The CERN particle-physics lab near Geneva is putting plans in place to build a successor to its Large Hadron Collider (LHC). At a meeting to be held at the University of Geneva next week, some 300 physicists and engineers – including current CERN boss Rolf-Dieter Heuer – will discuss a range of options for a possible future collider. This includes plans for a massive next-generation circular collider – with a circumference of 80–100 km – that would accelerate protons to energies of about 100 TeV.

While the 27 km-circumference LHC has been colliding protons at energies of up to 7 TeV in the hunt for new particles since it first switched on in 2008, for more than 30 years physicists have been carrying out R&D on linear colliders that could one day be the LHC’s successor. One leading design effort is the International Linear Collider (ILC), which would accelerate electrons and positrons to about 250 GeV and smash them together at a rate of five times per second. Funding for the $8bn, 31 km-long collider has yet to be found, but Japanese particle physicists are already making moves to host this next-generation particle smasher.

Meanwhile, a design for a higher-energy machine – the Compact Linear Collider (CLIC) – that could operate at 3 TeV is being developed by a team at CERN. Construction of the ILC and CLIC could begin in the coming decade and they would, if built, study the Higgs boson in great detail through the "clean" collisions that can be made from colliding electron and positrons rather than smashing protons together.

Yet it remains unclear whether these machines will be built and physicists have recently been coming up with other proposals that involve circular colliders similar to the LHC. Such colliders do have some advantages, not least that physicists have a lot of experience in building them. In particular, from 1989 to 2000 CERN operated the Large Electron–Positron Collider (LEP), which was located in the same tunnel that now houses the LHC and was used to study the Z and W bosons in detail. "We need to keep our options open about what the next particle collider will be," says John Ellis of Kings College London, who has been involved in designs for particle colliders beyond the LHC and will be speaking at next week’s meeting. "A bigger, more ambitious machine could offer us more capabilities."

Towards TLEP

Delegates at next week’s Geneva meeting will discuss the technologies needed to create these future machines. One leading design for a next-generation circular collider is "TLEP", which would be housed in an enormous new 80–100 km-circumference tunnel that would most likely be built in Geneva. It could initially collide electrons and positrons (as would both the ILC and CLIC) at energies of about 350–500 GeV. Most of the cost of such a machine would be in excavating the tunnel, with the accelerator itself only accounting for about one-third of the total.

Yet that same 100 km tunnel could then be used well into the future, eventually housing a proton–proton machine that could operate at an energy of up to 100 TeV, much in the same way as the LHC has used the LEP tunnel. This could then look for new particles – such as supersymmetric particles – that the LHC may yet discover. Researchers are planning to complete a conceptual design study for TLEP by 2017 as an input to the next review of the European strategy for particle physics.

Cost concerns

Although Ellis admits that the 100 km tunnel would involve an "enormous investment", he thinks that the advantages would outweigh such concerns in the long run. "LEP was first approved in 1981 with the original tunnel designed to include the LHC in the future, so that it would be an infrastructure that could serve the community for at least 50 years," says Ellis. "That is the same for the new tunnel: use it as an electron–positron machine and then later as a hadron collider."

Indeed, the 100 km tunnel housing the collider could even be built so that it could allow two machines – one electron–positron and one proton–proton – to operate simultaneously, if needed. Ellis says that a preliminary engineering report has already been done on the 100 km tunnel. He claims it threw up no "major show-stoppers", even if parts of it would be built underneath Lake Geneva. "The geology in the region is quite good for digging," adds Ellis.

Yet Lyn Evans, who masterminded the construction of the LHC and is now responsible for overseeing the development of the ILC and CLIC, says that, for the moment, the top priority for CERN is the full exploitation of the LHC and its upgrade programme that will include boosting the luminosity and energy of the collider. "A machine of [TLEP's] size will have a very high cost, so there must be a very strong scientific justification and international support," he told physicsworld.com.