The recently released recommendations of the European Strategy Group discuss Linear Colliders as technologically mature flagship projects which offer a competitive science program in Higgs and Top physics, while not ranking alternative projects beyond the FCC-ee. The next steps for Linear Colliders will be discussed at the LinearCollider@CERN Workshop in early January.
by Jenny List, Tatsuya Nakada and Steinar Stapnes
On the 12th of December, CERN released the Recommendations by the European Strategy Group (ESG) for the update of the European Strategy for Particle Physics (ESPPU). This process was launched by CERN Council in March 2024. Milestones have included the collection of community input until the end of March 2025, the Open Symposium in Venice in June 2025, the publication of the Physics Briefing Book in September 2025 and the ESG’s Assessment of large-scale projects proposed for CERN – and most recently the drafting session of the ESG, held in Ascona December 1-5. The resulting recommendations were presented to CERN Council last week. Council for now took note, and will assess them in the coming months, with aiming to take a decision in May 2026, at a dedicated Session to be held in Budapest.
With regard to a future large-scale project at CERN after the HL-LHC, the ESG was charged with identifying the preferred option for the next collider at CERN and prioritised alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive. As easily anticipated, the now published recommendations very clearly identify the circular electron-positron collider FCC-ee with a cost of about 15 billion Swiss Francs (BCHF) as the preferred option for the next collider at CERN. As preferred alternative option, they propose a descoped version of FCC-ee, aiming to reduce the costs by about 15% by decreasing the number of experiments from four to two, limiting the centre-of-mass energy to 240 GeV, and reducing the RF power by an unspecified amount. The recommendations refrain from ranking other options, however they point out that neither LEP3 nor LHeC would be a flagship collider, and would need to be complemented by an energy-frontier machine, such as a hadron collider.
With regard to Linear Colliders, the ESG recognized that they offer programmes in Higgs and Top-quark physics competitive to FCC-ee when operated at two energy stages and up to 550 GeV centre-of-mass energy, while stressing the absence of a competitive flavour physics program. They note that the CLIC technology could offer a more cost-effective implementation of a Linear Collider, while SCRF technology is more mature. Starting from a Linear Collider, they consider plasma wakefield acceleration as the only option for exploring the 10-TeV parton centre-of-mass energy scale – clearly a very different view from the one we expressed in the strategy submissions of the LCVision team, which consider a Linear Collider compatible with a variety of future paths and emphasize flexibility with regard to scientific and technological developments of the next decades as strong asset.
Following their prior assesment of large-scale projects, the ESG considers neither a hadron collider nor a muon collider, which is favoured as the future target by our colleagues in the US as the main option for their next collider project. However the ESG recommends that next to R&D on advanced superconducting and normal-conducting RF structures, efficient RF power sources and accelerator-quality high-field magnets, also energy recovery linacs and advanced technologies, such as high-gradient wakefield acceleration and those underpinning bright muon beams, should be supported at an appropriate level.
The ESG also stresses that the exploitation of HL-LHC remains the highest priority, and recommends support for a diverse spectrum of non-collider experiments, for detector R&D, theory, computing and technology transfer and comments on cooperation with other European particle physics labs, relations with other fields, sustainability, as well as outreach, communication and early career researchers.
What do these recommendations imply for the Linear Collider community? A definitive answer to this question will take time. The LinearCollider@CERN Workshop in early January will be an important opportunity to discuss the future, while it will take a few months to understand how CERN Council and the incoming CERN management will implement these recommendations. Nevertheless, let us already today share a few observations:
- With the descoped FCC-ee, the ESG chose as preferred alternative an option which was not among the inputs submitted to the ESPPU and thus neither included in the Physics Briefing Book, nor well documented in terms of scope, nor assessed by the ESG working groups. The scientific impact of the suggested measures – limiting the centre-of-mass energy to below the Top pair production threshold, reducing of the number of experiments and thus the integrated luminosity by about a factor of two, times a further reduction of the in instantaneous luminosity per interaction point from an unspecified decreasing of the radiofrequency (RF) system power – still needs to be presented in detail, but obviously will have significant consequences for Higgs physics, Top physics and multi-gauge boson production – all topics which for many years have been considered the highest priority for particle physics.
In terms of financial feasibility, a cost reduction of 15% is about half of the size of the uncertainty of the current cost estimates for FCC-ee[1] . Therefore this preferred alternative, despite the very significant physics descoping, does not offer a real alternative for the case that a substantially more affordable project is required. If costs were to rise, then an intended full FCC-ee may well anyhow default to this descoped version.
- A first-stage Linear Collider would cost around 7 BCHF in the very lowest-cost scenario and up to 9 BCHF when choosing the most mature technology and providing for higher luminosity as well as easier upgradability. This would offer a much more affordable path to deliver at least the Higgs physics program of the descoped FCC-ee. For the full price of the descoped FCC-ee, a Linear Collider could reach sufficient energy to pursue an excellent Top physics programme and to access di-Higgs production and thereby improve our understanding of the Higgs potential significantly beyond what we will learn from HL-LHC. The wish to build a tunnel infrastructure which could be used for a hadron collider in the future seems to carry a large weight in the choice of the ESG.
- The CERN press release explains that a decision by CERN Council on the possible construction of the FCC is expected in 2028. This acknowledges the strong wish in the particle physics community, and in particular among the younger generations, for a timely decision on the next flagship project for CERN – essential to minimize the “dark time” between the end of HL-LHC data-taking and the start of operation of a new collider. In order to avoid delays, it seems essential that until then, both the preferred flagship and a real alternative project are advanced to towards decision readiness. We strongly hope that this aspect will be considered in the implementation of the recommendations.
In summary, the “preferred alternative” nominated by the ESG is not really an alternative project, but rather a staged path to the full programme with a relatively small cost reduction in the first stage. On the other hand, recommendations on those projects that are actually different from FCC-ee, although not explicitely ranked by the ESG, make it quite clear that Linear Colliders are technologically mature and offer a competitive science program in Higgs and Top physics. We now need so see how this important insight will be reflected in the further discussions in CERN Council and in the first steps towards implementation of the strategy by CERN management.
LCVision team, CLIC and ILC IDT, consider realization of a linear collider is crucial for the future of particle physics. We would like to encourage you to join us in January for the LinearCollider@CERN workshop – in person or on zoom – to help substanciating the workplan for a Linear Collider Facility, taking into various technologies offering a forward-looking upgrade path. Until then we wish you a peaceful and relaxing end-of-the year break and hope to see you all next year in good spirit to discuss the future of Linear Colliders.
[1] FCC-ee cost estimate assessed as being class 3 (-10% / +30%) or class 4 (-15% / + 50%) for all major subsystems in ESG WG2a report.
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