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Next-generation LHC: CERN lays out plans for €21-billion supercollider

The proposed facility would be the most powerful collider ever built.

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Artistic impression of the Future Circular Collider

Artistic impression of the Future Circular Collider.Credit: CERN

CERN has unveiled its bold dream of building a new accelerator nearly 4 times as long as its 27-kilometre Large Hadron Collider (LHC) — currently the world’s largest collider — and up to 6 times more powerful.

CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, outlined the plan in a technical report released on 15 January.

The document offers several preliminary designs for a Future Circular Collider (FCC) — which would be the most powerful particle smasher ever built — with different types of collider ranging in cost from around €9 billion (US$10.2 billion) to €21 billion. It is the lab’s opening bid in a priority-setting process called the European Strategy for Particle Physics Update, which will take place over the next two years and will affect the field’s future well into the second half of the century.

“It’s a huge leap, like planning a trip not to Mars, but to Uranus,” says Gian Francesco Giudice, who heads CERN’s theory department and represents the laboratory in the Physics Preparatory Group of the strategy update process.

Since the LHC’s historic discovery of the Higgs boson in 2012, the collider has not discovered any new particles. This points to a need to push collider energies as high as possible, Giudice says. “Today, exploring the highest possible energies with bold projects is our best hope to crack some of the mysteries of nature at the most fundamental level.”

The potential for a machine such as the FCC is “very exciting”, says Halina Abramowicz, a physicist at Tel Aviv University in Israel who heads the European strategy update process. She adds that the FCC’s potential will be discussed in depth as part of that exercise and compared with other proposed projects.

The CERN Council, which includes scientists and government delegates from CERN’s member countries, will then make the final decision on whether to fund the project.

Too pricey?

Not everyone is convinced that the supercollider is a good investment. “There is no reason to think that there should be new physics in the energy regime that such a collider would reach,” says Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany. “That’s the nightmare that everyone has on their mind but doesn’t want to speak about.”

Hossenfelder says that the large sums involved might be better spent on other types of huge facility. For example, she says that placing a major radio telescope on the far side of the Moon, or a gravitational-wave detector in orbit, would both be safer bets than the collider in terms of their return on science.

Michael Benedikt, a CERN physicist who led the FCC study, says that a supercollider facility would be worth building regardless of the expected scientific outcome. “These kinds of largest scale efforts and projects are huge starters for networking, connecting institutes across borders, countries. All these things together make up a very good argument for pushing such unique science projects.”

But Hossenfelder says that a similar argument could be made in favour of other big-science projects.

The options

The FCC study started in 2014 and involved more than 1,300 contributors, according to CERN, with a financial contribution from the European Commission’s Horizon 2020 research-funding programme. Most of the scenarios that the study outlines involve a 100-kilometre tunnel being dug next to the existing LHC tunnel. The cost for this and for the related infrastructure on the surface would be around €5 billion, says CERN.

The FCC study prepared a conceptual design of a 100km long ring accelerator (red) using CERN's existing structure (blue)

The new collider would be housed in a 100km long ring (red) near CERN’s LHC (blue).Credit: CERN

A €4-billion machine built in such a tunnel could smash electrons and their antimatter counterparts, positrons, with energies of up to 365 gigaelectronvolts (GeV). Such collisions would enable researchers to study known particles, such as the Higgs boson, with greater precision than is possible at a proton–proton collider such as the LHC. This research programme would start by around 2040, after the LHC — and a planned upgraded version of it — has run its course.

Physicists have long planned to build an International Linear Collider (ILC) after the LHC has run its course, and this collider would also smash electrons and positrons. Japanese scientists made a pitch in 2012 to host the ILC. But the LHC’s failure to find any unpredicted phenomena has weakened the case for a linear collider. This is because the ILC would reach energies that are sufficient only for studying the Higgs boson but not for discovering any new particles that might exist at higher energies, as CERN’s planned collider might. The Japanese government is set to decide by 7 March whether it wants to host the ILC.

Another option outlined in the report is a €15-billion, 100-kilometre proton–proton collider (also known as a hadron collider), built in the same tunnel, that could reach energies of up to 100,000 GeV — much higher than the LHC’s maximum capability of 16,000 GeV. But it is more likely that the electron–positron machine will be built first, and the proton–proton collider later on, in the late 2050s. Either way, the higher-energy machine would look for entirely new particles, which could be more massive than known particles and would therefore require more energy to produce.

The hadron collider would be only 15% longer than the Superconducting Super Collider (SSC), a project in Texas that was abandoned over cost concerns in the 1990s when its tunnels were already in mid-construction. But because of technological improvements — notably, in the magnets that bend the protons’ path around the ring — the proposed hadron collider would smash particles at energies more than two times those predicted for the SSC.

Much research and development is still to be done, which is one reason it might make sense to build the lower-energy machine first. “If we had a 100-kilometre tunnel ready tomorrow, we could start building an electron–positron collider right away because the technology essentially exists already,” says Giudice. “But more research and development is needed for the magnets required by a 100-teraelectronvolt collider.”

China’s competitor

Wang Yifang, director of the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences in Beijing, says that he does not doubt that CERN could pull off such a project. “CERN has a long history of success. It has the technological capabilities, the management skills and good relationships with governments,” he says.

Wang is leading a similar project in China, and he says that both efforts have reassuringly come to essentially the same conclusions in terms of science goals and technical feasibility. In particular, it is a natural choice to do electron–positron collisions first and then move on to hadrons later, he says.

Much of the added cost for a hadron collider would come from the need for powerful superconducting magnets and huge helium cryogenic systems to keep them cold. The hadron-colliding FCC would aim at developing, building, and deploying 16-tesla magnets based on the superconducting alloy Nb3Sn, which would be twice as powerful as the LHC’s, while in principle requiring only slightly warmer temperatures. China is pushing for more advanced — but less proven — iron-based superconductors that could push temperatures even higher. “If you are able to do it at 20 kelvin, then you get huge savings,” Wang says.

Even if particle physicists agree that the world needs a 100-kilometre collider, it is unclear whether it needs two. Whichever side gets such a project going first will probably pre-empt efforts on the other side. Either collider would host experiments open to the broader international community, Wang says, so in terms of the science that will be done it will not make a difference which one ends up being built.

Nature 565, 410 (2019)

doi: 10.1038/d41586-019-00173-2

Additional reporting by Elizabeth Gibney.

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