomena;
  • Phase 2: (five to six decades from now) an exploratory proton-proton collider, like the Large Hadron Collider [LHC] but with a higher collision energy, and therefore capable of making discoveries of particles that either have higher mass or a lower production rate than what LHC can handle.

Importantly, any decision to fund this machine lies several years away, and in the meantime, a discovery at the LHC or elsewhere might completely change the conversation. So I don’t see the point in writing about this right now.

Instead, I refer you back to my two posts from 2014.

Some details are out of date — time scales are longer, and China is not what it was back then — but much of what I wrote is the same as what I would say today.

My opinion in a nutshell: Phase 1 makes perfect sense to me. Phase 2 seems far less obvious, but it’s decades away, and no final decision has to be made soon.

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16 Responses

  1. I do like the idea of a FCC stage 1 Higgs factory, especially since inflation looks like a (singlet) Higgs-like scalar field.

    In that regard I found the link back to your KITP Conference: “From the Renormalization Group to Quantum Gravity Celebrating the science of Joe Polchinski” both poignant and interesting due to your sharing the late Polchinski recognition of “anthropic argument”.* Polchinski commented on Sean Carroll’s blog that his reaction to Weinberg’s anthropic theory was roughly ‘he is correct!’

    But in this category you include a naturalness problem with the Higgs field being light, up to the point you find effective quantum field theory as a complete description questionable if only SM was found at LHC. (Which I think it effectively is by now.)

    There are several problems I have with this, not the ones of the other experts listening but much more simplistic of course.

    First with the idea that the math of string theory scan physically allowed systems. If the actually realized anthropic systems are a few (i.e. if strings are not physical), the amount of finetuning is not great.

    Second with the current knowledge of inflation field theory, if that is what inflation observation tell us is its nature. Then effective quantum field theory could stay below the Planck scale (without as severe a naturalness problem), the unstable inflation vacuum possibly overlapping with a quasistable Higgs vacuum in scale, and there would be no need for a UV theory. (Unless you claim general relativity is UV valid and we have to go there.)

    Third with the gravity coupling of dark matter making it too having an anthropic structure lifetime (enough time for galaxy formation). Why would a Higgs field have a much higher mass than dark matter can have? Dark matter needs to be relatively cold for galaxy formation.

    Fourth with the absence of chemistry in your hierarchical argument. We must have small objects as well.

    *) And informative, you – and independently another expert from other physics – claimed that there is already a standard model finetuning on the order 10^-4 to 10^-6. And the discussion let slip that the ‘reason’ the weak force breaks EW symmetry is because sphalerons would produce only neutrinos if the strong force does it, I didn’t know that either.

  2. Matt, is there any theoritical work you can reference me on quantum gravity, but one that tries to use the black hole physics to theoritize quantum gravity, i.e. are the dynamics of the atom similar to the black hole?

    PS: Is it correct to think of the atom like a super fast, speed of light, gryoscope?

  3. I’m sure the FCC will be there, but only for political reasons. The science is secondary since the first day of the existence of CERN.

    1. Thank you for the laugh; I assume you are joking, because you couldn’t possibly be serious. As you know, it is no accident that CERN’s discovery of and measurements of the W, Z and Higgs boson were awarded Nobel prizes, and that experimental techniques developed at CERN have also received Nobel prizes, as well as many other prizes. The science done at CERN has been extraordinary, the best in the world, and far beyond what you’d expect from a politically-motivated project.

      If what you mean is that the science has been secondary for the politicians, of course that is true; but is it also obvious, and true in every country in the world. Politics is primary for politicians.

      But the core of CERN is science — without the scientists, there is no CERN, no matter how much money politicians choose to lay out — and for the scientists, the science is primary. The achievements in particle physics and the underlying accelerator physics, magnet physics, and other material physics speak for themselves.

      1. Do read the old papers Matt. For example see this paper by Schrödinger where he talked about a wave in a closed path: http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Schrodinger_1926_Quantisation_II.pdf.

        To appreciate the relevance of this sort of thing, cut a long sinusoidal strip of paper (like this: https://5p277b.n3cdn1.secureserver.net/wp-content/uploads/2018/07/strip1a.png), give one end a 180° twist, then form it into a Möbius strip, sliding the two ends past one another to end up with 720° of rotation. Then check out Dirac’s belt and the Wikipedia Spinor article, and note that your Möbius strip is the same width all round. There is no width variation as you go round the Möbius strip, because at all locations round the loop there are two widths that add up to the same total width. The sinusoidal strip represents a photon, the Möbius strip represents an electron. It looks like a standing wave. Or a standing field if you prefer. IMHO this tells you something important about mass and charge.

      2. Matt, I think my comment is in your spam folder. There were 3 hyperlinks. Please delete this comment.

          1. It’s a bit late to delete anything now. Especially since they aren’t links to my personal ideas. They’re links to old papers by the likes of Erwin Schrödinger. I’m confident that they provide information that you would find extremely valuable.

            Do you remember in a previous post where you said “Or measure c directly” in a comment? I referred to an arXiv article by Magueijo and Moffat saying it’s a tautology, but you didn’t reply. Well, take a look at what Einstein said in 1920: https://einsteinpapers.press.princeton.edu/vol7-trans/156?highlightText=%22spatially%20variable%22. Then have a read of this 2014 article by Don Koks the PhysicsFAQ editor: https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html . There’s some great stuff out there.

  4. Interesting stuff Matt, particularly in the second article. I noted this: “Perhaps another Einstein will be needed to radically reshape the way we think about what we know. A dramatic rethink is both more exciting and more disturbing.” I don’t think you don’t need another Einstein, Matt. All you need to do is read the old papers by the likes of Einstein, Maxwell, de Broglie, Schrödinger, (Charles Galton) Darwin, and Born & Infeld. See for example https://www.nature.com/articles/119282a0.pdf.

  5. There’s recent news that the paper ‘Evidence for the Higgs Boson Decay to a Z Boson and a Photon at the LHC’ may point to new physics. Do you have anything to say about this, Matt?

    1. “The measured signal yield is 2.2±0.7 times the Standard Model prediction, and agrees with the theoretical expectation within 1.9 standard deviations.”

      According to the abstract of the paper, it is not even 2 standard deviations away from the Standard Model prediction. This happens all the time.

      Maybe you misread the 3.4 standard deviations evidence for an observation of the process to be a deviation of the evidence from Standard Model prediction?

      1. I didn’t read the paper. I picked up the news a few days ago from a popular science YouTuber who picked it up from an article on Phys.org that claimed:

        “The Atlas/CMS collaboration, work from more than 9,000 scientists, found a “branching ratio,” or fraction of decays of 34 times per 10,000 decays, plus or minus 11 per 10,000—2.2 times the theoretical value.”

        1. The YouTuber has made a correct statement, but has not apparently not pointed out to you that a result of 34+-11 per 10000, when compared to the theoretical value of 15 per 10000 [note 15*2.2 = 34], is less than two standard deviations from the prediction. The scientists themselves point this out in the abstract to their paper. 2 Standard Deviations from prediction is a commonplace and nothing to get excited about.

  6. The P5 report recommends funding research on muon collider that can obtain similar result to the FCC-ee collider. Do you think it is feasible? I doubt at least the luminosity.

    1. Recommending muon collider research seems reasonable to me, yes, though in the end, it is accelerator physicists who have to do the work to make it happen. We will not know whether it is truly feasible until more research is done. Like all entirely new approaches, it is not obvious that it will succeed, but the payoff would be great if it did, as it would open up a remarkable future and a whole host of new approaches to neutrino physics, Higgs physics and other weak-nuclear-force physics. Some investments in future technologies have to be made now, as current technologies are reaching their natural limit. CERN’s FCC represents an attempt to squeeze one more step out of them, but we have to consider whether it is the best option.

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