What comes after the Large Hadron Collider? With that question in mind, more than 1300 scientists have come up with a conceptual design for a €24 billion ($27.5 billion) Future Circular Collider. Released on 15 January by CERN, the FCC design consists of a 100 km ring in which protons would smash into each other at up to 100 TeV, nearly an order of magnitude higher energy than the LHC.
The FCC would pass below Lake Geneva, pushing the excavation cost for the 500-meter-deep tunnel to an estimated €5 billion, more than half the tab for the accelerator’s initial phase. The plan would be to start the machine around 2040 as an electron–positron collider to optimize the production of Higgs bosons. A €15 billion upgrade to a hadron collider would follow, with operations beginning in the late 2050s. As a hadron machine, the FCC would collide ions as well as protons.
The 100 TeV goal is based on the technologies that are expected to be available in the next two to three decades, says CERN physicist Michelangelo Mangano, one of the design study coordinators. “That’s the highest attainable energy—and it’s useful,” he says. No single experiment can go after every possible theory of dark matter, but 100 TeV is sufficient to establish whether dark matter is a weakly interacting massive particle. “And we could determine the nature of the phase transition that caused the breaking of the electroweak symmetry,” Mangano adds. That could reveal why matter dominates over antimatter. The FCC could probe Higgs self-coupling and also provide hints about how neutrinos get their mass.
Several proposals exist for electron–positron colliders to study the Higgs boson in detail. Japan will decide in March whether to pursue the International Linear Collider (ILC); CERN has a mature design, the multi-TeV Compact Linear Collider, or CLIC; and China recently unveiled a proposal for a Circular Electron Positron Collider (CEPC). If the ILC goes forward, says Halina Abramowicz, a high-energy experimentalist at Tel Aviv University, “it would take the pressure off the first phase of the FCC. And CLIC would not happen. I don’t think the community can afford more than one of these facilities.”
Abramowicz chairs the European Particle Physics Strategy process, which will evaluate and rank proposals for European projects and participation in global projects. The strategy is slated for completion in mid 2020; ultimately, decisions will be made by policymakers and funders. If the FCC gets a green light in the strategy, the next step would be a detailed technical design.
The main technical challenge to realizing the FCC lies in making stronger superconducting magnets. The plan is to use niobium-3-tin, which is highly brittle. “You have to wind it, while it’s still a powder inside copper tubes,” says Mangano. Then, through repeated heating and cooling cycles, the Nb3Sn develops the needed properties.
The method has been tested with less powerful magnets for the ongoing upgrade to the high-luminosity LHC, which is set to start up in 2026. The HL-LHC magnets reach about 12 T; the target for the FCC is 16 T, which will take about 20 years to develop, Mangano says. Magnet technologies and energy losses via synchrotron radiation limit the energy to which the current 27 km LHC ring could be pushed to perhaps 40 TeV. But he says that is a “far dream” that would require technology advancing to 24 T magnets.
China’s CEPC and the FCC design bear similarities. As envisioned they would both be 100 km in circumference and start as electron–positron colliders. The electron–positron FCC would go to higher luminosities and energies. At 365 GeV it could create top-quark pairs; the CEPC at 240 GeV could not. Still, says Abramowicz, for electron–positron collisions, a linear collider is the better investment. “A circular machine only makes sense if you [then move on] to a high-energy proton–proton collider.” She doesn’t expect both the FCC and the CEPC to be built. “I am assuming a global approach, sharing resources,” she says. Of course, the FCC would be funded by European and partner countries, whereas so far the CEPC is a solely Chinese project.
Mangano sees it differently: A decision in China to proceed could spur Europe to maintain its lead in the field. CERN is a “main driver of European intellectual and societal progress,” he says. “Everybody recognizes that with the end of the LHC in sight, we need to start preparing for the next big project. The challenges are to obtain the consensus of the community, political support, and funding.”