From the latest accelerator designs to their estimated cost and long-term societal returns, the Courier gathers the key takeaways so far from the Future Circular Collider feasibility study.
It’s exactly 10 years since 350 physicists and engineers met at the University of Geneva to kick-off the Future Circular Collider (FCC) study. A response to the 2013 European strategy for particle physics, the study initially examined options for an energy-frontier collider in a new 80–100 km-circumference tunnel. By late 2018 a conceptual design report (CDR) integrating the physics, detector, accelerator and infrastructure of a staged lepton (FCC-ee) and hadron (FCC-hh) collider was published. Two years of lengthy deliberations later, the 2020 European strategy recommended that the community investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an e+e– Higgs and electroweak factory as a possible first stage.
Studies show that the FCC would deliver benefits that outweigh its costs
After three years of work, mobilising the expertise of physicists and engineers from around the world, a mid-term report of the FCC feasibility study was completed in December 2023. Numerous technical documents and a 700-page overview of the results demonstrate significant progress across all project deliverables, including physics opportunities, the placement and implementation of the ring, civil engineering, technical infrastructure, accelerators, detectors and cost. No technical showstoppers have been identified, and the results were received positively by the CERN Council during a special session on 2 February. Here and in the some related articles, the Courier gathers the key take-aways.
A collider for the times
The scientific backdrop to the FCC is the existence of a 125 GeV Higgs boson together with no sign yet of new elementary particles at the TeV scale – transformational discoveries by the LHC that call for a broad and versatile exploration tool with unprecedented precision, sensitivity and energy reach (see “FCC: the physics case“). An unfathomable amount of work has led to an optimal placement of the FCC ring, surface sites and project implementation with CERN’s host states (see “Where and how“). The 90.7 km FCC tunnel, constituting a major global civil-engineering project in its own right, is well understood (see “Tunnelling to the future“). Assuming a decision to advance to the next stage is taken by the CERN Council after the next European strategy process, a preparatory phase (involving project authorisation, preparation of civil-engineering works, technical design for the collider, injectors and the detectors, further consolidation of physics cases and detector development) would take place from 2026 to 2032. Construction could then take place in 2033–2040, with the installation phase and transition to operation between 2038 and the mid-2040s.
The multi-energy lepton collider FCC-ee, which would produce huge quantities of Z, W and Higgs bosons, and ultimately top-quark pairs, over a period of about 15 years, builds on the remarkable success of LEP, which was instrumental in confirming the Standard Model and in guiding physicists to the discoveries of the top quark and the Higgs boson. Once thought to be the final word on circular e+e– colliders, advances in accelerator technology since LEP (such as top-up injection at B factories and synchrotron-radiation light sources, developments in superconducting RF, and novel beam-focusing techniques) offer collision rates more than two orders of magnitude larger. Boosting the FCC-ee luminosity further, a key outcome of the mid-term report is a new ring-layout that enables four interaction points.
Ideal springboard
The mid-term report confirms that FCC-ee is both a mature design for a Higgs, electroweak and top factory, and an ideal springboard for an energy-frontier collider, FCC-hh, for which it would provide a significant part of the infrastructure. Since the revised FCC-ee placement studies, the overall layout of FCC-hh has changed radically compared to the initial concept phase, with three key benefits: an optimal size of the experiment caverns, with the option of sharing detector components between the lepton and the hadron machines; a reduction in the number of surface sites; and a shorter tunnel for the transfer lines from the injector to the collider ring. The new layout is compatible with an injection scheme that delivers beams to the FCC-hh ring from the LHC or from an upgrade of the SPS.
The mid-term report addresses the challenging R&D for the high-field FCC-hh magnets. A key deliverable of the feasibility study is a summary of R&D plans based on Nb3Sn, high-temperature superconductors (HTS) and hybrid technologies. While Nb3Sn magnets are considered relatively low-risk, HTS technology would enable the most aspirational goals to be reached. Due to the sizable gap in technology readiness between the two options, however, the study team advises against an early decision. Instead, an adapted “phase-gate” process is proposed with regular review, steering and decision points every five years, and coordinated with the CERN high-field magnet programme. Taking into account the time needed to construct and operate FCC-ee and, in parallel, to develop the high-field dipole magnet technology, it is estimated that FCC-hh could begin physics operations in the early 2070s.
The cost of an FCC-ee with four interaction points is estimated to be CHF 15 billion, around a third of which is taken up by the tunnel. The reliability of the FCC-ee cost estimate will be improved following further development of the various accelerator systems and equipment required, along with the subsurface investigations starting in 2024. The final feasibility-study report will also address risk-management and the personnel resources required from project development to construction.
Power consumption is another topic of interest. The FCC-ee will be the largest particle accelerator ever built, with its RF, magnet and cryogenic systems drawing the main loads. The total CERN energy consumption throughout the FCC-ee scientific programme is estimated to vary between 2.0 and 2.8 TWh/year depending on the energy mode, to be compared with about 1.6 TWh/year during the High-Luminosity LHC era. The figures are hoped to be lowered as R&D (for example, to improve the performance of superconducting cavities and the efficiency of power sources) advances. The FCC study team is also working with regional authorities to identify ways in which part of this energy may be re-used for heating in local industries and public infrastructures.
Electrical power would be provided from the French electricity grid, and the system is designed such that no new sub-stations will need to be constructed between the different FCC-ee energy stages. Studies carried out in conjunction with McKinsey and Accenture indicate that by the time the FCC comes into operation, a low carbon footprint can be achieved with an energy mix that contains a large fraction of energy from renewable sources.
Return on investment
Beyond the creation of new knowledge, studies undertaken within the European Union co-funded FCC Innovation Study show that the FCC would deliver benefits that outweigh its cost. Impacts on industry from high-tech developments, the sustained training of early-stage researchers and engineers, the development of open and free software, the creation of spin-off companies, cultural goods and other factors lead to an estimated benefit/cost ratio of 1.66. The FCC project is linked to the creation of around 800,000 person-years of jobs, states the mid-term report, and the FCC-ee scientific programme is estimated to generate an overall local economic impact of more than €4 billion.
The digested mid-term report in summary: the FCC integrated programme is an ideal match for the uncharted physics territory ahead; its placement at CERN is geologically and territorially feasible; no technical showstoppers have been identified; the FCC would return more to society than it costs. Accelerator, detector, engineering and physics studies by the global FCC collaboration are continuing across more than 150 institutes in more than 30 countries, while new partners are sought to work on various R&D (see “The people factor” ). The final report of the FCC feasibility study is due in early 2025.
