A Muon Collider?

The US particle physics community has been going through a multi-year process designed to lead up this fall to a 10 year strategic plan to be presented to the DOE and the NSF. In particular, this will generate a prioritized list of what projects to fund over this period. The process began with the Snowmass self-study, concluded last year, and available here. Since last fall there have been two independent efforts going on:

  • A National Academies study has been holding meetings, materials available here.
  • A P5 (Particle Physics Project Prioritization Panel) is holding meetings, see here, and planning for a report to NSF and DOE by October.

Looking through all the materials relevant to particle theory, there seems to me little acknowledgement of the serious problems faced by the subject, or any new ideas for how to address these problems. Most of the effort though is devoted to where most of the money will be spent, on the experimental side. To a large degree, for the short-term it’s clear where funding has to go (to continue supporting the LHC into the HL-LHC era, and finish building the DUNE/LBNF US neutrino project). The longer-term is however very uncertain, as it is unclear whether there’s a viable energy-frontier project that could study higher energies than those accessible at the LHC.

Last week EPP2024 and P5 held Town Hall events at Fermilab, see here and here. There’s video of the EPP2024 event here. On the question of the long-term future, one issue that is getting a lot of attention is that of whether to prioritize development of a possible muon collider. In this presentation a young physicist gives a future timeline including their likely retirement and death dates, showing that a muon collider is their only hope for new energy frontier physics during their lifetime. For those of my age the situation is a bit different, since even a muon collider is not going to do the job. At the EPP2024 event (3:28 in the video) Nima Arkani-Hamed makes the case that:

I think the subject has not been so exciting for many, many decades, and at the same time our ability to experimentally address and solidly settle some of these very big questions has never been more uncertain. I don’t think it’s a normal time, it’s an inflection point in the history of the development of our subject, and it requires urgency… The confluence of the technical expertise for doing so and the enthusiasm amongst the young people who are willing to do it exists now and I very much doubt it will exist in 10 or 15 years from now. If we are going to do it, we have to start thinking about doing it now.

While his point is more general, he’s clearly making the case for starting a new energy frontier machine project soon, with the muon collider the one possibility for getting to higher energies than the LHC.

A few weeks ago there was a workshop at the KITP devoted to the muon collider question, with a news story here (anyone know why the video of the panel discussion is password-protected?). Arkani-Hamed gave a talk aimed at other physicists here. On the European front, a couple days ago there was this meeting.

Already twenty years ago when I was writing Not Even Wrong, it was clear that a muon collider was in principle a very attractive idea for how to get to higher energies and I wrote about this in the first chapter of the book. The much higher mass of the muon than the electron means that you don’t have the same synchrotron energy loss problem, so can build a much smaller storage ring at the same energy, or get to much higher energies with the same size. The problem though is that muons have a life-time of only 2.2 microseconds. This implies two serious difficulties:

  • You need to produce, store, accelerate and collide the muons in a very short period of time.
  • As the muons decay they’ll produce large numbers of high energy electrons and neutrinos, creating a difficult environment for detectors to operate in and significant radiation hazards.

Normally one thinks of neutrinos as virtually never interacting with anything, but the numbers and high energies of the neutrinos produced at a muon collider create a potential significant radiation hazard, one that cannot be dealt with by shielding.

While I might not be around to see the results from a muon collider, if such a thing is viable, I’d strongly support such a project, and would even buy a t-shirt. The US DOE HEP budget is about a billion dollars/year. One would think this should be enough to accommodate building demonstrator projects or a small collider ring on a 10 year timescale, and possibly even an energy-frontier ring on a 20 or more year timescale. What’s worrying me a bit is the fact that more visible progress on this hasn’t happened since I looked into it 20 years ago. Why no current demonstrator project? Have the potential radiation hazard issues found solutions? I’d be very curious to hear from anyone with expertise on these questions.

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11 Responses to A Muon Collider?

  1. Patrick Meade says:

    Hi Peter, for the most recent status you can look in the Snowmass muon collider forum report: https://arxiv.org/abs/2209.01318 there actually has been considerable progress recently which is why you see increased theory community support. I’m giving a Columbia HEP seminar next week on muon colliders so you’re welcome to attend and any questions not covered by the report can hopefully be answered!

  2. Severing Pappadeux says:

    Mu cooling needs to be demonstrated for collider to be feasible. MICE is not doing very good

  3. Mark says:

    The MICE collaboration demonstrated muon cooling with an experiment at the Rutherford Appleton Laboratory. I understand this is needed for a future muon collider, so there is not zero progress on topics needed to be demonstrated . Would a collider demonstrator have been useful (or even feasible at all) without first demonstrating we can cool the muons enough for a real system?

  4. Nathaniel Craig says:

    Thanks for taking a look at the KITP program! Regarding the panel — we wanted to encourage the panelists to engage in critical and constructive dialogue (half the panelists were asked to highlight challenges). We felt they would be able to speak most freely if the recording was not public. I think the panel was very successful, and we are working on a transcript for public release.

  5. Andrea Wulzer says:

    Thank you for the interest in muon collider activities. The International Muon Collider Collaboration (IMCC, https://muoncollider.web.cern.ch) is active, with mostly European participants for now. Here a link to the IMCC meetings and conferences (https://indico.cern.ch/category/11818/) and to the first IMCC report (https://arxiv.org/pdf/2303.08533.pdf). Section 3 of the report helps answering your questions.

  6. John Baez says:

    Can someone give me some figures about the neutrino emissions from a muon collider, and how they could be high enough to be dangerous? That’s surprising!

  7. Peter Woit says:

    For an early study, see
    something more recent

    This is surprising, one normally thinks about neutrinos going through the earth without interacting with anything. But neutrino cross-sections increase with energy, it’s high energy neutrinos that are the problem, and a TeV scale muon collider would produce a lot of them.

  8. Alon Faraggi says:

    There is a ready made solution to the US experimental particle physics program on the energy frontier on a time scale of 10 years. Resurrect the SSC. Possibly an upgraded version with a 100km circumference and 30TeV beam energy. The muon collider is bread & butter accelerator physics. The MICE experiment took a long while to deliver (~25 years). But eventually it did prove the feasibility of the muon beam phase space cooling (https://www.nature.com/articles/s41586-020-1958-9). As a physics machine the muon collider is a prospect of 50-100 years. Pure fantasy.

  9. Severin Pappadeux says:


    earliest is probably by N. V. Mokhov and A. Van Ginneken, “Muon Collider
    Neutrino Radiation”, Muon Collider Collaboration Meeting
    Book, Orcas Island, WA, May 1997.



  10. Severin Pappadeux says:

    MICE demonstrated that muons (being counted one-by-one) going through some LH2 follow well our multiple scattering, struggling, elastic etc models. That’s about it. What has to be done is (close to) full scale cooling cell, with intensive muon beam, material which could handle above mentioned beam, solenoid field, RF cavities etc.

    And it means there is ought to be reasonable target setup producing muons.

    We’re talking about serious accelerator test facility, and 10 years program on the scale of $50mln/year.

    Until such program is up, approved and running, all muon collider talks will be confined to conferences.

  11. John Baez says:

    Thanks, Peter, that’s fascinating. It’s interesting how being underground would increase the danger of these neutrino beams.

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