For many years now discussion in the HEP community of what might be the appropriate next machine to try and finance and build after the LHC has centered around the idea of a linear electron-positron collider. The logic has been that an electron-positron machine would provide a much better environment that the LHC for detailed studies of physics at the TeV scale. At these energies, synchrotron radiation losses when accelerating electrons are so high in a circular geometry that such a machine would have to be a linear collider to keep the power needed something plausible. The two main proposals under study have been the ILC (250 GeV + 250 GeV, later upgradeable to 500 GeV + 500 GeV) and, a less mature technology, CLIC (1.5 TeV + 1.5 TeV). These would be very expensive machines to build and operate ($10 billion and up?), requiring completely new technology, tunnels and detectors.
The operating assumption has been that initial results from the LHC would show the existence of new supersymmetric or other particle states in the region of multiple hundreds of GeV to small numbers of TeV, and the linear collider designs could then be chosen and optimized to study this new physics. The other main goal of such a collider would be detailed study of the Higgs, and knowing the mass of the Higgs is also highly relevant to what kind of linear collider to build.
The initial LHC results are now in (125 GeV Higgs, nothing new at the TeV scale) and they are rather discouraging for the linear collider idea, providing no strong motivation for studying electron-positron collisions around 1 TeV. A “Higgs Factory” capable of producing and studying large numbers of Higgs particles is an attractive idea, but the production cross-section for a 125 GeV Higgs is dominated by the process e+ + e- -> Z + H, which starts to get large around 220 GeV and reaches a maximum value around 255 GeV. So, for most Higgs studies, the right energy for an electron-positron machine is around 250 GeV, not 1 TeV.
This realization is driving a new proposal that is getting a lot of attention: the idea of going back to circular electron-positron colliders, building a new machine in the LHC tunnel, optimized as a Higgs factory, and designed to operate at 120 GeV + 120 GeV. This is being called “LEP3″, since it would be in many ways similar to LEP2, the predecessor machine to the LHC, which operated in the same tunnel, reaching an energy of 209 GeV. There would be huge cost advantages to building such a machine over the ILC or CLIC, since it can use the LHC tunnel, infrastructure, and, crucially, the CMS and ATLAS detectors (the detectors are a large part of the cost of a new accelerator).
Space was left in the LHC tunnel to allow another ring, so there are various possibilities for having a LEP3 and the LHC cohabitate. Until now, the assumption has been that the LHC would be upgraded to the”HL-LHC”, operating at higher luminosity throughout the 2020s, then perhaps an “HE-LHC”, operating at higher energies during the 2030s. This plan is being challenged by the LEP3 proposal, with the argument that it might turn out that Higgs physics is where the only action will be, and a long period of LHC operation at higher luminosities and modestly higher energies might be less worthwhile than building a LEP3 Higgs factory.
There’s a very good article about this at PhysicsWorld. For more detail about LEP3, see here, here and here. John Ellis is one of the co-authors of the latest document proposing study of the LEP3 possibility, and the Physics World article has him arguing that, after waiting to see if LHC14 turns up anything:
LEP3 could be a more secure option than the ILC if only a Higgs is discovered…If the LHC does not discover anything beyond the Higgs, then would you keep running it for years?
Lyn Evans, who led construction of the LHC and is now director of the linear-collider project argues against the LEP3 concept:
The first job is to fully exploit the LHC and all its upgrades, This is at least a 20 year programme of work, so I think that it is very unlikely that the LHC will be ripped out and replaced by a very modest machine with little scope apart from studying the Higgs.
The problem is, what if, as seems increasingly likely, “studying the Higgs” is the only new physics accessible in these energy ranges? Dreams of superpartners and extra dimensions may die hard.