This year’s Future Circular Collider (FCC) Week took place online from 28 June to 2 July, attracting 700 participants from all over the world to debate the next steps needed to produce a feasibility report in 2025/2026, in time for the next update to the European Strategy for Particle Physics in 2026/2027. The current strategy, agreed in 2020, sets an electron–positron Higgs factory as the highest priority facility after the LHC, along with the investigation of the technical and financial feasibility of such a Higgs factory, followed by a high-energy hadron collider placed in the same 100 km tunnel. The FCC feasibility study will focus on the first stage (tunnel and e+e– collider) in the next five years.
Although the FCC is a long-term project with a horizon up to the 22nd century, its timescales are rather tight. A post-LHC collider should be operational around the 2040s, ensuring a smooth continuation from the High-Luminosity LHC, so construction would need to begin in the early 2030s. Placement studies to balance geological and territorial constraints with machine requirements and physics performance suggest that the most suitable scenarios are based on a 92 km-circumference tunnel with eight surface sites.
The next steps are subsurface investigations of high-risk areas, surface-site initial-state analysis and verification of in-principle feasibility with local authorities. A “Mining the Future” competition has been launched to solicit ideas for how to best use the nine million cubic metres of molasse that would be excavated from the tunnel.
The present situation in particle physics is reminiscent of the early days of superconductivity
A highlight of the week was the exploration of the physics case of a post-LHC collider. Matthew Reece (Harvard University) identified dark matter, the baryon asymmetry and the origin of primordial density perturbations as key experimental motivations, and the electroweak hierarchy problem, the strong CP problem and the mystery of flavour mixing patterns as key theoretical motivations. The present situation in particle physics is reminiscent of the early days of superconductivity, he noted, when we had a phenomenological description of symmetry breaking in superconductivity, but no microscopic picture. Constraining the shape of the Higgs potential could allow a similar breakthrough for electroweak symmetry breaking. Regarding recent anomalous measurements, such as those of the muon’s magnetic moment, Reece noted that while these measurements could give us the coefficients of one higher dimension operator in an effective-field-theory description of new physics, only colliders can systematically produce and characterise the nature of any new physics. FCC-ee and FCC-hh both have exciting and complementary roles to play.
A key technology for FCC-ee is the development of efficient superconducting radio-frequency (SRF) cavities to compensate for the 100 MW synchrotron radiation power loss in all modes of operation from the Z pole up to the top threshold at 365 GeV. A staged RF system is foreseen as the baseline scenario, with low-impedance single-cell 400 MHz Nb/Cu cavities for Z running replaced by four-cell Nb/Cu cavities for W and Higgs operation, and later augmented by five-cell 800 MHz bulk Nb cavities at the top threshold.
As well as investigations into the use of HIPIMS coating and the fabrication of copper substrates, an innovative slotted waveguide elliptical (SWELL) cavity design was presented that would operate at 600 or 650 MHz. SWELL cavities optimise the surface area, simplify the coating process and avoid the need for welding in critical areas, which could reduce the performance of the cavity. The design profits from previous work on CLIC, and may offer a simplified installation schedule while also finding applications outside of high-energy physics. A prototype will be tested later this year.
Several talks also pointed out synergies with the RF systems needed for the proposed electron–ion collider at Brookhaven and the powerful energy-recovery linac for experiments (PERLE) project at Orsay, and called for stronger collaboration between the projects.
Machine design
Another key aspect of the study regards the machine design. Since the conceptual design report last year, the pre-injector layout for FCC-ee has been simplified, and key FCC-ee concepts have been demonstrated at Japan’s SuperKEKB collider, including a new world-record luminosity of 3.12 × 1034 cm–2 s–1 in June with a betatron function of βγ* = 1 mm. Separate tests squeezed the beam to just βγ* = 0.8 mm in both rings.
Other studies reported during FCC Week 2021 demonstrated that hosting four experiments is compatible with a new four-fold symmetric ring. This redundancy is thought to be essential for high-precision measurements, and different detector solutions will be invaluable in uncovering hidden systematic biases. The meeting also followed up on the proposal for energy-recovery linacs (ERLs) at FCC-ee, potentially extending the energy reach to 600 GeV if deemed necessary during the previous physics runs. First studies for the use of the FCC-ee booster as a photon source were also presented, potentially leading to applications in medicine and industry, precision QED studies and fundamental-symmetry tests.
Participants also tackled concepts for power reduction and power recycling, to ensure that FCC is sustainable and environmentally friendly. Ideas relating to FCC-ee include making the magnets superconducting rather than normal conducting, improving the klystron efficiency, using ERLs and other energy-storage devices, designing “twin” dipole and quadrupole magnets with a factor-two power saving, and coating SRF cavities with a high-temperature superconductor.
All in all, FCC Week 2021 saw tremendous progress across different areas of the study. The successful completion of the FCC Feasibility Study (2021–2025) will be a crucial milestone for the future of CERN and the field.
