FCC Physics, Experiments and Detectors: from Strategy Update to Project Preparation
by Patrick Janot, on behalf of the FCC PED Coordination Team
The 2026 European Strategy for Particle Physics Update (ESPPU) has changed our objective. We are no longer studying the FCC-ee. We are preparing to build it.
For everyone involved in the FCC Physics, Experiments and Detectors (PED) programme, this is much more than a semantic change. Fifteen years ago, the idea of a high-luminosity circular e+e– Higgs factory at CERN was little more than a bold proposal from a handful of enthusiasts, put forward even before the Higgs boson had been discovered. Since then, that vision has progressively grown into an ever-larger international effort, attracting broad institutional support, and has evolved into today’s FCC project. The Strategy update marked the culmination of that first chapter. Our task during the Reference Design Phase (RDP; 2026–2028) is now fundamentally different: to transform that vision into a project that can be confidently approved, successfully delivered and equipped to fully exploit FCC-ee’s extraordinary scientific potential.
This shift changes the priorities of the PED programme. Of course, physics studies remain our compass, but they must now increasingly inform concrete design decisions. The challenge is formidable: the FCC-ee detectors must satisfy an exceptionally broad set of requirements arising from both the physics programme and the collider itself. No single detector concept is expected to excel in every aspect, and the overall scientific programme will rely on the complementarity of four experiments.
It is precisely this diversity of requirements that motivates the coexisting studies of several detector concepts. Their primary purpose is to identify complementary detector solutions capable of satisfying these demanding, and often competing, requirements. By exploring different combinations of subsystem technologies, they expose the trade-offs between physics performance, engineering feasibility, machine integration and long-term operability. During the RDP, these concepts will progressively mature through detailed engineering, targeted R&D and realistic simulation within a common software framework, allowing meaningful cross-comparisons. The objective is no longer to demonstrate what the FCC-ee can achieve, but to ensure that it can actually deliver the unprecedented precision on which its scientific programme rests.
The response during the Feasibility Study was remarkable, with 39 preliminary proposals for FCC−ee detector development, highlighting the strength of the international base. Since then, a comprehensive working-group structure has been established, communication tools have matured, specialised workshops have multiplied and new detector concepts continue to emerge. Today, this endeavour is articulated around studies of six concepts, complemented by common subsystem activities that foster technological innovation while maintaining strong interactions across concepts.
This structure is designed to deliver the next major milestone: a broad portfolio of detector Expressions of Interest (EoIs) by the end of the RDP, ready for formal submission following project approval. These EoIs are not intended to determine the experiments that will ultimately be built. Rather, they will provide the framework within which ideas can mature, technologies can be compared, engineering constraints can be weighed against physics ambitions, and the necessary R&D can be identified. This evolution also provides a clear direction for the ECFA Detector R&D (DRD) collaborations. Following the overwhelming backing of the FCC-ee in the 2026 ESPPU, the DRD activities supporting future collider detectors are expected to increasingly align with the needs of the FCC-ee. The objective is no longer to develop technologies in isolation, but to understand and address the concrete challenges revealed by the FCC-ee detector concepts.
Closer and more regular interactions between the DRDs and the FCC detector subsystem and concept groups will ensure that R&D priorities are driven by well-defined experimental requirements and that successful developments can rapidly be incorporated into increasingly mature detector designs. Equally important is the sharing of geometries, digitisation and reconstruction algorithms, together with detailed test-beam data, within the common FCC software ecosystem
Perhaps the most important challenge, however, is no longer technical. It is organisational.
Detector concepts must not yet be regarded as experiment collaborations. Just as during the preparation of the LHC experiments, today’s detector concepts are expected to provide the foundations from which tomorrow’s experiment collaborations will emerge. In this respect, ECFA can again play a uniquely important role. The history of the LHC demonstrates the value of ECFA workshops open to the entire field, where detector concepts are critically discussed, common solutions emerge and collaborations naturally form, as happened in Evian in 1992. A similar process, following project approval and detector EoIs, would provide the forum needed to converge towards four robust detector collaborations and Letters of Intent by the beginning of 2030, which would be fully consistent with the FCC timeline.
Having first served as an incubator, FCC PED is now designed to nurture this evolution; ECFA can help bring it to fruition. Announcing such a roadmap early would also send a strong signal to the funding agencies and help mobilise the additional resources that the project requires. The HL-LHC upgrade experience teaches us that detectors of such complexity take about two decades from conception to installation. If the FCC-ee detectors are to be operating in the mid-2040s, the time to launch the necessary R&D, begin detailed detector design and lay the foundations for future experiment collaborations is not tomorrow – it is now.
The Strategy update has also introduced the possibility of a descoped FCC-ee as an alternative implementation scenario. This should not be viewed simply as a reduction of the project but as an invitation to explore more effective strategies. The guiding principle must be the preservation of scientific ambition while reducing the initial investment and the associated risks whenever possible. In this context, reversible staging, using time as a strategic resource, deserves articular attention. Some features, notably the top-energy run and the ultimate collider performance, can be implemented progressively, provided that the necessary upgrade paths are preserved from the outset. Others, such as reducing the number of interaction regions from four to two, are effectively irreversible, carry far-reaching consequences and should not be considered staging options.
More generally, the RDP provides an opportunity to explore a broad range of optimisations that could reduce technical risks, programme duration and construction cost. Taken together, they may offer alternatives, or valuable complements, to the proposed illustrative descoping scenario, while remaining fully consistent with the scientific objectives.
The coming two years will therefore be decisive. Detector technologies must mature into realistic engineering solutions, common software and analysis tools must become fully operational, international participation must continue to grow, and staging/contingency options must be evaluated, without compromising the long-term scientific vision. The success of the RDP will be measured not only by the quality of the technical designs that emerge, but also by our ability to bring together the talent, creativity and commitment of everyone willing to contribute.
Ultimately, the RDP is as much a human endeavour as it is a technical challenge. Technology builds detectors. People build experiments. If we aspire to four outstanding FCC-ee experiments in the mid-2040s, we must start building them today.
As it did so successfully three decades ago for the LHC, ECFA can once again act as a catalyst, working together with CERN and with institutes and universities worldwide.
This article by Patrick Janot was originally published in the 17th ECFA Newsletter (Summer 2026).