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The future circular colliders considered under the FCC study compared to previous circular colliders.

The Future Circular Collider (FCC) is a proposed particle accelerator with an energy significantly above that of previous circular colliders, such as the Super Proton Synchrotron, the Tevatron, and the Large Hadron Collider (LHC).[1][2] The FCC project is considering three scenarios for collision types: FCC-hh, for hadron-hadron collisions, including proton-proton and heavy ion collisions, FCC-ee, for electron-positron collisions, and FCC-eh, for electron-hadron collisions.[3]

In FCC-hh, each beam would have a total energy of 560 MJ. With a centre-of-mass collision energy of 100 TeV (vs 14 TeV at LHC) the total energy value increases to 16.7 GJ. These total energy values exceed the present LHC by nearly a factor of 30.[4]

CERN hosted an FCC study exploring the feasibility of different particle collider scenarios with the aim of significantly increasing the energy and luminosity compared to existing colliders. It aims to complement existing technical designs for proposed linear electron/positron colliders such as the International Linear Collider and the Compact Linear Collider.

The study explores the potential of hadron and lepton circular colliders, performing an analysis of infrastructure and operation concepts and considering the technology research and development programmes that are required to build and operate a future circular collider. A conceptual design report was published in early 2019,[5] in time for a scheduled update of the European Strategy for Particle Physics.

Background

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The CERN study was initiated as a direct response to the high-priority recommendation of the updated European Strategy for Particle Physics, published in 2013 which asked that "CERN should undertake design studies for accelerator projects in a global context, with emphasis on proton-proton and electron-positron high-energy frontier machines. These design studies should be coupled to a vigorous accelerator R&D programme, including high-field magnets and high-gradient accelerating structures, in collaboration with national institutes, laboratories and universities worldwide". The goal was to inform the next Update of the European Strategy for Particle Physics (2019–2020) and the wider physics community for the feasibility of circular colliders complementing previous studies for linear colliders as well as other proposal for particle physics experiments.

The launch of the FCC study was also in line with the recommendations of the United States' Particle Physics Project Prioritization Panel (P5) and of the International Committee for Future Accelerators (ICFA), a working group of the International Union of Pure and Applied Physics.

The discovery of the Higgs boson at the LHC, together with the absence so far of any phenomena beyond the Standard Model in collisions at centre of mass energies up to 8 TeV, has triggered an interest in future circular colliders to push the energy and precision frontiers complementing studies for future linear machines. The discovery of a "light" Higgs boson with a mass of 125 GeV revamped the discussion for a circular lepton collider[6] that would allow detailed studies and precise measurement of this new particle. With the study of a new 80–100 km circumference tunnel (see also VLHC),[7][8] that would fit in the Geneva region, it was realized that a future circular lepton collider could offer collision energies up to 400 GeV (thus allowing for the production of top quarks) at unprecedented luminosities. The design of FCC-ee (formerly known as TLEP (Triple-Large Electron-Positron Collider[9])) was combining the experience gained by LEP2 and the latest B-factories.

Two main limitations to circular-accelerator performance are energy loss due to synchrotron radiation, and the maximum value of magnetic fields that can be obtained in bending magnets to keep the energetic beams in a circular trajectory. Synchrotron radiation is of particular importance in the design and optimization of a circular lepton collider and limits the maximum energy that can be reached as the phenomenon depends on the mass of the accelerated particle. To address these issues a sophisticated machine design along with the advancement of technologies like accelerating (RF) cavities and high-field magnets are needed.

Future "intensity and luminosity frontier" lepton colliders like those considered by the FCC study would enable the study with very high precision of the properties of the Higgs boson, the W and Z bosons and the top quark, pinning down their interactions with an accuracy at least an order of magnitude better than today. The FCC-ee could collect 1012 Z bosons, 108 W pairs, 106 Higgs bosons and 4 · 105 top-quark pairs per year. As a second step, an "energy frontier" collider at 100 TeV (FCC-hh) could be a "discovery machine" offering an eightfold increase compared to the current energy reach of the LHC.

The FCC integrated project, combining FCC-ee and FCC-hh, would rely on a shared and cost effective technical and organizational infrastructure, as was the case with LEP followed by LHC. This approach improves by several orders the sensitivity to elusive phenomena at low mass and by an order of magnitude the discovery reach for new particles at the highest masses. This will allow to uniquely map the properties of the Higgs boson and Electroweak sector and broaden the exploration for different Dark Matter candidate particles complementing other approaches with neutrino beams, non-collider experiments and astrophysics experiments.

Motivation

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The LHC has advanced the science of matter and the Standard Model (SM). The discovery of the Higgs boson completed the particle-related component of the Standard Model of Particle Physics, the theory that describes the laws governing most of the known Universe. Yet the Standard Model cannot explain several observations, such as:

The LHC has inaugurated a new phase of detailed studies of the properties of the Higgs boson and the way in which it interacts with the other SM particles. Future colliders with a higher energy and collision rate will largely contribute in performing these measurements, deepening our understanding of the Standard Model processes, test its limits and search for possible deviations or new phenomena that could provide hints for new physics.

The Future Circular Collider (FCC) study develops options for potential high-energy frontier circular colliders at CERN for the post-LHC era. Among other things, it plans to look for dark matter particles, which account for approximately 25% of the energy in the observable universe.[10] Though no experiment at colliders can probe the full range of dark matter (DM) masses allowed by astrophysical observations, there is a very broad class of models for weakly interacting massive particles (WIMPs) in the GeV – tens of TeV mass scale, and which could be in the range of the FCC.

FCC could also lead the progress in precision measurements of Electroweak precision observables (EWPO). The measurements played a key role in the consolidation of the Standard Model and can guide future theoretical developments. Moreover, results from these measurements can inform data from astrophysical/cosmological observations. The improved precision offered by the FCC integrated programme increases the discovery potential for new physics.

Moreover, FCC-hh will enable the continuation of the research programme in ultrarelativistic heavy-ion collisions from RHIC and LHC. The higher energies and luminosities offered by FCC-hh when operating with heavy-ions will open new avenues in the study of the collective properties of quarks and gluons.[11]

The FCC study also foresees an interaction point for electrons with protons (FCC-eh).[12] These deep inelastic scattering measurements will resolve the parton structure with very high accuracy providing a per mille accurate measurement of the strong coupling constant. These results are essential for a programme of precision measurements and will further improve the sensitivity of search for new phenomena particularly at higher masses.

Five percent of the matter and energy in the Universe is directly observable. The Standard Model of Particle Physics describes it precisely. What about the remaining 95%?

Scope

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The FCC study originally put an emphasis on proton-proton (hadron or heavy-ion) high-energy collider that could also house an electron/positron (ee) high-intensity frontier collider as a first step. However after assessing the readiness of the different technologies and the physics motivation the FCC collaboration came up with the so-called FCC integrated programme foreseen as a first step FCC-ee with an operation time of about 10 years at different energy ranges from 90 GeV to 350 GeV, followed by FCC-hh with an operation time of about 15 years.

The FCC collaboration has identified the technological advancements required for reaching the planned energy and intensity and performs technology feasibility assessments for critical elements of future circular colliders (i.e. high-field magnets, superconductors, Radio-frequency cavities cryogenic and vacuum system, power systems, beam screen system, a.o). The project needs to advance these technologies to meet the requirements of a post-LHC machine but also to ensure the large-scale applicability of these technologies that could lead to their further industrialization. The study also provides an analysis of the infrastructure and operation cost that could ensure the efficient and reliable operation of a future large-scale research infrastructure. Strategic R&D has been identified in the CDR[13] over the coming years will concentrate on minimising construction costs and energy consumption, whilst maximising the socio-economic impact with a focus on benefits for industry and training.

Scientists and engineers are also working on the detector concepts needed to address the physics questions in each of the scenarios (hh, ee, he). The work programme includes experiment and detector concept studies to allow new physics to be explored. Detector technologies will be based on experiment concepts, the projected collider performances and the physics cases. New technologies have to be developed in diverse fields such as cryogenics, superconductivity, material science, and computer science, including new data processing and data management concepts.

Colliders

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The FCC study developed and evaluated three accelerator concepts for its conceptual design report.

FCC-ee (electron/positron)

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A lepton collider with centre-of-mass collision energies between 90 and 350 GeV is considered a potential intermediate step towards the realisation of the hadron facility. Clean experimental conditions have given e+e− storage rings a strong record both for measuring known particles with the highest precision and for exploring the unknown.

More specifically, high luminosity and improved handling of lepton beams would create the opportunity to measure the properties of the Z, W, Higgs, and top particles, as well as the strong interaction, with increased accuracy.[14][15]

It can search for new particles coupling to the Higgs and electroweak bosons up to scales of Λ = 7 and 100 TeV. Moreover, measurements of invisible or exotic decays of the Higgs and Z bosons would offer discovery potential for dark matter or heavy neutrinos with masses below 70 GeV. In effect, the FCC-ee could enable profound investigations of electroweak symmetry breaking and open a broad indirect search for new physics over several orders of magnitude in energy or couplings.

Realisation of an intensity-frontier lepton collider, FCC-ee, as a first step requires a preparatory phase of nearly 8 years, followed by the construction phase (all civil and technical infrastructure, machines and detectors including commissioning) lasting 10 years. A duration of 15 years is projected for the subsequent operation of the FCC-ee facility, to complete the currently envisaged physics programme. This makes a total of nearly 35 years for construction and operation of FCC-ee

FCC-hh (proton/proton and ion/ion)

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A future energy-frontier hadron collider will be able to discover force carriers of new interactions up to masses of around 30 TeV if they exist. The higher collision energy extends the search range for dark matter particles well beyond the TeV region, while supersymmetric partners of quarks and gluons can be searched for at masses up to 15–20 TeV and the search for a possible substructure inside quarks can be extended down to distance scales of 10−21 m. Due to the higher energy and collision rate billions of Higgs bosons and trillions of top quarks will be produced, creating new opportunities for the study of rare decays and flavour physics.

A hadron collider will also extend the study of Higgs and gauge boson interactions to energies well above the TeV scale, providing a way to analyse in detail the mechanism underlying the breaking of the electroweak symmetry.

In heavy-ion collisions, the FCC-hh collider allows the exploration of the collective structure of matter at more extreme density and temperature conditions than before.[16][17]

Finally, FCC-eh adds to the versatility of the research programme offered by this new facility. With the huge energy provided by the 50 TeV proton beam and the potential availability of an electron beam with energy of the order of 60 GeV, new horizons open up for the physics of deep inelastic scattering. The FCC-he collider would be both a high-precision Higgs factory[18] and a powerful microscope that could discover new particles, study quark/gluon interactions, and examine possible further substructure of matter in the world.

In the FCC integrated scenario, the preparatory phase for an energy-frontier hadron collider, FCC-hh, will start in the first half of the FCC-ee operation phase. After the stop of FCC-ee operation, machine removal, limited civil engineering activities and an adaptation of the general technical infrastructure will take place, followed by FCC-hh machine and detector installation and commissioning, taking in total about 10 years. A duration of 25 years is projected for the subsequent operation of the FCC-hh facility, resulting in a total of 35 years for construction and operation of FCC-hh.

The staged implementation provides a time window of 20–30 years for R&D on key technologies for FCC-hh. This could allow alternative technologies to be considered e.g. high-temperature superconducting magnets, and should lead to improved parameters and reduced implementation risks, compared to immediate construction after HL-LHC.

High-Energy LHC

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A high-energy hadron collider housed in the same tunnel but using new FCC-hh class 16T dipole magnets could extend the current energy frontier by almost a factor of 2 (27 TeV collision energy) and delivers an integrated luminosity of at least a factor of 3 larger than the HL-LHC. This machine could offer a first measurement of the Higgs self-coupling and directly produce particles at significant rates at scales up to 12 TeV – almost doubling the HL-LHC discovery reach for new physics. The project reuses the existing LHC underground infrastructure and large parts of the injector chain at CERN.

It is assumed that HE-LHC will accommodate two high-luminosity interaction-points (IPs) 1 and 5, at the locations of the present ATLAS and CMS experiments while it could host two secondary experiments combined with injection as for the present LHC.

The HE-LHC could succeed the HL-LHC directly and provide a research programme of about 20 years beyond the middle of the 21st century.

Technologies

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As the development of a next generation particle accelerator requires new technology the FCC study has studied the equipment and machines that are needed for the realization of the project, taking into account the experience from past and present accelerator projects.[19]

The FCC study drives the research in the field of superconducting materials.

The foundations for these advancements are being laid in focused R&D programmes:

  • a 16-tesla high-field accelerator magnet and related super-conductor research,
  • a 100 MW radiofrequency acceleration system that can efficiently transfer power from the electricity grid to the beams,
  • a highly efficient large-scale cryogenics infrastructure to cool down superconducting accelerator components and the accompanying refrigeration systems.
The CERN magnet group produced a 16.2-tesla peak field magnet – nearly twice that produced by the current LHC dipoles – paving the way for future more powerful accelerators.
New superconducting radiofrequency (RF) cavities are developed to accelerate particles to higher energies.

Numerous other technologies from various fields (accelerator physics, high-field magnets, cryogenics, vacuum, civil engineering, material science, superconductors, ...) are needed for reliable, sustainable and efficient operation.

Magnet technologies

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High-field superconducting magnets are a key enabling technology for a frontier hadron collider. To steer a 50 TeV beam over a 100 km tunnel, 16 tesla dipoles will be necessary, twice the strength of the magnetic field of the LHC.

Evolution of superconducting niobium-titanium magnets for particle accelerator use.

The main objectives of R&D on 16 T Nb3Sn dipole magnets for a large particle accelerator is to prove that these types of magnets are feasible in accelerator quality and to ensure an adequate performance at an affordable cost. Therefore the goals are to push the conductor performance beyond present limits, to reduce the required "margin on the load line" with consequent reduction of conductor use and magnet size and the elaboration of an optimized magnet design maximizing performance with respect to cost.[20][21]

The magnet R&D aims to extend the range of operation of accelerator magnets based on low-temperature superconductors (LTS) up to 16 T and explore the technological challenges inherent to the use of high-temperature superconductors (HTS) for accelerator magnets in the 20 T range.

Superconducting radiofrequency cavities

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The beams that move in a circular accelerator lose a percentage of their energy due to synchrotron radiation: up to 5% every turn for electrons and positrons, much less for protons and heavy ions. To maintain their energy, a system of radiofrequency cavities constantly provides up to 50 MW to each beam. The FCC study has launched dedicated R&D lines on novel superconducting thin-film coating technology will allow RF cavities to be operated at higher temperature (CERN, Courier, April 2018),[22][23] thereby lowering the electrical requirement for cryogenics, and reduce the required number of cavities thanks to an increase in the accelerating gradient. An ongoing R&D activity, carried out in close cooperation with the linear collider community, aims at raising the peak efficiency of klystrons from 65% to above 80%. Higher-temperature high-gradient Nb-Cu accelerating cavities and highly-efficient RF power sources could find numerous applications in other fields.

Cryogenics

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Liquefaction of gas is a power-intensive operation of cryogenic technology. The future lepton and hadron colliders would make intensive use of low-temperature superconducting devices, operated at 4.5 K and 1.8 K, requiring very large-scale distribution, recovery, and storage of cryogenic fluids.

Improving refrigeration cycle efficiency from 33% to 45% leads to 20% reduced cost and power.

As a result, the cryogenic systems that have to be developed correspond to two to four times the presently deployed systems and require increased availability and maximum energy efficiency. Any further improvements in cryogenics are expected to find wide applications in medical imaging techniques.

The cryogenic beam vacuum system for an energy-frontier hadron collider must absorb an energy of 50 W per meter at cryogenic temperatures. To protect the magnet cold bore from the heat load, the vacuum system needs to be resistant against electron cloud effects, highly robust, and stable under superconducting quench conditions.

It should also allow fast feedback in the presence of impedance effects. New composite materials have to be developed to achieve these unique thermo-mechanical and electric properties for collimation systems. Such materials could also be complemented with the ongoing exploration of thin-film NEG coating that is used in the internal surface of the copper vacuum chambers.

Collimation

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A 100 TeV hadron collider requires efficient and robust collimators, as 100 kW of hadronic background is expected at the interaction points. Moreover, fast self-adapting control systems with sub-millimeter collimation gaps are necessary to prevent irreversible damage of the machine and manage the 8.3 GJ stored in each beam.

To address these challenges, the FCC study searches for designs that can withstand the large energy loads with acceptable transient deformation and no permanent damage. Novel composites with improved thermo-mechanical and electric properties will be investigated in cooperation with the FP7 HiLumi LHC DS and EuCARD2 programmes.

Timescale

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The Large Hadron Collider at CERN with its High Luminosity upgrade is the world's largest and most powerful particle accelerator and is expected to operate until 2036. A number of different proposals for a post-LHC research infrastructure in particle physics have been launched, including both linear and circular machines.

The FCC study explores scenarios for different circular particle colliders housed in a new 100 km circumference tunnel, building on the tradition of the LEP and LHC, which are both housed in the same 27 km circumference tunnel. A time-frame of 30 years is appropriate for the design and construction of a large accelerator complex and particle detectors.

The experience from the operation of LEP and LHC and the opportunity to test novel technologies in the High Luminosity LHC provide a basis for assessing the feasibility of a post-LHC particle accelerator. In 2018, the FCC collaboration published the four volume Conceptual Design Report (CDR)[13] as input to the next European Strategy for Particle Physics.[4] The four volumes focus on: (a) "Vol. 1 Physics Opportunities";[24] (b) "Vol. 2 FCC-ee: The lepton collider";[25] (c) "Vol. 3 FCC-hh: The hadron collider";[26] and (d) "Vol. 4 The High-Energy LHC".[27]

The significant lead time of approximately twenty years for the design and construction of a large-scale accelerator calls for a coordinated effort.[clarification needed]

Organisation

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The FCC study, hosted by CERN is an international collaboration of 135 research institutes and universities and 25 industrial partners from all over the world.

The FCC study was launched following a response to the recommendation made in the update of the European Strategy for Particle Physics 2013, adopted by CERN's council. The study is governed by three bodies: the International Collaboration Board (ICB), the International Steering Committee (ISC), and the International Advisory Committee (IAC).

The organization of the FCC Study

The ICB reviews the resource needs of the study and finds matches within the collaboration. It so channels the contributions from the participants of the collaboration aiming at a geographically well-balanced and topically complementary network of contributions. The ISC is the supervisory and main governing body for the execution of the study and acts on behalf of the collaboration.

The ISC is responsible for the proper execution and implementation of the decisions of the ICB, deriving and formulating the strategic scope, individual goals and the work programme of the study. Its work is facilitated by the Coordination Group, the main executive body of the project, which coordinates the individual work packages and performs the day-to-day management of the study.

Finally, the IAC reviews the scientific and technical progress of the study and shall submit scientific and technical recommendations to the International Steering Committee to assist and facilitate major technical decisions.

Philanthropic funding

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In December 2025, CERN announced that private sector philanthropists led by telecom and internet magnates Yuri Milner and Eric Schmidt (via the Breakthrough Prize Foundation and Schmidt Innovation Fund), along with John Elkann and Xavier Niel had pledged €860 million (US$1 billion) to fund CERN's bid to get its member states to approve the FCC. The philanthropists argued that the FCC would contribute not only new insights into the Standard Model but profound impacts on new technologies. CERN said the pledge was the first of its kind in the institution's history. As of then, the approval was expected to take place in 2028.[28][29]

Criticism

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The FCC's proposed particle accelerator has been criticized for costs, with the cost for the energy-frontier hadron collider (FCC-hh) variant of this project projected to be over 20 billion US dollars.[30]

Layout and placement scenario PA31-4.0 of the FCC.
Layout and placement scenario PA31-4.0[31] of the FCC.

Physicist, author, content creator Sabine Hossenfelder[32] criticized a relevant promotional video for outlining a wide range of open problems in physics, despite the fact that the accelerator will likely only have the potential to resolve a small part of them. She noted that (as of 2019) there is "no reason that the new physical effects, like particles making up dark matter, must be accessible at the next larger collider".[33][34]

Response to this criticism came both from within the physics community as well as from philosophers and historians of science who emphasized the exploratory potential of any future large-scale collider.[35] A detailed physics discussion is included in the first volume of the FCC Conceptual Design Report. Gian Giudice, Head of CERN's Theoretical Physics Department wrote a paper on the "Future of High-Energy Colliders"[36] while other commentary came from Jeremy Bernstein, Lisa Randall, James Beacham,[37] Harry Cliff[38][39] and Tommaso Dorigo[40][41] among others. In a recent[when?] interview theorist for the CERN Courier, Nima Arkani-Hamed described the concrete experimental goal for a post-LHC collider: "While there is absolutely no guarantee we will produce new particles, we will definitely stress test our existing laws in the most extreme environments we have ever probed. Measuring the properties of the Higgs, however, is guaranteed to answer some burning questions. [...] A Higgs factory will decisively answer this question via precision measurements of the coupling of the Higgs to a slew of other particles in a very clean experimental environment."[42] Moreover there has been some philosophical responses to this debate, most notably one from Michela Massimi who emphasised the exploratory potential of future colliders: "High-energy physics beautifully exemplifies a different way of thinking about progress, where progress is measured by ruling out live possibilities, by excluding with high confidence level (95%) certain physically conceivable scenarios and mapping in this way the space of what might be objectively possible in nature. 99.9% of the time this is how physics progresses and in the remaining time someone gets a Nobel Prize for discovering a new particle."[43]

Studies for linear colliders

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A high-luminosity upgrade of the LHC [HL-LHC] has been approved to extend its operation lifetime into the mid-2030s. The upgrade will facilitate the detection of rare processes and improve statistical measurements.

The Future Circular Collider study complements previous studies for linear colliders. The Compact Linear Collider (CLIC) was launched in 1985 at CERN.[44] CLIC examines the feasibility of a high-energy (up to 3 TeV), high-luminosity lepton (electron/positron) collider.

The International Linear Collider is a similar to CLIC project, planned to have a collision energy of 500 GeV. It presented its Technical Design Report in 2013.[45] In 2013, the two studies formed an organisational partnership, the Linear Collider Collaboration (LCC) to coordinate and advance the global development work for a linear collider.[46]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Future Circular Collider

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from Grokipedia
The Future Circular Collider (FCC) is a proposed next-generation project led by , consisting of a 90.7-kilometer circumference underground ring designed to enable high-precision measurements and high-energy collisions exceeding those of the (LHC). The project envisions a staged implementation, beginning with the FCC-ee collider for electron-positron interactions serving as a Higgs factory and precision physics instrument, followed by the FCC-hh targeting proton-proton collisions at a center-of-mass energy of 100 TeV to explore potential physics beyond the Standard Model. Located in a new averaging 200 meters deep around the region spanning and , the facility would incorporate advanced technologies such as high-field superconducting magnets and cryogenic systems to achieve these capabilities. A released in March 2025, involving over 1,000 experts, confirmed the technical viability of the infrastructure, including , accelerators, and experiments, while addressing environmental and territorial challenges through multiple site scenarios. If approved by the Council around 2028, construction could commence in the early , with FCC-ee operations potentially starting in the late 2040s after a 12-year build phase. The estimated cost for the FCC-ee stage alone is approximately 15 billion Swiss francs, drawn largely from 's budget, prompting debates within the physics community about the project's scientific priority amid uncertain prospects for groundbreaking discoveries following the LHC's confirmation of the without evidence of new particles or forces. Critics argue that the substantial investment may yield given the absence of beyond-Standard-Model signals at current energies, advocating instead for alternative approaches or diversified funding in research.

Origins and Development

Initial Proposal and Conceptualization

The Future Circular Collider (FCC) concept emerged in 2014 amid discussions at on successors to the (LHC), which had confirmed the in 2012 but highlighted the need for higher-energy probes of fundamental physics. initiated the FCC study to evaluate options for a new circular accelerator infrastructure capable of achieving center-of-mass energies up to 100 TeV in proton-proton collisions, far exceeding the LHC's 14 TeV design capability. The proposal envisioned a staged approach, beginning with an electron-positron collider (FCC-ee) for precision measurements at the Z, W, Higgs, and top-quark poles, followed by a high-luminosity (FCC-hh). The formal kickoff of the FCC study occurred with an international workshop held September 9–11, 2014, marking the start of systematic conceptualization efforts involving over 1,000 scientists from more than 100 institutions worldwide. This meeting built on preliminary ideas circulated earlier in 2014, focusing on feasibility for a 100 km circumference tunnel—approximately four times larger than the LHC's 27 km ring—to accommodate advanced superconducting magnets and enable unprecedented luminosity and energy scales. Key conceptual drivers included the potential to explore weakly coupled new physics beyond the Standard Model, such as dark matter candidates and hierarchy problem solutions, through direct production at high energies or indirect precision tests. Early conceptualization emphasized technical challenges, including the development of 16 Tesla dipole magnets using high-temperature superconductors like niobium-tin, and site evaluations near to reuse existing infrastructure while minimizing environmental impact. The study's timeline targeted a Conceptual Design Report (CDR) by 2018 to inform the 2018 update of the European Strategy for Particle Physics, though delivery slipped to January 2019 due to the scope's complexity. This phase established the FCC as a multi-decade project, with initial operations projected for the 2040s, contingent on international funding and geopolitical stability.

Feasibility Studies and Key Milestones

The Future Circular Collider (FCC) feasibility studies originated from initial conceptual explorations launched in 2014, involving worldwide collaboration across physics, experiments, and accelerator technologies, with the goal of producing a Conceptual Design Report (CDR) by 2018. These early efforts evaluated options for a post-Large Hadron Collider (LHC) facility, focusing on a ~100 km circumference tunnel to enable higher energies than the LHC's 27 km ring. The CDRs for the electron-positron (FCC-ee) and hadron (FCC-hh) configurations were published in December 2018, outlining baseline parameters such as FCC-ee operations up to 365 GeV center-of-mass energy and FCC-hh up to 100 TeV, alongside preliminary infrastructure needs including high-field magnets and cryogenic systems. Following the 2020 update to the European Strategy for Particle Physics, initiated a dedicated FCC Feasibility Study in 2020 to assess technical, financial, and environmental viability, building on the CDRs. This phase included subsurface investigations starting in 2023 to evaluate geological conditions for the proposed 90.7 km ring, averaging 200 m depth, spanning and . Key technical assessments covered accelerator designs, detector concepts, and enabling technologies like 16 T superconducting magnets, confirming baseline feasibility for staged implementation: FCC-ee in the mid-2040s followed by FCC-hh. The Feasibility Study culminated in a comprehensive report released on March 31, 2025, comprising three volumes on physics/experiments, accelerators, and civil engineering/infrastructure. It estimated costs at 15 billion Swiss francs for the FCC-ee stage, distributed over ~12 years from the early 2030s, encompassing civil engineering, accelerators, detectors, and power systems, while noting shared infrastructure with existing CERN facilities to optimize expenses. Subsequent milestones include independent expert reviews, a CERN Council discussion in November 2025, and a potential project approval decision around 2028, informing the next European Strategy update. Annual FCC Week conferences, such as the 2021 virtual edition and the 2025 event in Vienna (May 19–23), have served as platforms for progress updates and international input.

Scientific Rationale

Fundamental Physics Goals

The Future Circular Collider (FCC) seeks to probe fundamental questions in that remain unresolved by the , including the nature of electroweak , the origin of particle masses, and the absence of new phenomena at Large Hadron Collider (LHC) energies. A central objective is to measure the Higgs boson's trilinear self-coupling with sufficient precision to test whether it aligns with predictions or reveals deviations indicative of extended scalar sectors, potentially linked to the or early-universe phase transitions. In the FCC-ee phase, operating at center-of-mass energies from 90 to 365 GeV, single and double Higgs production processes could yield sensitivities to modifications in this coupling at the level of 20-50% relative uncertainty, building on LHC constraints through higher event statistics exceeding 10^6 Higgs events. Searches for constitute another core goal, targeting weakly interacting massive particles () as candidates, supersymmetric particles to stabilize the Higgs mass, and heavy neutral leptons explaining oscillations. The FCC-hh configuration, with proton-proton collisions at 100 TeV, would extend direct discovery reach to masses up to 40 TeV for colored particles and cover much of the thermally produced WIMP parameter space, complementing indirect probes from cosmology. Precision electroweak measurements in FCC-ee, including production of trillions of Z bosons and precise determinations of W and properties at parts-per-million accuracy, aim to uncover subtle deviations from predictions that could signal new dynamics at higher scales. Additional aims include investigating matter-antimatter asymmetry through enhanced sensitivity to CP-violating processes and exploring flavor anomalies in and sectors via rare decays at high . These objectives leverage the FCC's staged approach—starting with collisions for baseline precision before transitioning to hadronic mode for energy frontier exploration—to maximize discovery potential while minimizing theoretical biases in interpretation, as validated by simulations in reports.

Justification from Empirical Data and First-Principles

The (LHC) has validated the (SM) to extraordinary precision, culminating in the 2012 discovery of the with a of approximately 125 GeV, yet extensive searches up to center-of-mass energies of 13.6 TeV have yielded no evidence for physics beyond the SM, such as supersymmetric particles or heavy vector bosons, excluding many models up to masses of 1-2 TeV depending on quantum numbers and couplings. This empirical null result underscores the SM's effectiveness as an effective field theory (EFT) valid to at least the TeV scale but highlights its incompleteness, as it fails to incorporate observed phenomena like non-baryonic comprising ~27% of the universe's energy density or the matter-antimatter asymmetry. From first principles, the SM's structure as a implies ultraviolet (UV) completion at higher scales to avoid inconsistencies. Radiative to the Higgs mass parameter exhibit quadratic sensitivity, δm_H² ≈ (3 y_t² / (8 π²)) Λ² log(Λ/m_t), where y_t is the top Yukawa and Λ the cutoff, necessitating extreme fine-tuning between bare mass and to yield the observed m_H ≪ M_Pl unless stabilizing new physics intervenes near the electroweak scale—a resolution unverified by LHC , which instead constrains such mechanisms to higher energies. Similarly, partial wave unitarity in electroweak processes like longitudinal W_L W_L scattering bounds new dynamics or resonances to ~1-10 TeV to prevent growth violating tree-level unitarity, with LHC limits pushing viable scales beyond direct reach and motivating probes at 100 TeV to test EFT validity or uncover strong regimes. These considerations compel higher-energy colliders like the FCC, targeting proton-proton collisions at √s = 100 TeV, to extend direct search sensitivity by factors of 5-7 in mass reach for particles (e.g., gluinos up to 20 TeV) and enable precision tests of SM parameters at per-mille level via integrated e⁺e⁻ operations, potentially revealing indirect BSM effects through deviations in Higgs couplings or rare processes unattainable at LHC luminosities. Empirical gaps, such as neutrino masses implied by data requiring mechanisms or sterile neutrinos, further demand enhanced sensitivity to low-mass weakly interacting states, where FCC's projected 20 ab⁻¹ integrated dwarfs LHC's capabilities. Thus, advancing beyond LHC empirically tests the persistence of SM EFT behavior and first-principles expectations for UV physics resolving naturalness and stability issues.

Technical Design

Ring Infrastructure and Site Considerations

The Future Circular Collider (FCC) ring infrastructure centers on a new underground circular tunnel with a circumference of 90.7 km, designed to accommodate both the electron-positron collider (FCC-ee) and the subsequent hadron collider (FCC-hh) in a shared infrastructure, following the precedent set by CERN's LEP and LHC. The tunnel features a primary internal diameter of 5.5 m for most of its length, enabling the installation of superconducting magnets, beam pipes, and associated systems, while requiring extensive civil engineering for excavation and support structures. Access to the ring is provided via shafts with depths ranging from 180 to 400 m, supporting eight surface sites for utilities, injection/extraction lines, and four primary experimental caverns for detectors. Site considerations emphasize geological stability and proximity to CERN's existing campus near , with the proposed alignment spanning the Switzerland-France border, including submersion under and passage through the and . Feasibility studies, initiated in 2022 and advancing to field investigations by February 2023, involve stratigraphic, sedimentological, and petrographical analyses in French departments such as and to evaluate , conditions, and seismic risks at depths up to 400 m. The average tunnel depth of 200 m necessitates advanced tunneling methods, projecting significant material excavation volumes comparable to major projects like the but scaled for precision requirements. Infrastructure planning incorporates energy-efficient designs and principles to minimize environmental footprint, including optimized surface facilities for , power distribution, and management, while addressing challenges like in densely populated regions. The March 2025 feasibility report confirms the layout's viability, reducing the original 97.8 km concept to 90.6 km for practical alignment, though full implementation depends on international approvals and detailed geotechnical validations.

Collider Configurations

The Future Circular Collider (FCC) is proposed to operate in a staged, integrated programme featuring distinct collider configurations to address complementary physics objectives, beginning with precision electroweak and Higgs measurements before transitioning to high-energy explorations of new phenomena. The primary configurations include the electron-positron (FCC-ee) for collisions at varying centre-of-mass energies, followed by the (FCC-hh) for proton-proton and heavy-ion interactions at 100 TeV, with an optional electron-hadron mode (FCC-eh) leveraging the shared . These setups would utilize the same 90.7 km circumference tunnel, with FCC-ee injecting beams via upgrades to existing injectors and FCC-hh requiring advanced superconducting magnets for higher rigidity. FCC-ee operates as a circular electron-positron optimized for high and precision, running sequentially at four energy stages: the Z-pole at 91 GeV for electroweak precision tests with projected luminosities exceeding 10^12 inverse femtobarns; the WW threshold at 160 GeV; the Higgs-strahlung (ZH) mode at 240 GeV serving as a Higgs factory; and the top-quark threshold at 365 GeV. Transitions between the lower-energy modes (Z, WW, ZH) can occur without hardware changes, enabling flexible operation over a decade-long phase starting potentially in the 2040s, while top mode requires RF system upgrades. Two interaction points would host detectors for comprehensive data collection, emphasizing top-up injection to maintain beam currents and mitigate synchrotron radiation losses inherent to high-energy lepton rings. In the subsequent FCC-hh configuration, protons collide at 100 TeV centre-of-mass energy (50 TeV per beam) to probe TeV-scale physics beyond the Standard Model, with integrated luminosities targeting hundreds of inverse femtobarns over extended runs into the late 21st century. Heavy-ion collisions, such as lead-lead at reduced energies around 40 TeV per nucleon pair, would investigate quark-gluon plasma dynamics, building on LHC capabilities but with order-of-magnitude higher interaction rates due to enhanced luminosity. Proton-ion and the FCC-eh mode, involving 50 GeV electrons from a linear accelerator colliding with 50 TeV protons extracted from FCC-hh, would enable deep-inelastic scattering studies of nucleon structure at unprecedented scales, though this requires additional infrastructure like a bypass insertion for beam crossing. These configurations prioritize maximal physics reach while sharing cryogenic and vacuum systems, though FCC-hh demands 16 T dipole magnets versus FCC-ee's 6 T fields.

Enabling Technologies

Magnet and Superconducting Systems

The magnet systems of the Future Circular Collider (FCC) are essential for steering and focusing high-energy particle beams within a 91 km circumference ring, with designs tailored to the requirements of both the electron-positron (FCC-ee) and proton-proton (FCC-hh) phases. For FCC-ee, over 9,000 superconducting magnets, including dipoles for beam bending and quadrupoles for focusing, enable precise control to achieve high at interaction points. In the subsequent FCC-hh phase, the system demands advanced high-field dipoles capable of generating 16 tesla to bend 50 TeV proton beams, necessitating novel superconducting materials beyond those used in the (LHC). The baseline technology for FCC-hh dipole magnets employs niobium-tin (Nb3Sn), a low-temperature superconductor (LTS) that supports fields exceeding the 8.3 tesla limit of the LHC's NbTi magnets, with operation at 1.9 using superfluid helium cooling. Nb3Sn coils are fabricated via a challenging "wind-and-react" process, where cables are wound before to form the superconducting phase, followed by impregnation and assembly into structures to manage the material's brittleness. A significant milestone was reached in March 2020, when a Nb3Sn demonstrator achieved a peak field of 16.5 tesla at 1.9 , validating the feasibility of FCC-hh requirements for 100 TeV collision energies in a ~100 km tunnel. This test, conducted as part of international R&D efforts, also demonstrated 16.3 tesla at 4.5 , highlighting potential operational margins. Ongoing research explores high-temperature superconductors (HTS), such as tapes, to enable fields above 16 tesla, reduce cryogenic demands, or enhance reliability, with conceptual designs targeting up to 20 tesla in hybrid LTS-HTS configurations. Facilities like the FRESCA2 test magnet, upgraded for high-field cable evaluation, support this by verifying Nb3Sn performance at large apertures (e.g., 13.3 tesla sustained for hours at 10 cm) and preparing for HTS insertions. Key challenges include achieving field quality in 50 mm apertures, optimizing cost through material procurement and manufacturing scalability, and integrating quench protection to mitigate risks from sudden superconducting transitions. These systems form part of broader superconducting technologies, including corrector magnets and beam separation elements, all requiring precise multipole field control for accelerator stability. The FCC , completed in March 2025, confirms the technical viability of these magnet advancements within the project's timeline.

Acceleration and Beam Management Technologies

The acceleration systems for the Future Circular Collider (FCC) encompass an upgraded injector complex leveraging existing CERN infrastructure, such as Linac4, the (PS), and (SPS), to deliver beams at injection energies suitable for both the electron-positron (FCC-ee) and (FCC-hh) configurations. For FCC-ee, dedicated and linacs accelerate beams to approximately 20 GeV using S-band radiofrequency (RF) structures, followed by damping rings to minimize emittance before injection into the main ring. In FCC-hh, protons are ramped from lower energies in the injectors to about 3.3 TeV for multi-turn injection, enabling efficient filling of the 50 TeV per beam storage. These systems prioritize energy efficiency and beam quality to support luminosities exceeding those of the (LHC) by factors of up to five. Superconducting RF cavities form the core of in-ring acceleration and beam maintenance, with designs tailored to the distinct operational demands of FCC-ee and FCC-hh. For FCC-ee, operations at the Z-pole require single-cell 400 MHz niobium-on-copper (Nb/Cu) cavities to handle high-current, low-voltage beams (around 1-2 MV per cavity), while higher-energy phases (W and top) employ two-cell superconducting cavities for greater voltage gradients and efficiency. Crab cavities, also at 400 MHz, enable the crab-waist scheme to achieve nanometer-scale vertical beam sizes at interaction points, enhancing luminosity without excessive synchrotron radiation losses. In FCC-hh, the RF system must compensate for synchrotron radiation losses during energy ramping and store 0.5 A beams, necessitating high-power klystrons, cavity detuning for multi-bunch stability, and beam loading compensation to minimize bunch-by-bunch variations in parameters like energy spread. Ongoing R&D focuses on cryomodule integration and high-efficiency power sources to address thermal loads exceeding 100 MW in total RF dissipation. Beam management technologies emphasize stability, low emittance, and precise control to mitigate instabilities in the 91 km circumference ring. in FCC-ee provides natural damping, augmented by wigglers to achieve 10% beam polarization within 12 hours at the Z-pole, while feedback systems correct transverse and chromatic aberrations via optimized . For FCC-hh, fast injection kickers and helical beam screens manage high-intensity proton trains, preventing emittance growth during transfer from the SPS, with local in magnets and advanced diagnostics ensuring collision parameters like 100 TeV center-of-mass and integrated of 20-30 ab⁻¹ over the operational lifetime. Critical systems modeling highlights RF and injection as high-availability components, with failure rates informed by LHC data to target >80% uptime. These approaches draw on empirical scaling from prior accelerators but require validation through prototypes to confirm performance under FCC-scale beam currents and energies.

Organizational and Timeline Aspects

CERN's Governance and International Involvement

CERN functions as an intergovernmental organization governed by the CERN Council, its highest policy-making body, which consists of two delegates from each representing both governmental and scientific interests. The Council approves the organization's strategic direction, budget, and major projects, ensuring alignment with the contributions of Member States, whose financial commitments are scaled according to their net national income. This structure facilitates collaborative decision-making among European nations while maintaining CERN's foundational mission of advancing fundamental research. In the context of the Future Circular Collider (FCC), the Council has played a pivotal role by mandating a comprehensive following endorsements in the European Strategy for updates. This study, completed in March 2025, evaluates the technical, financial, and organizational viability of the project, with a potential decision on construction authorization expected around 2028 after further strategic deliberations scheduled for 2026. The Council's oversight ensures that FCC proposals undergo rigorous scrutiny, balancing scientific ambition with fiscal responsibility shared among Member States. International involvement in the FCC extends beyond CERN's 23 Member States through a global collaboration network encompassing over 100 research institutions, universities, and industrial partners from non-Member States. Contributions from entities in the United States, , and other regions have historically supported CERN's large-scale endeavors, such as the , and are similarly anticipated for FCC development, including technology prototyping and expertise sharing. The has provided targeted funding for the FCC feasibility phase under programs, underscoring broader multinational commitment, while non-Member State participation remains voluntary and often leverages CERN's infrastructure for mutual scientific benefit.

Projected Phases and Cost Projections

The Future Circular Collider (FCC) is planned as a two-stage project to maximize scientific return while sharing infrastructure. The first stage involves the FCC-ee, an electron-positron collider designed for high-precision measurements of known particles, including serving as a Higgs, electroweak, and top-quark factory at varying center-of-mass energies such as the Z pole, WW threshold, ZH peak, and top/anti-top threshold. This phase would produce vast datasets, such as over 6 trillion Z bosons and around 3 million Higgs bosons over approximately 15 years of operation. The second stage, FCC-hh, would upgrade the infrastructure for a proton-proton collider reaching collision energies of about 100 TeV, enabling explorations of new physics beyond current reach, with support for heavy-ion and lepton-hadron collisions at luminosities 5-10 times higher than the High-Luminosity LHC. Construction for the shared 90.7 km circumference and initial is projected to begin in the early 2030s, following a Council decision expected around 2028 after the 2025 completion. FCC-ee operations are anticipated to commence in the late 2040s, succeeding the LHC's runout around 2040, with the phase lasting about 15 years. The FCC-hh stage would follow, with construction leveraging the existing and starting post-FCC-ee, leading to operations in the 2070s for approximately 25 years. These timelines assume approval and funding, with preparatory technical design work ongoing from 2025 to 2027. Cost projections for the FCC-ee stage, including civil engineering for the tunnel (accounting for about one-third of the total), accelerators, technical infrastructure, and four interaction point detectors, are estimated at 15 billion Swiss francs (CHF), distributed over roughly 12 years of construction starting in the early . This estimate draws from the 2025 feasibility study involving around 1,500 contributors and covers the initial investment primarily from CERN's annual budget, similar to the LHC model. Full programme costs, incorporating the FCC-hh upgrade, remain preliminary as they depend on phased implementation and future technological developments, but the shared infrastructure aims to optimize overall expenditure.

Debates and Criticisms

Economic Viability and Resource Allocation

The proposed Future Circular Collider (FCC) electron-positron stage is estimated to cost 15 billion Swiss francs (approximately €15 billion or $17 billion), including , , and accelerators, spread over about 12 years of construction. This figure arises from CERN's March 2025 , involving contributions from around 1,500 physicists and engineers, which details no insurmountable technical barriers but highlights substantial financial demands. Full implementation, including subsequent proton stages and detectors, could exceed this by additional billions, with total investment and operations over a 25-year lifespan potentially reaching higher figures based on modeling of phased development. Funding for the FCC would primarily draw from 's existing annual budget of around 1.2 billion Swiss francs, supported by contributions from 23 member states proportional to their GDP, supplemented by potential increases or international partners. management has indicated that a significant portion—up to two-thirds—of the initial costs could be absorbed within current budgetary envelopes by reallocating resources from LHC operations post-2040, though this would require approval and possible host-state investments for site-specific infrastructure near . Critics, including the German government in June 2024, have deemed the project unaffordable amid fiscal constraints, arguing it strains public resources without assured returns comparable to the Large Hadron Collider's discovery, which cost about 4.75 billion Swiss francs over its lifecycle. Resource allocation debates center on opportunity costs, with proponents estimating economic multipliers from construction jobs, supply chains, and technological spin-offs—such as advanced superconductors and —potentially generating equivalent to several times the investment through and diffusion, as modeled in economic impact assessments. However, skeptics contend that diverting funds from diverse fields like , , or yields diminishing marginal returns, given the FCC's reliance on unproven high-energy physics breakthroughs amid global priorities such as energy transitions and poverty alleviation. Comparisons to lower-cost alternatives, like China's estimated at $5.15 billion, underscore allocation inefficiencies, as CERN's model demands multinational consensus that has historically delayed projects. Empirical precedents from LHC operations suggest indirect benefits like skilled workforce training, but causal attribution remains contested, with some analyses attributing only modest GDP boosts relative to direct expenditures.

Environmental and Sustainability Realities

The Future Circular Collider (FCC) proposes a 90.7 km circumference underground tunnel, with depths averaging 200 meters and access shafts ranging from 180 to 400 meters, situated primarily beneath rural areas in the cantons of and ( and ). This configuration limits surface to eight sites for experiments and infrastructure, reducing relative to surface-heavy designs, though geological surveys indicate potential challenges from seismic activity and in the Jura region's karstic terrain. Excavation would displace approximately 20-25 million cubic meters of rock, comparable to LHC volumes scaled up, necessitating spoil management and potential landscape alterations during tunneling. Operational energy demands are anticipated to exceed the Large Hadron Collider's (LHC) 1.3 TWh per physics run, with estimates for FCC electron-positron stages (FCC-ee) implying several TWh annually during high-luminosity operations, driven by superconducting magnets requiring cryogenic cooling and high-power RF systems. CERN's regional grid, sourcing over 90% of from low-carbon nuclear and hydroelectric power, yields a projected operational near 0 Mt CO₂e after 2040, assuming grid decarbonization trends continue; per-Higgs production, this equates to roughly 1.8-3 MWh of and 0.1 tonnes CO₂ equivalents under current conditions. Construction-phase emissions, dominated by and for tunnel lining, are forecasted at 1.056 Mt CO₂e, partially offset by low-carbon variants reducing emissions by up to 26%. Sustainability initiatives integrated into the FCC feasibility study emphasize principles, including tunnel reuse across collider phases (e.g., FCC-ee followed by FCC-hh), heat recuperation from for , and reduced water consumption via closed-loop systems. Rare-earth materials for high-field magnets pose supply chain risks, but plans procurement from recycled sources and eco-friendly alternatives to mitigate mining impacts. Detector operations, akin to LHC's, contribute additional footprints from refrigerants with high , though ongoing R&D targets low-GWP substitutes like C₃F₆ replacements. Critics highlight the opportunity costs of such resource intensity amid global decarbonization imperatives; the Swiss environmental group Noe21 has labeled the FCC "excessive," citing its electricity demands as disproportionate to potential gains when redirectable to renewables or . Independent analyses affirm FCC's lower per-particle footprint versus linear colliders or Asian circular proposals like CEPC, but underscore variability—up to a factor of 100—tied to site-specific grid carbon intensity and construction efficiencies. These concerns underscore causal trade-offs: while yields indirect technological spillovers (e.g., LHC-enabled grid tech), direct environmental loads from megaprojects warrant scrutiny against empirical benchmarks like per-kWh emissions thresholds for .

Scientific Necessity and Alternative Approaches

The Future Circular Collider (FCC) is proposed to address fundamental limitations of the (LHC), which operates at center-of-mass energies up to approximately 14 TeV and has confirmed the Standard Model's but yielded no clear signals of physics beyond it. Proponents at argue that the FCC's staged design—beginning with the electron-positron FCC-ee at energies of 90–365 GeV for precision measurements, followed by the proton-proton FCC-hh at up to 100 TeV—would enable unprecedented probes into open questions such as the Higgs boson's self-coupling, electroweak , candidates, masses, and the matter-antimatter asymmetry. Specifically, FCC-ee could produce over 10¹² bosons and millions of Higgs events, achieving part-per-million precision on parameters like and boson masses, far surpassing LHC capabilities in clean lepton collisions. This, they contend, is essential to test subtle deviations from the that may indicate weakly coupled new physics inaccessible at hadron colliders. Critics, however, question the empirical necessity of such a massive undertaking, given the LHC's null results for beyond-Standard-Model (BSM) phenomena despite extensive searches up to TeV scales. Particle Matt Strassler acknowledges FCC-ee's value for high-statistics Higgs studies—potentially revealing rare decays or low-mass particles missed by prior experiments—but views FCC-hh's high-energy phase as premature, lacking concrete predictions tied to current data and risking resources on speculative reaches to 40 TeV direct discovery limits. Similarly, Sabine Hossenfelder argues that colliders like the FCC offer diminishing returns, as theoretical motivations such as naturalness have failed empirically, and higher energies may only refine constants without breakthroughs, especially since BSM scales could lie orders of magnitude higher near the Planck energy. She notes the absence of reliable, testable predictions for new particles in the FCC's range, contrasting with historical successes like the Higgs, predicted decades in advance. Alternative approaches emphasize more targeted or compact technologies to pursue similar physics goals with potentially lower costs and risks. colliders, under development at facilities like , could achieve effective energies of 3–10 TeV in rings as small as 4.5–10 km by colliding short-lived s, offering cleaner probes of Higgs properties and BSM particles without the quark-gluon debris of proton collisions, though challenges remain in muon cooling and production. Linear electron-positron colliders, such as the proposed (ILC) at 500 GeV over 31 km in , provide precision Higgs studies in a hadron-free environment, with operations feasible by the at lower upfront investment than FCC. Emerging plasma wakefield acceleration techniques aim to shrink accelerator sizes dramatically by using laser-driven plasma waves, enabling high-gradient fields for future compact colliders that could rival FCC energies without vast circumferences. Beyond accelerators, non-collider efforts in precision measurements (e.g., anomalies) and multi-messenger offer complementary paths to and early-universe insights, potentially yielding discoveries at fractions of FCC's projected scale. These options reflect a causal view that progress in may hinge more on theoretical refinement and diverse empirical strategies than on sheer energy escalation, given the Standard Model's resilience.

Anticipated Outcomes

Potential Discoveries and Knowledge Gains

The Future Circular Collider (FCC) encompasses two primary phases: FCC-ee, an electron-positron designed for high-precision measurements at centre-of-mass energies ranging from 90 GeV to 365 GeV, and FCC-hh, a proton-proton targeting 100 TeV collision energies for direct searches of new phenomena. FCC-ee prioritizes luminosity-driven precision studies, producing vast samples such as 6 × 10¹² Z bosons and 2.4 × 10⁸ W boson pairs, enabling tests of the (SM) at levels unattainable by the (LHC). In contrast, FCC-hh leverages its sevenfold energy increase over the LHC to probe particle masses up to 40 TeV, potentially revealing direct signatures of physics beyond the SM. FCC-ee's precision programme would advance Higgs boson characterization, yielding per-mil-level accuracy on couplings like g_{HZZ} and the total width via processes such as H → Z⁰Z⁰, while producing ~2 × 10⁶ Higgs events to constrain the trilinear self-coupling and assess the electroweak phase transition's (first- or second-order). These measurements could detect deviations from SM predictions indicative of new physics, such as invisible decays down to branching ratios of 2.5 × 10⁻⁴ or exotic channels like H → μτ at <10⁻⁴. Electroweak observables would benefit from 50-fold improvements, including Z width precision at 10 ppm and W mass at 7 ppm, indirectly accessing energy scales of tens of TeV for phenomena like or composite Higgs structures. Top quark studies would refine mass determinations to tens of MeV, and flavour physics would yield ~200,000 rare B⁰ → K⁰*e⁺e⁻ decays to probe and quark sector anomalies with enhanced sensitivity. FCC-hh's high-energy reach would facilitate searches for supersymmetry, extending discovery limits by an order of magnitude to gluinos or squarks at 15–20 TeV and charginos up to 10 TeV, far beyond LHC projections. Dark matter candidates, including weakly interacting massive particles (WIMPs) in doublet or triplet representations, could be covered up to thermal relic density limits, with indirect sensitivity to 100 TeV scales via missing energy signatures. It would also explore dark sectors, such as dark photons or axions, and sterile neutrinos linked to mass generation, with sensitivities to mixing angles |Θ_{νN}|² ≈ 10⁻¹¹. Producing 5 × 10¹⁰ Higgs bosons and 10¹² top quarks, FCC-hh would enable detailed spectroscopy of SM particles under extreme conditions, potentially uncovering hierarchical structures or unification mechanisms. Overall, the FCC's integrated approach combines FCC-ee's indirect constraints on high-scale physics with FCC-hh's direct exploration, offering a pathway to resolve open questions like the , matter-antimatter asymmetry, and masses, while providing null results that refine effective field theories if no new particles emerge. This dual strategy maximizes discovery potential by cross-validating precision anomalies against high-energy signals, with FCC-ee sensitivities to weakly coupled sectors complementing FCC-hh's brute-force searches.

Broader Technological and Economic Impacts

The development of high-field superconducting magnets capable of sustaining 16 tesla fields represents a core technological advancement pursued for the FCC's hadron collider phase, utilizing materials like niobium-tin (Nb₃Sn) or high-temperature superconductors to enable 100 TeV proton collisions. These magnets demand innovations in coil winding, quench protection, and mechanical support structures, with potential spin-offs to fusion energy projects requiring similar compact, high-field designs, as seen in parallels with ITER's magnet technology. Enhanced cryogenic systems for helium cooling at scale, alongside efficient radiofrequency acceleration, further drive progress in energy-efficient cooling and power systems applicable to industrial refrigeration and large-scale computing facilities. Economically, the FCC-ee construction is estimated at CHF 15.3 billion over approximately 15 years, with the full project spanning investment, upgrades, and operations potentially totaling CHF 21.1 billion in 2019 prices across 25 years from 2028 to 2057. This investment is projected to yield an annualized global value added of CHF 1.4 billion, supported by multipliers from direct CERN expenditures, indirect effects, and induced spending from wages and up to 300,000 annual visitors. Employment impacts include around 26,000 jobs worldwide, comprising 11,000 direct positions in and operations, 6,700 indirect roles in industry suppliers, and 8,200 induced jobs from economic ripple effects. Local economic benefits for the FCC-ee are forecasted to exceed €4 billion, including roughly 800,000 person-years of through collaborations with over 140 institutes and firms across more than 30 countries. Industry involvement in co-construction, such as tunnel boring and infrastructure, fosters and spin-off companies, mirroring LHC patterns where societal returns reached €1.2 per €1 invested via knowledge diffusion and service sector growth. Overall, analyses indicate a positive socio-economic benefit-cost , with project-driven innovations in software, materials, and efficiency outweighing direct costs through long-term productivity gains.

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