The Future Circular Collider (FCC) is envisaged to be a multi-stage facility for exploring the energy and intensity frontiers of particle physics. An initial electron–positron collider phase (FCC-ee) would focus on ultra-precise measurements at the centre-of-mass energies required to create Z bosons, W-boson pairs, Higgs bosons and top-quark pairs, followed by proton and heavy-ion collisions in a hadron-collider phase (FCC-hh), which would probe the energy frontier directly. As recommended by the 2020 update of the European strategy for particle physics, a feasibility study for the FCC is in full swing. Following the submission to the CERN Council of the study’s midterm report earlier this year (CERN Courier March/April 2024 pp25–38), and the signing of a joint statement of intent on planning for large research infrastructures by CERN and the US government (CERN Courier July/August 2024 p10), FCC Week 2024 convened more than 450 scientists, researchers and industry leaders in San Francisco from 10 to 14 June, with the aim of engaging the wider scientific community, in particular in North America. Since then, more than 20 groups have joined the FCC collaboration.
SLAC and LBNL directors John Sarrao and Mike Witherell opened the meeting by emphasising the vital roles of international collaboration between national laboratories in advancing scientific discovery. Sarrao highlighted SLAC’s historical contributions to high-energy physics and expressed enthusiasm for the FCC’s scientific potential. Witherell reflected on the legacy of particle accelerators in fundamental science and the importance of continued innovation.
CERN Director-General Fabiola Gianotti identified three pillars of her vision for the laboratory: flagship projects like the LHC; a diverse complementary scientific programme; and preparations for future projects. She identified the FCC as the best future match for this vision, asserting that it has unparalleled potential for discovering new physics and can accommodate a large and diverse scientific community. “It is crucial to design a facility that offers a broad scientific programme, many experiments and exciting physics to attract young talents,” she said.
International collaboration, especially with the US, is important in ensuring the project’s success
FCC-ee would operate at several centre-of-mass energies corresponding to the Z-boson pole, W-boson pair-production, Higgs-boson pole or top-quark pair production. The beam current at each of these points would be determined by the design value of 50 MW synchrotron-radiation power per beam. At lower energies, the machine could accommodate more bunches, achieving 1.3 amperes and a luminosity in excess of 1036 cm–2 s–1 at the Z pole. Measurements of electroweak observables and Higgs-boson couplings would be improved by a factor of between 10 and 50. Remarkably, FCC-ee would also provide 10 times the ambitious design statistics of SuperKEKB/Belle II for bottom and charm quarks, making it the world-leading machine at the intensity frontier. Along with other measurements of electroweak observables, FCC-ee will indirectly probe energies up to 70 TeV for weakly interacting particles. Unlike at proposed linear colliders, four interaction points would increase scientific robustness, reduce systematic uncertainties and allow for specialised experiments, maximising the collider’s physics output.
For FCC-hh, two approaches are being pursued for the necessary high-field superconducting magnets. The first involves advancing niobium–tin technology, which is currently mastered at 11–12 T for the High-Luminosity LHC, with the goal of reaching operational fields of 14 T. The second focuses on high-temperature superconductors (HTS) such as REBCO and iron-based superconductors (IBS). REBCO comes mainly in tape form (CERN Courier May/June 2023 p37), whereas IBS comes in both tape and wire form. With niobium-tin, 14 T would allow proton–proton collision energies of 80 TeV in a 90 km ring. HTS-based magnets could potentially reach fields up to 20 T, and centre-of-mass energies proportionally higher, in the vicinity of 120 TeV. If HTS magnets prove technically feasible, they could greatly decrease the cryogenic power. The development of such technologies also holds great promise beyond fundamental research, for example in transportation and electricity transmission.
FCC study leader Michael Benedikt (CERN) outlined the status of the ongoing feasibility study, which is set to be completed by March 2025. No technical showstoppers have yet been found, paving the way for the next phase of detailed technical and environmental impact studies and critical site investigations. Benedikt stressed the importance of international collaboration, especially with the US, in ensuring the project’s success.
The next step for the FCC project is to provide information to the CERN Council, via the upcoming update of the European strategy for particle physics, to facilitate a decision on whether to pursue the FCC by the end of 2027 or in early 2028. This includes further developing the civil engineering and technical design of major systems and components to present a more detailed cost estimate, continuing technical R&D activities, and working with CERN’s host states on regional implementation development and authorisation processes along with the launch of an environmental impact study. FCC would intersect 31 municipalities in France and 10 in Switzerland. Detailed work is ongoing to identify and reserve plots of land for surface sites, address site-specific design aspects, and explore socio-economic and ecological opportunities such as waste-heat utilisation.
