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.”