The recent Future Circular Collider (FCC) workshop, held online from 9 to 13 November, brought together roughly 500 scientists, engineers and stakeholders to prepare a circular-collider-oriented roadmap towards the realisation of the vision of the European strategy for particle physics: to prepare a Higgs factory followed by a future hadron collider with sensitivity to energy scales an order of magnitude higher than at the LHC.
The meeting combined the fourth FCC physics week with the kick-off event for the EU-funded Horizon 2020 FCC Innovation Study (FCCIS). A successor to the previous EuroCirCol project, which was completed in 2019 and supported the preparation of the FCC conceptual design report (CDR), it will support the preparation of a feasibility study of a 100 km-circumference collider that could host an intensity- frontier electron–positron Higgs and electroweak factory (FCC-ee), followed by a 100 TeV energy-frontier hadron collider (FCC-hh) – an integrated scheme that EuroCirCol showed to be doable “in principle”. Key advantages of the FCC design are the multiple interaction points, high beam luminosities and long-term science mission covering both precision and energy frontiers over several decades (see FCC-ee: beyond a Higgs factory). The design must now be validated. “The feasibility study of FCC is particularly challenging and will require the hard work, dedication and enthusiasm of the full FCC community,” noted CERN Director-General Fabiola Gianotti.
The main goal of the study, said FCC-study project-leader Michael Benedikt, is to demonstrate the practical feasibility of delivering the unprecedented luminosities and precise energy-calibration capabilities of the proposed electroweak factory in a modular fashion. The study will also incorporate a socio-economic impact analysis and an implementation plan for an infrastructure that could fit in the global research landscape, he said. The feasibility study – a “CDR++” – will be prepared by 2025/2026, in time for the next strategy update.
A key consideration for FCC-ee that was discussed at the meeting is the development of a complete collider design with full beam-dynamic simulations and a complete injector. Continuous top-up injection, from a full-energy booster ring installed next to the collider, will lead to stable operation and maximum integrated luminosity, offering availability for physics runs of more than 80%. A series of tests in research facilities around Europe, including at PETRA-III (DESY), KARA (KIT), DAΦNE (Frascati), and potentially other facilities such as VEPP-4M (BINP), will provide the opportunity to validate the concepts. Developing a staged superconducting radio-frequency system is another major challenge. Multi-cell 400 MHz Nb/Cu cavities required for the Higgs-factory operation mode will be available within five years, alongside a full cryomodule. A mock-up of a 25 m-long full-arc half-cell of the FCC-ee is expected for 2025. Such cells will cover about 80 km of FCC-ee’s 100 km circumference.
Physics-analysis questions were also at the forefront of participants’ minds. “We are confronted with three deep and pressing questions when we observe our universe,” noted ECFA chair Jorgen D’Hondt. “What is the mechanism responsible for the transition from massless to massive particles? What are the processes that lead to the breaking of symmetry between particles and antiparticles? And how is the observed universe connected to what remains invisible to us?” Theorist Christopher Grojean (DESY) showed that electroweak, Higgs and flavour data from FCC-ee, in conjunction with astrophysical and cosmological observations, have the potential to break through the armour of the Standard Model and begin to tackle these questions. Discussions explored the need to halve theoretical uncertainties and hone detector designs to match the high statistical precision offered by the FCC-ee, and the possibility of complementing FCC-ee with a linear collider such as the proposed International Linear Collider, which could access higher energies.
The November FCC workshop paved the way for progress beyond the state-of-the-art in a variety of areas that could ensure the sustainable and efficient realisation of a post-LHC collider. A strong message from the workshop was that the FCC feasibility study must be a global endeavour that attracts industrial partners to co-develop key technologies, and inspires the next generation of particle physicists.