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.