Comparative evaluation of future collider options

In anticipation of the completion of the High-Luminosity Large Hadron Collider (HL-LHC) programme by the end of 2041, CERN is preparing to launch a new major facility in the mid-2040s. According to the 2020 update of the European Strategy for Particle Physics (ESPP), the highest-priority next collider is an electron-positron Higgs factory, followed in the longer term by a hadron-hadron collider at the highest achievable energy. The CERN directorate established a Future Colliders Comparative Evaluation working group in June 2023. This group brings together project leaders and domain experts to conduct a consistent evaluation of the Future Circular Collider (FCC) and alternative scenarios based on shared assumptions and standardized criteria. This report presents a comparative evaluation of proposed future collider projects submitted as input for the Update of the European Strategy for Particle Physics. These proposals are compared considering main performance parameters, environmental impact and sustainability, technical maturity, cost of construction and operation, required human resources, and realistic implementation timelines. An overview of the international collider projects within a similar timeframe, notably the CEPC in China and the ILC in Japan is also presented, as well as a short review of the status and prospects of new accelerator techniques.

💡 Research Summary

The CERN Future Colliders Comparative Evaluation Working Group presents a systematic, side‑by‑side assessment of the major accelerator proposals that could be operational in the mid‑2040s to early 2050s, and of longer‑term concepts extending beyond 2050. The study is framed by the 2020 European Strategy for Particle Physics, which prioritises an electron‑positron Higgs factory followed, in the longer term, by the highest‑energy hadron collider. Using a common set of criteria—physics performance (energy, luminosity, number of interaction points), environmental impact (carbon footprint, electricity consumption), technical readiness (Technology Readiness Level, required R&D), construction and installation cost (with defined cost‑class uncertainties), human‑resource requirements (full‑time‑equivalent years), project schedule (key decision points, design‑construction‑commissioning timeline), and operational cost (power and personnel)—the report evaluates each option on an equal footing.

Circular e⁺e⁻ colliders (FCC‑ee)
FCC‑ee is a 100 km circular machine covering centre‑of‑mass energies from the Z‑pole (≈91 GeV) to the top‑pair threshold (≈350 GeV). Its design delivers unprecedented integrated luminosities (up to 10³⁵ cm⁻² s⁻¹ at the Z‑pole) thanks to multiple interaction points and a kHz‑scale revolution frequency. The synchrotron‑radiation loss limits the maximum energy to the t‑t̄ threshold, but within that range the collider provides a seamless energy scan and a factor of two to three higher luminosity per unit of electricity compared with linear designs. The estimated construction cost is about €30 billion, dominated by civil engineering (≈40 %) and the 16 T high‑field superconducting magnet system (≈30 %). Annual electricity demand ranges from 1.2 to 1.9 TWh depending on the operating energy, which is comparable to the current CERN consumption because the French grid supplies low‑carbon power. The technical maturity is assessed at TRL 7‑8, with detailed cost and site studies already completed (cost class 3). Human‑resource needs for construction are 10 000–15 000 FTE‑years, largely scheduled after the HL‑LHC programme ends. A realistic schedule places the start of construction around 2032, with commissioning targeted for 2045.

Linear e⁺e⁻ colliders (CLIC and LCF)
CLIC is a two‑beam, high‑frequency (12 GHz) linear accelerator designed for an initial 380 GeV stage and upgrade paths to 1.5 TeV and 3 TeV. Its projected luminosity is 1.5 × 10³⁴ cm⁻² s⁻¹, and the power consumption is about 0.8 TWh per year. The baseline construction cost is roughly €22 billion, the lowest among the three options, but any energy upgrade would require additional civil works. Technical readiness is also TRL 7‑8, with key challenges in maintaining nanometre‑scale beam sizes at the final focus and controlling beam‑strahlung‑induced luminosity spectra.

LCF (Linear Collider Facility) builds on the ILC design but adopts the 1.3 GHz superconducting RF technology already industrialised for the European XFEL and LCLS‑II. Its baseline energy is 250 GeV with an upgrade to 500 GeV. Because the SRF cavities are at TRL 9, LCF enjoys the highest technology readiness among the linear options. Power consumption is similar to CLIC (≈0.8 TWh/yr) and the cost is comparable, with the added benefit of a mature industrial supply chain.

Both linear concepts face common R&D issues: high‑gradient RF structures, positron source intensity, damping‑ring emittance preservation, and ultra‑stable final‑focus optics. Their operation avoids synchrotron‑radiation limits, allowing higher centre‑of‑mass energies, but the need for extremely small beam spots and the broadening of the luminosity spectrum due to beam‑strahlung remain critical technical risks.

Long‑term hadron collider (FCC‑hh)
FCC‑hh is a 100 km circular proton‑proton collider targeting a centre‑of‑mass energy of 100 TeV. The design requires a new generation of 16 T (potentially 20 T with HTS) superconducting dipoles, massive cryogenic infrastructure, and a power demand of 3–4 TWh per year. The projected construction cost exceeds €30 billion, with civil engineering and magnet production as the dominant items. The schedule envisions a design phase through the early 2030s, construction from 2035 to 2055, and commissioning thereafter. Technical readiness is at TRL 6‑7; substantial R&D on high‑field magnets, beam‑dump systems, and radiation protection is still required.

Muon Collider
A muon‑based collider offers the prospect of multi‑TeV lepton collisions in a relatively compact ring because muons radiate far less than electrons. The concept studied here targets a centre‑of‑mass energy of 10 TeV. However, the short muon lifetime imposes stringent requirements on rapid acceleration, ionisation cooling, and precise beam handling. The current TRL is 5–6, and a realistic operational date lies beyond 2060. The report highlights the need for a dedicated international R&D programme on muon production, cooling channels, and high‑gradient acceleration.

Re‑using the LHC tunnel (LEP3, HE‑LHC, LHeC)
The group also evaluates options that would reuse the existing 27 km LHC tunnel. LEP3 would be an e⁺e⁻ Higgs factory, HE‑LHC a higher‑energy proton collider (≈27 TeV), and LHeC an electron‑proton collider. These concepts reduce civil‑engineering costs but inherit limitations from the tunnel circumference, especially synchrotron‑radiation for circular e⁺e⁻ machines and magnetic field limits for hadron machines.

International landscape
The report compares the European proposals with the Chinese Circular Electron‑Positron Collider (CEPC) and the Japanese International Linear Collider (ILC). CEPC aims for a similar performance to FCC‑ee but with a different cost‑sharing model and a target start‑up around 2035‑2040. ILC has completed its Technical Design Report and is awaiting governmental approval; its schedule is uncertain, but the technology (SRF) aligns closely with LCF.

New acceleration techniques
Emerging concepts such as plasma wakefield acceleration (PWFA) and high‑temperature superconductors (HTS) are surveyed. PWFA promises GV/m gradients, potentially shrinking future collider lengths and costs, but remains at the proof‑of‑principle stage. HTS could enable >20 T dipoles, reducing the number of magnets needed for a given energy, yet large‑scale manufacturing and quench protection are open challenges. The group recommends sustained R&D investment to keep these options viable for the post‑FCC‑hh era.

Conclusions and roadmap
The Working Group proposes a staged European strategy:

1. Phase 1 (2045‑2050): Build and operate a circular e⁺e⁻ Higgs factory (FCC‑ee) or, alternatively, the LCF linear collider, to deliver high‑precision Higgs measurements, electroweak studies, and a platform for detector and computing innovations.

2. Phase 2 (2055‑2065): Construct the 100 TeV proton‑proton collider (FCC‑hh) to explore the energy frontier, leveraging the infrastructure, expertise, and trained workforce developed during Phase 1.

3. Parallel R&D: Continue development of CLIC, muon‑collider concepts, and advanced acceleration techniques (PWFA, HTS) to diversify the long‑term portfolio and mitigate technical risk.

The report emphasizes that FCC‑ee currently offers the best balance of scientific reach, technical maturity, cost certainty, and environmental sustainability, while providing a natural stepping stone to the ultimate hadron‑collider goal. International collaboration with CEPC and ILC projects is encouraged to maximise synergies in technology, detector development, and human resources. The recommended timeline calls for decisive governance and funding decisions between 2028 and 2030 to keep the 2045 target realistic.