The Next Machine

For the last week or so US HEP physicists have been meeting in Minneapolis to discuss plans for the future of  US HEP.  Some of the discussions can be seen by looking through the various slides available here.   A few days earlier Fermilab hosted TLEP13, a workshop to discuss plans for a new very large electron-positron machine.   There is a plan in place (the HL-LHC) for upgrading the LHC to higher luminosity, with operations planned until about 2030.   Other than this though, there are no current definite plans for what the next machine at the energy frontier might be. Some of the considerations in play are as follows:

  • The US is pretty much out of the running, with budgets for this kind of research much more likely to get cut than to get the kinds of increases a new energy frontier machine would require. Projects with costs up to around $1 billion could conceivably be financed in coming years, but for the energy frontier, one is likely talking about $10 billion and up.
  • Pre-LHC, attention was focused on prospects for electron-positron linear colliders, specifically the ILC and CLIC projects. The general assumption was that LEP, which reached 209 GeV in 2000, was the last circular electron-positron collider. The problem is that, at fixed radius, synchrotron radiation losses grow as the fourth-power of the energy, and LEP was already drawing a sizable fraction of the total power available at Geneva. Linear accelerators don’t have this problem, but they do have problems achieving high luminosity since one is not repeatedly colliding the same stored bunches.

    The hope was that the LHC would discover not just the Higgs, but all sorts of new particles. Once the mass of such new particles was known, ILC or CLIC technology would give a design of an appropriate machine to study such new particles in ways that not possible at a proton-proton machine. These hopes have not worked out so far, making it now appear quite unlikely that there are such new particles at ILC/CLIC accessible energies. It remains possible that the Japanese will decide to fund an ILC project, even without the appealing target of a new particle besides the Higgs to study.

  • The LHC has told us the Higgs mass, making it now possible to consider what sort of electron-positron collider would be optimal for studying the physics of the Higgs, something one might call a “Higgs factory”. It turns out that a center of mass energy of about 240 GeV is optimal for Higgs production. This is easily achievable with the ILC, but since it is not that much higher than LEP, there is now interest in the possibility of a circular collider as a Higgs factory. There is a proposal called LEP3 (discussed on this blog here) for putting such a collider in the LHC tunnel, but it is unclear whether such a machine could coexist with the LHC, and no one wants to shutdown the LHC before a 2030 timescale.
  • Protons are much heavier than electrons, so synchrotron radiation losses are not the problem, but the strength of the dipole magnets needed to keep them in a circular orbit is. To get to higher proton-proton collision energies in the same tunnel, one needs higher strength magnets, with energy scaling linearly with field strength. The LHC magnets are about 8 Tesla, current technology limit is about 11 Tesla for appropriate magnets. The possibility of an HE-LHC, operating at 33 TeV with 20 Tesla magnets is under study, but this technology is still quite a ways off. Again, the time-scale for such a machine would be post-2030.
  • The other way to get to higher proton-proton energies is to build a larger ring, with energy scaling linearly with the size of the ring (for fixed magnet strength). Long-term thinking at CERN now seems to be focusing on the construction of a much larger ring, of size 80-100 km. One could reach 100 TeV energies with either 20 Tesla magnets and an 80 km ring, or 16 Tesla magnets and a 100 km ring (such a machine is being called a VHE-LHC). If such a tunnel were to be built, one could imagine first populating it with an electron-positron collider, and this proposal is being called TLEP. It would operate at energies up to 350 GeV and would be an ideal machine for precision studies of the Higgs. It could also be used to operate at very high luminosity at lower energies, significantly improving on electroweak measurements made at LEP (the claim is that LEP-size data sets could be reproduced in each 15 minutes of running). Optimistic time-lines would have TLEP operating around 2030, replaced by the VHE-LHC in the 2040s.
  • For more about TLEP, see the talks here. The final talk of the TLEP workshop wasn’t about TLEP, but Arkani-Hamed on the VHE-LHC (it sounds like maybe he’s not very interested in the Higgs factory idea). He ends with

    EVERY student/post-doc/person with a pulse (esp. under 35) I know is ridiculously excited by even a glimmer of hope for a 100 TeV pp collider. These people don’t suffer from SSC PTSD.

    Looking at the possibilites, I do think TLEP/VHE-LHC looks like the currently most promising route for the future for CERN and HEP physics (new technology might change this, i.e. a muon collider). Maybe I don’t have a pulse though, since I can’t say that I’m ridiculously excited by just a glimmer of VHE-LHC hope for a time-frame past my life-expectancy.

    A 100 km tunnel would be even larger than the planned SSC tunnel (89 km) and one doesn’t have to suffer from SSC post-traumatic-stress-disorder to worry about whether a project this large can be successfully funded and built (In very rough numbers I’d guess one is talking about costs on the scale of $20 billion). My knowledge of EU science funding issues is insufficient to have any idea if the money for something on this scale is a possibility. On the other hand, with increasing concentration of all wealth in the hands of an increasingly large number of multi-billionaires, perhaps this just needs the right rich guy for it to happen.

    Someone is going to have to do a better job than Arkani-Hamed in terms of finding an argument that will sell this to rest of the scientific community. His main argument is that such a machine would allow us to improve the ultimate LHC number of “fine-tuning” being at least 10-2 to a number like 10-4, or maybe finally see some SUSY particles. I don’t think this argument is going to get $20 billion: “we thought we’d see all this stuff at the LHC because we were guessing some number we don’t understand was around one. We saw nothing and turns out the number is small, no bigger than one in a hundred. Now we’d like to spend $20 billion to see if it’s smaller than one in a hundred, but bigger than one in ten thousand.”

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21 Responses to The Next Machine

  1. uair01 says:

    As a normal citizen I’m a bit dismayed by all these plans and amounts. Why spend it on esoteric and far-out theory research? Why not spend it on something that we need more, like nuclear fusion research? I would heartily support that. But this feels more like the weird luxury hobby of a very small elite. We are all in the aftershocks of a financial crisis, people are losing their jobs and you guys are playing with Higgs bosons. What good will that do for the rest of us?

  2. Peter Woit says:

    I see that even before I’d finished re-reading this to proof read it, the usual first comment that comes in whenever I write anything about a possible HEP experiment has appeared.

    Enough, all further comments from people who want to argue not about the physics but about the desirability of spending money at all for this purpose will be ruthlessly deleted. This argument has gone on here many times, I don’t see any possibility of anything new being said.

  3. Low Math, Meekly Interacting says:

    About muon colliders…I’ve read a bit about the latest research into hypothetical machine design, but can glean no sense of where the bets are on feasibility. Seems like there’s been serious discussion since 2009, so I’m wondering if the HEP community is coming to some consensus on whether or not it’s a remotely viable idea.

  4. Peter Woit says:


    There’s a long history of work on the muon collider idea, see for instance
    As far as I can tell, the current situation is still one of research into determining whether such a thing is truly feasible, with an answer to that question still years in the future.

  5. srp says:

    I wouldn’t appropriate money for any of these proposals until some of the ideas here

    had been thoroughly investigated. A billion dollars and five years to figure out whether there isn’t a better way (that also may give more-useful spinoffs) would be a very sensible investment. It might even turn out to shorten the time needed to achieve much higher energies.

    If it turns out that these concepts are all 20+ years off or are impossible, then OK, it’s reasonable to discuss accelerators that would swallow up small countries. Without such a conclusion, these proposals (except maybe the muon collider) all have the air of calling for “more cowbell.”

  6. lun says:

    By far the cheapest and fastest option to explore an extra big chunk of as yet unknown parameter space is to put the LEP back in the LHC tunnel, and study ep collisions at much higher space in x-Q^2 than ever before. One can do this with equipment already built.

    I cannot help but think that the only reason this is not even mentioned as a main alternative is theoretical prejudice: Theorists are indicating the way, most theorists dont like technicolor and/or substructure theories, therefore such theories (for which an eLHC would be optimized) are not being explored. Note that more often than not the argument against these approaches w.r.t. SUSY is that they are intractable to analytical calculation, something very different from saying they are unlikely to happen.

    There is a general point to be made here – experiments and accelerators cost a lot of money, so the pressure is there to go on a road where theorists say youre likely to make a discovery, rather than blindly explore and hope to find something new.
    Yet it is the latter road that has often given the big discoveries.
    And, more to the point, in this case going off the beaten track (eLHC) would be much cheaper than the standard approach (ILC/VLHCs), and the most quoted theory predictions (SUSY) are being roundly falsified. One needs to invent an experimental strategy that is less risk-averse, which for such expensive toys as those the physicists use is not simple.

  7. M says:

    Theorists too suffer PTSD because LHC did not confirm their theories of the past 30 years. It is great that nature can give such surprises.

    A bigger circular collider for a precise study of the Higgs and later an exploration up to 100 TeV would surely the best option. Setting rhetoric aside, exploring is the only way in which we can understand what is the weak scale.

  8. fuzzy says:

    before planning the future, i feel that we should make an effort to remind the discussions before the lhc was built, which arguments have been collected, who had the right to speak and who did not. it is not enough to note the failure of the plans, one should analyze the causes, in order not to repeat it.

    personally, i have a strong impression that the lust for machines is larger than the will of discussing what we are searching for. and even when we discuss of physics, the greatest effort is to win the argument or to maintain the position rather than getting to the point. this should change.

    finally, i would suggest to invest into gamma and neutrino astronomy, measurements concerning neutrinos, studies about the gravity, understanding astrophysical objects, search for dark matter and alike. these researches gave a lot to physics in the past years and will continue to do so for a long time.

  9. Zathras says:

    This is off-topic for this post, but this article on how science journalists should be skeptical of scientific claims is really fascinating, as well as being quite germane to this blog.

  10. Z says:

    It looks like a muon collider is far off. Results from MICE (e.g. aren’t expected until the 2020s, which means it probably won’t be ready for a collider planned for the early 2030s. If it does happen though, you could have a 50-70 TeV lepton machine on a 100km ring before you run into synchrotron losses.

  11. Alex says:

    fuzzy’s last point is an interesting one. What is the evidence that there must be a theory of fundamental interactions beyond the Standard Model? Well, among other things, there’s the existence of dark matter (which we know from astronomical observations), the existence of dark energy (again, known from astronomy), the existence of gravity (something we can study in terrestrial experiments but we understand much better because of astronomical observations), and the existence of neutrino oscillations (something we can study in terrestrial experiments but we first learned about from solar astronomy). For that matter, how did we learn that antiparticles exist? Somebody found positrons in cosmic rays. How did we learn that there’s more than one generation of leptons? Somebody found muons in cosmic rays.

    I wonder if it might make more sense to invest in understanding neutrinos, looking for dark matter candidates in labs, doing astronomical observations that can constrain particle physics beyond the Standard Model, looking for unexpected things in cosmic rays, and looking for anomalies in dipole moments and whatnot. Those sorts of observations have the potential to either rule out some models or make some models more promising. With that sort of information in hand, one could push for an accelerator with a clearer idea of what it needs to look for.

  12. Peter Woit says:


    The things you suggest aren’t being ignored, they’re more or less exactly the topics that US HEP is now concentrating on, given that an energy frontier machine is not plausible. Even the one main accelerator project (Project X) under discussion is motivated largely by neutrino physics.

  13. Alex says:

    Yeah, I didn’t mean to say they’re being ignored. I know they aren’t. I’m mostly wondering if, given our current lack of knowledge of where to go, these are actually more valuable than a large accelerator.

  14. Sammy says:

    The HEP community might help the case for massive further HEP funding by demonstrating that a by-product could be improved design and analysis of PET scanning, SPECT scanning and Proton Beam Therapy or even the development of Higgs Knives (my second cousin’s uncle-in-law’s deep brain tumor just got zapped clean away with a Gamma Knife). Then there are smoke alarms and all kinds of industrial probes. Don’t forget that the space program gave us nonstick frypans.

  15. Pingback: Never Give Up | Not Even Wrong

  16. Zathras says:


    Pointing to these invention is the conventional response to the cost of science, but I do not think it will be enough. To show this value, the LHC better come up with similar concrete innovations. Otherwise, you’re just talking ancient history. There is still time for the LHC to do so, but it is not a given that it will happen.

  17. harryb says:

    I work in the oil industry – and the annual – annual – project capital expenditure globally is now over 1 Trillion dollars (google it). Typically, in any given year, there are 5-15 > $20bn projects underway. I am involved in two currently in this range – one in construction, one in late design. A $20bn project in the oil and gas world – lets call it mature applied physics – is unremarkable at any given moment now. Maybe Peter is right – get these billionaires to invest a proportion in HEP projects rather than largesse to vogue theorists (eg Milner Foundation) and access immortality that way. It would be a refreshing change in perspective to invest heavily in experiment.

  18. Sammy says:

    Zathras: what’s ancient history is e.g. the theory (Bethe 1932-33 plus frills) presently used to shape pulses in Proton Beam Therapy. Why not bring it up to the level of modern HEP?

    Dirac postulated the positron when my father was a medical student. Now PET scanning is routine clinical imaging. These things take time, and lay people do understand that.

  19. Yatima says:

    Considering the way “western” economies stay mired in inflationism, debt traps and welfare-warfare statism — if China manages to make a soft-ish landing for their overleveraged, bubbling, export-slanted and cheap-labor-from-the-countryside dependent economy (as well as not fall into the trap of internal or external armed conflict), they might be a contender for a projet of a 2030 or later timeframe. They are flush with foreign cash (or rather, US gov’nment IOUs) and looking for national prestige so they could start on design work immediately. Is anyone talking to them?

  20. Loren Petrich says:

    A muon collider seems like a nice idea: the cleanness of a lepton and a high mass. But the muon’s mean life is about 2.2 microseconds. That means that there isn’t much time to accelerate and collide a muon before it decays. I’ve seen some discussions of muon colliders, but I haven’t found out how they expect to get around that problem.

  21. Peter Woit says:

    Loren Petrich,

    See the comment by Z above. The paper linked to has some relevant discussion. The problem isn’t so much acceleration but “cooling”: the muons you are producing come with a wide spread of momenta, and you need narrow that considerably before you can accelerate and collide. The question is whether you can do this on time scales of less than a microsecond.

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