The first stage of the Future Circular Collider (FCC) Integrated Project is a high-luminosity, high-energy electron-positron collider, serving as a Higgs, top and electroweak factory. The second stage is an energy frontier hadron collider, with a centre-of-mass energy of at least 100 TeV. This programme matches the highest priority future requests issued by the 2020 Update of the European Strategy for Particle Physics. In 2021, with the support of the CERN Council, a five-year FCC Feasibility Study was launched.

The Future Circular Collider (FCC) study has adopted an iterative approach, in collaboration with the CERN Host States, France and Switzerland, for the development of a balanced and feasible implementation scenario. The scenario considers three key aspects, namely scientific excellence delivering a world-leading research programme, territorial compatibility (respecting territorial constraints and leveraging assets as well as building synergies with local stakeholders) and finally the technical feasibility and cost control, both tightly linked to the previous two elements.

Following eight years of intense study of the implementation scenario, one specific configuration was identified out of some 100 variants as being particularly suitable to meet the three key criteria previously discussed. This scenario has a total circumference of about 90.7 km, has eight surface sites and permits installation of up to four experiments.

During 2023, this reference scenario has been reviewed with the relevant regional and local stakeholders and today serves as a baseline for further validation activities such as geophysical and geotechnical investigations. These investigations will confirm the stability of the subsurface, the connection to the national high voltage grid, the access to water for cooling purposes, the connection to major road and railway infrastructures, the development of a concrete and localised re-use and deposit plan for excavated materials, the development of synergies with local projects and the hosting regions, landscape integration and the development of sustainable mitigation measures.

Following the identification of the baseline variant discussed in the previous sections, the FCC technical and infrastructure systems have been adapted to the new layout with a 90.7 km circumference and 8 surface sites. Four of the eight surface sites are assigned to experiments (PA, PD, PG and PJ). PA and PG are large experiment areas for FCC-hh, while PD and PJ are optional experiment points that may house smaller hadron-collider experiments. The same points are also suitable for hosting the much smaller experiments of the lepton collider (FCC-ee). The other four points are technical sites. Point PB has been chosen to house the beam dump system. Points PL and PH have been selected to house the booster and collider RF, respectively. Point PF was found less appropriate to house the RF due to difficult access conditions, but it could house the collimation system. The assignment of the experimental and technical points was made based on constraints from accelerator physics, constraints at the surface areas and infrastructure constraints (accessibility, resources, electrical network connection and powering). Besides a well-defined R&D programme on Superconducting RF cavities (SRF), FCC-ee R&D plans have been formulated for several accelerator systems including warm and cold magnets, vacuum system, controls, beam intercepting devices, beam transfer systems, beam diagnostics, power converters, survey and alignment, machine protection, availability, robotics, and engineering software.

For the FCC-ee collider, the baseline scenario foresees four main running modes with centre-of-masss collision energies of about 91 GeV (Z), 160 GeV (WW), 240 GeV (ZH) and 365 GeV (t ̄t) being considered. In the scenario with 4 interaction points, the design luminosity per interaction point is about 1.40×10³⁶ cm⁻²s⁻¹ on the Z pole, 5.0 × 10³⁴ cm⁻²s⁻¹ at the Higgs (ZH) production peak, and 1.25 × 10³⁴ cm⁻²s⁻¹ at or above the t ̄t threshold. The corresponding integrated luminosity values are 17, 0.6 and 0.15 ab⁻¹ per IP and per year.

The different energy runs have their virtues and synergistically complement each other. For example, the precise determination of the W mass needs the improved knowledge of a_QED(m_Z²) from the Z-pole run, but the m_W measurement is of little use unless we have a much better measurement of m_top which itself relies on the improved knowledge of a_S(m_Z²)  from lower data points. The baseline scientific outcome will become available when all these energy settings are exploited with the maximum possible integrated luminosity (irrespective of their chronological sequence).

One of the great advantages of FCC-ee is the possibility of serving several interaction points simultaneously with a net overall gain both in integrated luminosity and luminosity per TWh (thus increasing the collider sustainability while in operation), and with a multi-faceted improvement of the science value for the overall investment. The key to success for FCC-ee is a blend of high luminosity, redundancy, and careful preparation of detector setups. The challenges arise from the very richness of the FCC-ee physics programme, and the studies already indicate that the variety of detector requirements may not be satisfied by one or even two detectors. Four experiments allow instead for a range of detector solutions to cover all physics opportunities, thus broadening the FCC attractiveness to an extended scientific community. These observations led, at the beginning of the Feasibility Study, to an optimisation of the ring layout with a new four-fold periodicity, further increasing the synergies with FCC-hh with a common set of four experimental areas for both FCC stages.

In the first half of the Feasibility Study, a preliminary list of detector requirements was derived with fast simulation from many case studies inspired by a set of adequately chosen physics benchmark processes at FCC-ee. So far, interesting results have been obtained mostly with Higgs and flavour studies, but the dominant challenge for the FCC-ee detector design remains the precision with which the basic electroweak observables can be measured. The work on the hadronic, dilepton, and diphoton cross-section measurements has begun, but much remains to be done to extract detector specifications from these studies. The necessary next step will require full simulation of the proposed detector concepts and the development of the full event reconstruction, which in turn requires a substantially larger participation of the community at large, if a detector concept evaluation is to be completed by the end of the Feasibility Study.

Theoretical challenges

Much of the theoretical and Monte Carlo code development since LEP times has been focused on hadron colliders. The FCC-ee physics programme presents a number of key and specific theoretical challenges. Generally speaking, the aim is either to provide the tools to compare experimental observations to theoretical predictions at a level of precision similar to or better than the (statistical) experimental uncertainties; or to identify the additional calculations, tools, observables, or experimental inputs that are required to achieve this level of precision. Another essential line of research to be followed jointly by theorists and experimenters is to identify observables, or ratios of observables, for which experimental and/or theoretical uncertainties can be reduced. Finally, the relative impact of the various measurements on the search for new physics should be evaluated. A community of theorists has already risen to the precision challenges, especially at the Z peak, with remarkable first results. We conclude that a clear and feasible plan is emerging for achieving the required precision in theoretical predictions and Monte Carlo event generators for the challenging demands of FCC-ee. This plan will require a great deal of advancement from the precision calculation community and support from the broader particle physics community, with adequately planned resources until the end of the Feasibility Study and for the decades that follow.

Progress in software

To cope with the unprecedented large amount of data to be collected, the experiments will need to rely on solid software tools and computing infrastructure. The driving consideration has been to develop one software “ecosystem” and one common data format for all use cases, adaptable to any future collider (e.g., FCC-ee, FCC-hh, CLIC) and usable by any future experiment (e.g., the four FCC-ee collaborations), and based on the most modern software provided by running experiments (e.g., LHC) and ongoing R&D projects (e.g., AIDA). This is being realised, and the software is now regularly used by the particle physicists for their everyday work (fast simulation, event reconstruction, and data analysis, in particular), exceeding the performance of algorithms previously used for linear collider studies. Further outstanding developments are due to comprehensively simulate the interaction region (a.k.a. Machine-Detector Interface) considering the evaluation and mitigation of the beam-related backgrounds; to proceed with the full simulations of the relevant sub-detectors currently considered, to enable the study of a large variety of detector concepts; and to evaluate the need for computing resources until the end of the Feasibility Study, for the preparation of the detector TDRs, and during the FCC operation. These tasks require a strong core team at CERN and sustained participation from the community, as well as adequate computing resources.

Coda

Civil engineering work for the new infrastructure could start in the 2030s with the first collisions in FCC-ee expected in 2048, just after the end of the HL-LHC programme. To meet this timeline, the particle physics community will need to submit, in time for the next European Strategy Update, expressions of interest for sub-detector technologies towards the realisation of four experiments. The efforts will be progressively transferred from the small central group dedicated to delivering the Feasibility Study to the wider particle physics community. Meanwhile, it is the task of the physics, experiments, and detectors (PED) central group to prepare the grounds for the community work. One of the PED's main missions has been, is, and will be to support and encourage the creation of a worldwide consortium of scientific contributors and national contacts that can reliably commit resources to the development of the FCC-ee science project.

Ten years after the discovery of the Higgs boson, FCC-ee has become a mature Higgs/electroweak/flavour/top factory project with a broad and sharp physics case. Such a machine offers ideal conditions (luminosity, centre-of-mass energy calibration, possibly monochromatisation) for the study of the four heavy particles of the Standard Model with a flurry of opportunities for precision measurements, searches for rare or forbidden processes, and the possible discovery of feebly coupled particles. The FCC-ee is also a perfect springboard for FCC-hh, for which it provides a great part of the infrastructure as well as the necessary knowledge of the low-energy landscape to fully exploit the physics potential of the wealth of data to be collected at higher energies.

For reasons that are more historical than scientific, the particle physics community is today scattered over multiple Higgs factory projects. If not addressed today, this confusing situation could prevent convergence towards a strong recommendation from the next update of the ESU, which possibly pernicious effects on the future of particle physics in Europe.