The discovery of the Higgs boson at the LHC in 2012 completed the Standard Model but left many fundamental questions unanswered. The High-Luminosity LHC upgrade will extend the LHC’s data-taking into the mid-2030s, improving precision for known processes. However, the LHC’s 14 TeV collision energy and hadron-collision environment impose limitations. Despite extensive searches, no new particles beyond the Higgs have been conclusively observed at the LHC. This motivates the need for future colliders that can either reach higher energies or achieve significantly higher precision. New machines could directly access mass scales beyond LHC’s reach or detect subtle effects of new physics through precision measurements.

Several compelling physics drivers shape proposals for next-generation colliders. These include exploring the nature of electroweak symmetry breaking in detail, and searching for physics Beyond the Standard Model such as dark matter particles, heavy neutrinos, or evidence of new forces. A high-precision “Higgs factory” would allow percent- to sub-percent-level measurements of Higgs, W, Z, and top-quark properties, far exceeding the precision achievable at the LHC. On the other hand, an energy-frontier collider would vastly expand the direct search reach for new particles up to tens of TeV in mass. Moreover, some proposed colliders aim to do both in stages – first a precision machine, then a high-energy successor using the same infrastructure.

The motivations are reinforced by the LHC’s current results. The absence of clear new-physics signals at the LHC has prompted a strategic shift toward machines that can either increase the energy by an order of magnitude or scrutinize known particles in unprecedented detail. In parallel, the community recognizes that building any new collider is a long-term endeavor: designing, approving, and constructing a new accelerator can span decades. To avoid a “discovery gap” after the LHC program ends around 2040, planning must begin now for machines that could operate in the 2040s and beyond. The following sections analyze the major next-generation collider proposals under international consideration, examining their designs, timelines, physics goals, costs, and challenges.

Analysis of Major Collider Proposals

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The Future Circular Collider is a two-stage proposal at CERN for a ∼100 km circumference tunnel (∼3 × 10⁵ m) to host consecutive machines. The first stage, FCC-ee, is a circular $$e^+ e^-$$ collider operating at center-of-mass energies from the Z-boson pole ($$\sim91.2 , \text{GeV}$$) up to the top-quark pair threshold ($$\sim350 \text{GeV}$$). It would provide extremely high luminosities—for example, (\sim2 \times 10^{34} , \text{cm}^{-2} \text{s}^{-1}) ((\sim 2 \times 10^{-4} , \text{fb}^{-1} \text{s}^{-1})) at (\sqrt{s} = 240 , \text{GeV}) and up to (\sim1.6 \times 10^{35} , \text{cm}^{-2} \text{s}^{-1}) ((\sim 1.6 \times 10^{-3} , \text{fb}^{-1} \text{s}^{-1})) at the Z pole—enabling the collection of (\sim10^6) Higgs bosons in 10 years and trillions ((\sim10^{12})) of Z bosons. The accelerator design features a double-ring layout with separate ( e^- ) and ( e^+ ) storage rings to accommodate many bunches and continuous top-up injection.

Following FCC-ee, the plan is to install FCC-hh, a proton–proton collider in the same tunnel, with a collision energy of 100 TeV ((10^5 , \text{GeV})) using next-generation 16 T ((1.6 \times 10^4) G) superconducting magnets. FCC-hh would reach a peak luminosity on the order of (10^{35} , \text{cm}^{-2} \text{s}^{-1}) ((10^{-3} , \text{fb}^{-1} \text{s}^{-1})), yielding an integrated dataset of perhaps 20–30 ab(^{-1}) ((2 \times 10^4 - 3 \times 10^4 , \text{fb}^{-1})) over 25 years of operation. The FCC infrastructure is also being designed to allow hadron–electron collisions by injecting a high-energy electron beam into the hadron ring, further broadening the physics program.

A comprehensive international feasibility study for FCC is underway and due to report by the end of 2025. The envisioned timeline would have a construction start in the 2030s, first FCC-ee collisions by the mid-2040s, a ~15-year FCC-ee program, then installation of FCC-hh with first 100 TeV collisions by the 2070s, but mid-century remains a best-case scenario. The stepwise approach allows technical risk reduction – e.g. the tunnel is built and utilized by FCC-ee while the challenging high-field magnet technology for FCC-hh matures. A decision on FCC’s go-ahead is expected around 2027–28, aligned with the next update of Europe’s strategy for particle physics . Feasibility considerations include the civil engineering of a 90–100 km tunnel, development of efficient high-power RF systems for FCC-ee, and crucial R&D on Nb₃Sn or high-temperature superconductor magnets for FCC-hh to achieve 16 Tesla fields at sustainable costs. The FCC collaboration of over 150 institutes has already produced a four-volume Conceptual Design Report in 2019, demonstrating no show-stoppers in principle. A staged upgrade path is built-in: FCC-ee could later be upgraded to higher luminosity or energy, and FCC-hh itself might be upgraded with even higher-field magnets late in the century.

Scientific Goals & Physics: As a Higgs factory, FCC-ee would scrutinize the 125 GeV Higgs boson with extraordinary precision. With O Higgs events, most Higgs couplings could be measured to the sub-percent level , an order of magnitude better than HL-LHC’s expected ~5–10% precision . It would also produce ~10^12 Z bosons , enabling vastly improved electroweak measurements and sensitive searches for rare Z decays . Running at the W pair threshold and top quark threshold would allow high-precision W mass and top quark measurements and an independent extraction of the Higgs self-coupling via loop effects . FCC-ee’s clean environment and high statistics make it a discovery machine for tiny deviations from Standard Model predictions – e.g. indirect signs of new physics in loop processes or forbidden decays.

The FCC-hh phase would then be the ultimate energy-frontier machine. At 100 TeV, it could produce new particles with masses up to ~30–50 TeV directly , far beyond the LHC . It would also greatly extend the reach for high-mass phenomena like multi-TeV resonances, microscopic black holes, or composite structure of quarks . Even in absence of new resonances, FCC-hh would enable precision measurements of phenomena at high momentum transfer – for example, probing the Higgs self-interaction by producing double-Higgs events in large numbers, or studying rare top quark processes. The FCC program is synergistic: FCC-ee would refine the Standard Model “baseline” with precision measurements, thereby sharpening the interpretation of any deviations seen at FCC-hh . It also guarantees a rich physics output even if new heavy particles are very elusive, by fully profiling the Higgs, W, Z, and top. Additionally, FCC-hh would explore heavy-ion collisions and could even be used for proton–electron collisions to probe high-$Q^2$ deep inelastic scattering regimes .

Cost & Collaboration: The scale of FCC makes it a global project. Preliminary cost estimates in the CDR put the tunnel and FCC-ee construction on the order of tens of billions of CHF/€, and the subsequent FCC-hh installation around an additional ~15 billion EUR . CERN and its Member States would be expected to fund a substantial portion, but significant contributions from non-member nations would likely be needed. The project has been presented as a worldwide undertaking, with over 1300 scientists contributing to the design . International review and endorsement – for example, through the European Strategy Update and discussions in Asia and the US – will be critical in securing commitments. An important aspect of cost is the dual-use nature of the infrastructure: the same tunnel and site would host two flagship colliders spanning the entire twenty-first century’s needs . This is part of the cost-benefit rationale: while upfront investment is large, it enables 50+ years of physics and multiple experiments.

Technical Challenges & Risks: Both stages of FCC face significant technical hurdles. For FCC-ee, achieving very high luminosity at ~100 GeV per beam requires handling intense beam currents and power. Synchrotron radiation in a 100 km ring at 45 GeV is substantial and the design must include robust cryogenic and power systems to continuously recover and manage this energy ) ). The beamstrahlung and ultra-fine beam focusing at the interaction points demand cutting-edge accelerator tuning and stability. Maintaining beam collisions for many hours will require rapid top-up injection and perhaps novel feedback systems. FCC-hh brings the challenge of manufacturing and installing approximately 1000 high-field dipole magnets – well beyond the 8–10 T LHC magnets. This necessitates advances in superconductors and magnet cooling. The stored energy in the beams is enormous – roughly 20 times the LHC’s – so beam dump systems and machine protection must be redesigned for 16 GJ of beam energy . Handling routine operation at 100 TeV will stress detector technology too: experiments must withstand higher radiation doses and event rates than ever before, requiring new rad-hard materials, faster sensors, and perhaps AI-assisted triggers to deal with the deluge of data. The power consumption of FCC-hh is projected to be in the few-hundred megawatt range ), raising environmental and cost concerns. Mitigating this will involve improving the energy efficiency of RF power sources, cryogenics, and possibly load-balancing with local power grids or renewable energy sources – areas highlighted for R&D . In summary, while no fundamental physics obstacles prevent FCC’s realization, the engineering and financial challenges are substantial. A failure to secure broad international funding or unexpected technical show-stoppers in magnet development are primary risks. CERN’s strategy to address these involves a vigorous R&D program in magnet and accelerator technology this decade, and a detailed feasibility report to inform a decision by 2028 .

2.2 International Linear Collider

Technical Specifications & Design: The International Linear Collider is a proposed linear e^+?e⁻ collider that would produce high-precision collisions without the energy losses from synchrotron radiation that circular machines suffer at high energy . The ILC design uses superconducting RF cavities to accelerate electrons and positrons in opposite directions along a straight 20–40 km tunnel, colliding them head-on. The baseline design envisions an initial √s = 500 GeV machine upgradable to 1 TeV . However, a staged approach has been emphasized: an initial stage at 250 GeV center-of-mass as a Higgs factory, with later upgrades to 500 GeV and 1 TeV by extending the linacs and adding more accelerating cavities . The ILC would operate with polarized beams to enhance certain physics measurements. The design luminosity is of order 1.8×10^34 cm^−2s^−1 at 500 GeV, and similarly high at 250 GeV, yielding several ab^−1 over a 20-year run . Because beams do not recirculate, the ILC will use damping rings to produce low-emittance bunches and nanometer-scale final focus systems to achieve extremely small beam spot sizes at the collision point . Two large detectors are foreseen, alternating in data-taking . A notable advantage of a linear design is easy extensibility by adding modules, and the inherently cleaner collision environment .

Timeline & Feasibility: Among proposed colliders, the ILC is one of the most technically mature. A complete Technical Design Report was published in 2013 , incorporating decades of R&D from precursor projects in the US and Europe . The technical basis – superconducting RF at 1.3 GHz – has been proven at scale by facilities like DESY’s European XFEL. Japan has been the most likely host: the ILC’s siting efforts since 2013 have focused on the Kitakami mountains in Iwate, northern Japan . The Japanese government has expressed interest, with indications it might fund about half the project , seeking the remainder from international partners. Initially, hopes were to start construction by the early 2020s; however, government approval in Japan has not yet been secured, causing schedule delays. If an endorsement comes within a few years, a possible timeline could be: site preparation late 2020s, construction ~8–10 years, and first beams by late 2030s. The ILC’s feasibility is generally high from a technical standpoint – the accelerator technology is well understood and several prototype components have been tested. A remaining technical challenge is the positron source system: the ILC plans to produce positrons by sending the high-energy electron beam through a helical undulator to create gamma rays that then convert to e⁺e⁻ pairs. This requires a sufficiently high-energy drive beam and a robust target that can handle the intense photon beam. Solutions have been designed and tested on a smaller scale, but engineering it reliably is on the to-do list. In terms of upgrades, the machine can be staged – after a 250 GeV Higgs program, additional linac sections can be installed to reach 500 GeV, and eventually new high-gradient technology could push it to 1 TeV. The timeline for upgrades would depend on physics needs and funding, likely adding another decade for each major upgrade stage.

Scientific Goals & Potential: The ILC’s primary mission is as a Higgs and electroweak precision factory. At 250 GeV, it sits at the maximum of the Higgsstrahlung cross-section , ideal for producing large samples of Higgs bosons. Studies show the ILC could measure the Higgs boson’s couplings to particles like W, Z, b, c, τ with accuracy around 0.5% or better, and even the Higgs total width and invisible decays to <0.5%, by leveraging techniques like recoil mass measurements in ZH events . With beam polarization, the ILC can separately measure how the Higgs couples to left- vs right-handed electrons, adding insight into potential new physics that differentiates chiral couplings. At higher energy , the ILC would measure the top quark’s properties and increase sensitivity to rare processes like e^+?e⁻ → ZHH . If pushed to 1 TeV, it could produce pairs of Higgs bosons in significant numbers and extend the direct search for new particles with electroweak charges up to ~500 GeV–1 TeV . Even at 250–500 GeV, the ILC has discovery potential for “hidden sector” particles through precision – e.g. by detecting slight deviations in processes like e^+?e⁻ → f\bar{f}, or through missing energy signals from exotic decays. Another important role of ILC is to provide model-independent determinations of Higgs properties. Because e^+?e⁻ collisions are much cleaner than pp, and the kinematics are well-known, one can measure absolute quantities like the Higgs coupling constants without assuming, say, that no invisible channels exist . The ILC detectors are designed to reconstruct complex final states with excellent resolution – for instance, separating hadronic decays of W vs Z, or identifying heavy flavor jets – which is crucial for isolating Higgs decay modes. In summary, the ILC promises a broad physics program focused on high precision: from measuring m_W and m_top to a few MeV, to pinning down the Higgs couplings at sub-percent level, to searching for subtle signs of BSM physics in electroweak processes . While it is not an energy-frontier machine beyond 1 TeV, it could discover new particles that couple to electrons up to multi-TeV scales indirectly by observing deviations in how the Standard Model particles interact.

Cost & International Collaboration: Building the ILC is estimated to cost on the order of $7–8 billion for the full 500 GeV machine . The initial 250 GeV stage would correspond to roughly half that length and cost – a ballpark of ~$5 billion . These figures exclude certain in-kind contributions like site-specific civil engineering, and also the ongoing operational costs. Japan’s interest in hosting the ILC is partly driven by the opportunity to create a global science center; the Japanese government has signaled willingness to cover about 50% of the construction cost . The remainder would need to come from international partners. Potential contributors include CERN , the United States, Canada, Korea, China and others – all of whom have large particle physics communities that would benefit from a Higgs factory. Indeed, the international physics community has coordinated on ILC R&D through the Linear Collider Collaboration, uniting prior regional efforts into one team since 2013 . Political and funding discussions for ILC have been ongoing: the project has high scientific merit according to international reviews, but each country must weigh the significant cost. If Japan formally proposes the ILC, it’s expected that a new global oversight body would be formed to manage contributions . Cost-sharing models might include contributions of specific components in lieu of direct cash transfer. The cost-benefit tradeoff for ILC is often discussed in terms of its precision physics output versus the high price tag for a machine that does not itself reach the energy frontier. Proponents note that a guaranteed extensive Higgs/Top/EW program is worth the investment, especially given the complementarity with energy-frontier hadron colliders .

Technical Challenges & Risks: While the ILC is relatively mature, some risks remain. Achieving and maintaining the required beam quality over tens of kilometers is non-trivial – the beams must be focused to ~nanometer sizes and collide with pinpoint precision. Ground motion, vibrations, and tiny fabrication misalignments could spoil the luminosity; hence active stabilization systems and feedback loops are needed. Work on “global accelerator network” feedback and alignment systems is ongoing to mitigate this. Another challenge is the positron source: the ILC’s helical undulator scheme for positron production is elegant, but if it underperforms , a fallback is to build a conventional positron source which could impact the polarization or require more power. Detector backgrounds at a linear collider are generally low, but one must handle beamstrahlung-induced backgrounds and ensure the vertex detectors can cope with intense but short beam pulses. The ILC also requires a very robust cryogenic infrastructure to cool its ~16,000 SRF cavities to 2 K. The operational efficiency and achievable duty cycle will determine if the luminosity goals per year are met; any unforeseen bottleneck in, say, klystron power systems or cryogenics could reduce performance. A broader risk is project scope creep or funding shortfall – if major partners do not join, Japan might not proceed alone, and delays could demoralize the community. The technical risk, however, is considered lower than for other proposals: indeed, the international review panels have largely validated the ILC design as ready to proceed to construction, pending political go-ahead . In sum, ILC’s challenges are surmountable through careful engineering and have been studied extensively, but the project’s realization hinges on securing a global coalition to share the costs and responsibilities.

2.3 Compact Linear Collider

Technical Specifications & Design: CLIC is another linear electron-positron collider concept, pursued by CERN and international partners, but unlike the superconducting RF approach of ILC, CLIC employs a novel high-gradient two-beam acceleration scheme. It aims for multi-TeV capabilities in a comparatively “compact” footprint: staged operation at √s = 380 GeV, 1.5 TeV, and up to 3 TeV, with a corresponding site length from about 11 km to 50 km - Project Implementation Plan]). The key innovation is using a high-current, low-energy “drive beam” that runs parallel to the main beam. This drive beam is decelerated in special structures to generate intense RF power, which is transferred to accelerate the main e⁻/e⁺ beams. This method allows CLIC’s normal-conducting X-band cavities to reach accelerating gradients of ~100 MV/m – roughly three times higher gradient than ILC’s superconducting cavities . The 380 GeV first stage would consist of a central linac ~11–12 km long , producing luminosity about 1.5×10^34 cm^−2s^−1. In the second stage, additional modules extend the linac to ~30 km for 1.5 TeV, and finally ~50 km for 3 TeV. Two interaction points are foreseen so that two detectors can operate alternately. CLIC’s beam parameters call for extremely small bunch sizes and a modest bunch train repetition . The beam pulse structure and fast acceleration minimize synchrotron radiation . Like ILC, CLIC would use damping rings to reduce emittance before injection into the main linac, and sophisticated final-focus optics to attain the required tiny beam spot. The technology for two-beam acceleration has been demonstrated in principle at the CLIC Test Facility, achieving the design gradient and power transfer. An alternate option for the first stage is to use conventional klystron RF sources at X-band instead of a drive-beam .

Estimated Timeline & Feasibility: CLIC’s development has paralleled the FCC and ILC studies in recent years. A Project Implementation Plan was published in 2018, updating the initial 2012 CDR - Indico Global]). The plan laid out a technical readiness by ~2026, meaning that if a decision to build were made, construction could start in the later 2020s - Project Implementation Plan]). The earliest first beams at 380 GeV were projected around 2035 - Project Implementation Plan]). This timeline is similar to ILC’s, but CLIC’s realization is tied to CERN’s strategy: the European Strategy Update placed a higher priority on FCC feasibility study, with CLIC as a backup linear-collider approach. Thus, unless FCC is deemed infeasible or significantly delayed, CLIC might not be approved in the near term. However, CERN continues R&D to keep CLIC viable. The feasibility of the 380 GeV stage is considered high – X-band RF technology has advanced and a prototype drive-beam facility showed the concept works. The main challenges for the higher-energy stages are to maintain efficiency and beam quality as length increases. Staging helps mitigate risk: the 380 GeV machine can be built with existing technology . By operating initially at a lower energy, the project can gain experience and justify further upgrades. Technical readiness: Many components have reached TRL ~4–6, meaning prototypes exist. Still, scaling to thousands of structures will require an industrialization effort. Feasibility studies also examined the geology around CERN for a 50 km linear tunnel . No insurmountable geological issues are known, but detailed surveys would be needed. In terms of staged upgrades, after the first 8–10 years of 380 GeV operations, an interruption of a couple of years might be needed to extend the linacs for 1.5 TeV, and similarly for the final stage. Each extension would be a major project but less than the initial construction since injector systems and infrastructures are already in place.

Scientific Goals & Physics Potential: At 380 GeV, CLIC serves as a Higgs/top factory very much like the ILC at 500 GeV or FCC-ee at 240–365 GeV. Its goals include precision Higgs measurements – e.g. couplings at the sub-percent level, Higgs mass to ~tens of MeV, and observation of rarer processes like Higgs decays to muons . It also aims to measure the top quark mass in a threshold scan with 20 MeV precision and study top-quark couplings in detail . Because CLIC’s detectors would run without the trigger constraints of hadron colliders , it can record all e^+?e⁻ interactions and thereby be sensitive to unexpected signatures . The real power of CLIC comes with the multi-TeV stages: at 3 TeV, CLIC would be a discovery machine for BSM particles that are out of range of the LHC and even FCC-hh in some cases, if those particles couple electroweakly. For instance, CLIC could pair-produce new heavy leptons or supersymmetric particles up to near 1.5 TeV in mass . It can also probe contact interactions or composite scales up to tens of TeV via deviations in scattering angular distributions. Uniquely, certain scenarios like “WIMP” dark matter might be directly visible at CLIC: a pair of dark matter particles could be produced with an associated photon, giving a characteristic mono-photon + missing energy signal in a clean environment, potentially observable if the WIMP mass is < TeV. Precision-wise, CLIC at 1.5–3 TeV could measure the Higgs self-coupling by observing double Higgs production . It could also measure rare decays like H→ Zγ or test the top-Higgs Yukawa coupling at 1.5 TeV via the process e^+?e⁻ → t\bar{t}H. Another interesting aspect: CLIC’s high energy gives it reach to indirectly test physics at scales well beyond 3 TeV through loop effects. Some BSM models would produce slight differences in processes such as two-fermion to two-fermion scattering at high energy; with enough luminosity, CLIC can detect these. In summary, CLIC’s physics program evolves: Stage 1 focuses on Higgs and top with high precision . Stage 2 and 3 push the energy frontier for lepton colliders, giving a direct window into new physics up to ~1.5 TeV in mass and indirect sensitivity well beyond that . It is a unique interplay of precision and energy: at 3 TeV, certain measurements become possible or more precise than at lower-energy machines.

Cost & International Collaboration: The cost estimate for CLIC is about 5.9 B CHF in 2018 prices - Project Implementation Plan]). The later stages would add additional costs . This price is comparable to the ILC for similar energy, though CLIC’s initial stage is a bit higher energy hence slightly more expensive than a 250 GeV ILC. CERN would likely host CLIC at or near the Geneva site, so CERN member states’ contributions would form the backbone of funding. Nevertheless, a project of this scale would seek worldwide participation. The CLIC collaboration includes over 70 institutes globally , and many of those groups also participate in ILC R&D – in fact, the Linear Collider Collaboration ensures knowledge exchange between ILC and CLIC efforts. If CLIC were chosen as the next collider at CERN, we could expect contributions from non-European countries as well, potentially in exchange for access or roles in governance. One cost-benefit point for CLIC is that it can reach multi-TeV energies, offering a higher discovery reach in return for the investment, whereas circular e^+?e⁻ colliders top out at a few hundred GeV. However, its operation at high energy comes with very high power consumption, which is a recurring cost. The collaboration is working on improving energy efficiency . The decision on CLIC vs. other projects is inherently tied to strategy: Europe will compare the cost and physics potential of CLIC with FCC and decide where to commit resources. At present, CLIC is positioned as an alternative should a large circular collider not go forward; it remains “on the shelf” ready to proceed if called upon, with a relatively well-defined cost and schedule - Project Implementation Plan]) .

Technical Challenges & Risks: CLIC’s ambitious acceleration method comes with unique challenges. The synchronization between the drive beam and main beam must be exquisitely maintained – any phase jitter translates to energy jitter in the main beam. This requires novel RF phase control and stability systems. The drive beam generation itself is complex: it involves creating a long train of electron bunches, running them through a bunch compression and delay loop system to interleave bunches and increase current , then decelerating them in energy extraction structures. All these steps have to work reliably at high power. There is also a challenge in beam focusing and alignment over tens of kilometers: like ILC, CLIC needs alignment to the micron level and active steering to collide beams of nanometer size. CLIC’s beam pulses are much shorter and more frequent than ILC’s, which means its detectors have to deal with more instantaneous background . Detector technologies need to handle these intense bursts without significant dead time or damage. Radiation from beamstrahlung at 3 TeV will create a field of incoherent pairs and secondary particles; detailed simulations and a detector design have shown it’s manageable, but it’s a harsh environment. The power consumption is a serious consideration: ~168 MW for 380 GeV operation and possibly ~400 MW or more at 3 TeV - Project Implementation Plan]) . Without mitigation, this could strain operating budgets and raise sustainability issues. R&D into more power-efficient RF sources and energy recovery is ongoing to reduce these numbers. From a risk perspective, one major uncertainty is scaling the X-band structures production: thousands of precisely machined copper structures are needed, and they must withstand high electric fields without breakdown. So far, test stands achieved the required gradient with acceptable breakdown rates, but ramping up industrial production while maintaining quality is a project in itself. Overall, the primary risks for CLIC lie in the successful integration of many advanced subsystems and the availability of funding in a scenario where it might be the second choice behind a bigger machine. If funding or political will falters, CLIC could remain a well-developed concept that isn’t built. Technically, however, no fundamental show-stopper has appeared – it’s a matter of complex engineering and coordination to make it a reality.

2.4 Circular Electron–Positron Collider and Super Proton–Proton Collider

Technical Specifications & Design: China has proposed an ambitious two-stage collider project similar in scope to CERN’s FCC. The Circular Electron–Positron Collider is a 100 km circular e^+?e⁻ collider planned as a Higgs and electroweak factory, to be followed by the Super Proton–Proton Collider, a 75–125 TeV-class hadron collider in the same tunnel . The latest CEPC design ) envisions a double-ring collider in a 100 km circumference tunnel, with a full-energy injector booster co-located in the tunnel . CEPC would operate at several energies: a Higgs run at √s = 240 GeV, a Z-pole run at 91 GeV, a W threshold run ~160 GeV, and a top-pair threshold run around 360 GeV . Two large detectors are foreseen. Luminosity targets are on the same order as FCC-ee: about 2×10^34 cm^−2s^−1 at 240 GeV, and up to 16×10^34 cm^−2s^−1 at the Z pole . The CEPC design uses 30 MW of synchrotron radiation power per beam at 120 GeV as a design cap , and includes continuous top-up injection to keep luminosity constant despite beam losses . Notably, CEPC’s design also considers using it as a synchrotron light source and a γγ collider via laser backscattering in parasitic modes , to maximize its utility. The follow-on SppC would be a proton collider initially targeting 75 TeV c.m. with an upgrade to >100 TeV possible if 20 T magnets are realized . The SppC could possibly run concurrently with CEPC in the tunnel . SppC’s design luminosity goal is around 1×10^35 cm^−2s^−1 . To achieve 75 TeV with 12 T magnets, SppC will rely on iron-based high-temperature superconductors, a newer technology which potentially offers higher critical fields at higher temperatures than Nb₃Sn . The tunnel cross-section is planned to accommodate both rings .

Timeline & Feasibility: The timeline put forward for CEPC–SppC is aggressive. Chinese planning documents have suggested CEPC construction in the 2020s, operation in the 2030s for about 10 years, and a transition to SppC construction by ~2040, aiming for SppC completion in the mid-2040s . Indeed, CEPC was identified as the top priority future project by the Chinese Academy of Sciences in 2023 . A pre-conceptual design report was completed in 2015 , and the Conceptual Design Report in 2018 . Now with the Technical Design Report done , the project is moving into a phase of seeking governmental approval and international partnership. Building a 100 km tunnel in China is deemed feasible – several candidate site locations have been geologically studied . China has recent experience with large tunnel projects , which lends confidence to the civil engineering. The CEPC accelerator technology – large-scale RF systems, cryogenics, precision magnets for the double-ring – will require significant industrial ramp-up, but no novel fundamental tech beyond what FCC-ee plans. SppC, on the other hand, faces the magnet R&D challenge. The decision was made to start SppC at 12 T with new superconductors to not delay the project until 16 T Nb₃Sn is ready; this places a heavy R&D load on developing practical iron-based superconductor cables and 12 T dipoles. A staged approach for SppC is built-in: first 75 TeV, then later push to 20 T magnets for ~125 TeV . The feasibility of that second stage is uncertain – high-temperature superconductors at 20 T in accelerator-quality magnets is a cutting-edge research topic worldwide. In terms of global feasibility, China has expressed that CEPC should be an international project, inviting collaborators. If realized purely nationally, it could face delays if funding is limited, but if opened to international contributions, it may progress faster. Political will in China will be crucial – the project will likely be approved only if seen as aligning with national science and technology goals. As of the latest reports, CEPC had strong support at the scientific level, but a full government funding decision was still pending. If approval comes by mid-2020s, a construction period of ~8–10 years could indeed see CEPC operational by ~2035. SppC would then be a follow-on decision ~ decade later.

Scientific Goals & Physics Potential:CEPC’s physics mission is very similar to FCC-ee and ILC in the precision electroweak and Higgs domain. Running at 240 GeV, CEPC would produce about a million Higgs bosons in 10 years , allowing measurements of Higgs couplings to ~0.1–1% accuracy . Particularly, the Higgs coupling to b-quarks, W, Z can be measured to better than 1%, and even to gluons, τ, c to a few percent. By lowering to the Z-pole energy , CEPC can collect an enormous sample of Z bosons – numbers on the order of 5×10^11 to 10^12 in a couple of years – far beyond the LEP sample. This enables a rich program of precision electroweak measurements: e.g. the Z boson mass and width with unprecedented precision, improved measurements of sin^2θ_W, and searches for rare decays like Z→µµγ that could signify new physics. As a “W factory” at 160 GeV, it can measure the W mass to a few MeV by threshold scanning. And as a “top factory” at ~350–360 GeV, CEPC could collect ~10^5 top quark pairs to measure the top mass to tens of MeV and study top electroweak couplings. All these would surpass what the LHC/HL-LHC can do, providing a deeper test of the Standard Model. The high statistics of Higgs allows sensitivity to exotic or rare decays ). It also allows a search for potential Higgs CP-violation or other anomalies via precision differential measurements. In summary, CEPC is a quintessential intensity-frontier machine for e^+?e⁻ physics, filling in knowledge of Higgs and electroweak in great detail.

SppC, as a proton collider at 75–125 TeV, targets the discovery of new physics at the highest energies. Its initial 75 TeV energy would greatly extend the direct mass reach: for example, it could potentially produce particles of tens of TeV in mass . The jump from LHC to SppC is slightly less than LHC to FCC-hh , but still transformative. SppC would thoroughly explore any TeV-scale new physics scenarios that the LHC might hint at. If dark matter is a weak-scale particle, SppC could either produce it directly or, if dark matter is heavier, explore the physics up to the scale where we might see quantum effects of heavy particles . It also offers the possibility to examine QCD in new regimes – for instance, to study the behavior of the strong force at extremely high momentum transfer, or heavy-ion collisions at far higher energy than LHC, potentially probing quark–gluon plasma under different conditions. As with FCC-hh, SppC would allow precise measurements of Higgs self-interactions by producing high rates of double-Higgs events once it reaches the full 100+ TeV scope. The combination of CEPC + SppC mirrors FCC’s philosophy: first, a no-ambiguity precision mapping of the Standard Model , then a wide-net search for new particles. Additionally, CEPC and SppC together enable unique opportunities: for example, a CEPC–SppC electron-proton run could be realized in that same infrastructure , enabling deep-inelastic scattering studies at previously unreachable energy and momentum-transfer, which would be extremely valuable for QCD and parton distribution knowledge relevant to the proton collider. China’s strategy also emphasizes that CEPC/SppC would make the country a focal point of high-energy physics, potentially attracting many international users and facilitating technology exchange.

Cost & Collaboration: One attractive feature of CEPC as presented is its cost relative to other projects. The official estimate for CEPC is 36.4 billion RMB, approximately $5.15 billion USD . This includes about RMB 19B for the accelerator, 10.1B for infrastructure, 4B for detectors . By comparison, this is on par with the estimated cost of ILC or CLIC first stages, largely due to China’s lower construction and labor costs, and design optimizations such as a partial double-ring option considered to save money . The cost for SppC has been less formally quoted, but given FCC-hh is ~€15B, one might expect SppC in the same range . China so far has mostly self-funded the design phases . For construction, Chinese funding agencies would cover a large fraction, but they have expressed that international contributions are desired – both for cost-sharing and for integrating global expertise. The International Committee for Future Accelerators has noted support for global Higgs factory proposals, including CEPC . Potential partners in CEPC could be European institutes , the US , and neighboring Asian countries. A tricky point is that at the same time, Japan has ILC in consideration and Europe has FCC – the global community will need to balance these. It’s possible that if Japan does not proceed with ILC, some of that effort might pivot to CEPC. Conversely, if CEPC moves fast, Europe might reconsider the necessity of FCC-ee and focus on the hadron collider later. In terms of cost-benefit, CEPC offers a relatively cost-efficient Higgs factory – a major selling point – but it doesn’t by itself reach energy frontier. The combination with SppC gives a complete package but doubles the overall timeline and cost. It’s conceivable that China would fund CEPC largely on its own and then seek a broader coalition for SppC. Politically, the CEPC/SppC project aligns with China’s desire to lead big science projects. Economically, however, it has to compete with other domestic priorities. The stated cost is significant but spread over many years it might be palatable; SppC later would require another large investment. We should note that for context, China is currently constructing a new fusion reactor facility, a new space station, etc., showing willingness to invest in large-scale science.

Technical Challenges & Risks: Many of the technical challenges of CEPC/SppC echo those of FCC. For CEPC: handling high beam currents and synchrotron radiation, producing and controlling ultra-low emittance beams, and managing a long operational cycle with constant luminosity via top-up injection are key challenges . Achieving the design luminosity at the Z pole might be limited by beam instabilities or beam-beam effects – the CEPC team has considered a partial double-ring option to reduce cost, but it would cap luminosity at the Z pole . Ensuring mechanical and electrical stability across 100 km of circumference is non-trivial. For SppC, the big challenge is clearly the superconducting magnet development. Iron-based high-Tc superconductors are relatively new and have not yet been made in long, high-current cables suitable for accelerator magnets. The SppC plan assumes they can achieve 12 T with such materials , which will require intensive R&D and likely international collaboration with experts in magnet technology. There’s a risk that the desired performance or cost-efficiency of these magnets might not materialize on the expected timeline, which could force SppC to fall back to Nb₃Sn technology at slightly lower field . Another risk area is the detectors, especially for SppC: like FCC-hh, the detectors must survive higher radiation and event rates than LHC. China has less experience with building huge collider detectors ; ramping up that expertise is part of the plan, but it’s a learning curve. On the CEPC side, designing detectors that can leverage the enormous Z sample for flavor and QCD studies will push technology to combine precision with very fast readout. Environmental and operational challenges also exist: a 100 km tunnel with multiple surface sites means significant environmental impact , which requires careful planning and community engagement. The power consumption for CEPC is projected to be similar to FCC-ee , and for SppC possibly a couple hundred MW – ensuring sustainable energy supply and cooling is a challenge . An interesting risk factor is concurrency: if CEPC runs and SppC is built simultaneously in the same tunnel , coordination will be complex. Overall, the CEPC/SppC project carries moderate technical risk on CEPC and higher risk on SppC. If CEPC goes well but the advanced magnets for SppC lag, there could be a significant gap or need for redesign for the hadron phase. To mitigate this, the project is doing magnet R&D in parallel and considering a phased approach where even if SppC is delayed, CEPC’s physics output justifies itself in the interim.

2.5 Muon Collider

Technical Specifications & Design Overview: A Muon Collider is a novel concept that seeks to collide µ⁺ and µ⁻ at very high energy. Muons are 200 times heavier than electrons, which dramatically suppresses synchrotron radiation – meaning muons can be accelerated in a circular ring to multi-TeV energies without the catastrophic energy loss that electrons would suffer . This property offers the tantalizing prospect of an energy-frontier lepton collider that could reach 10 TeV or more in a relatively compact ring . A 10 TeV muon collider would have a collision energy well beyond any planned e^+?e⁻ machine, with the clean interaction of fundamental leptons . However, muons are unstable, with a lifetime of 2.2 µs at rest. Any realistic muon collider design must create, accelerate, and collide muons fast – on the timescale of microseconds to milliseconds . The broad design involves: producing muons via an intense proton beam hitting a target , collecting and cooling those muons , accelerating them quickly , and injecting into a collider ring before too many decay. Because muon beams decay, they produce a large flux of neutrinos in the direction of the beam . At TeV energies, those decay neutrinos can radiate and pose a hazard if pointed at the ground or detectors. Thus, lattice designs for muon colliders include carefully managing the geometry to mitigate neutrino-induced radiation . The baseline parameters under study by the International Muon Collider Collaboration include a 3 TeV demonstrator collider and an eventual 10 TeV collider, with luminosities in the order of 1–5×10^34 cm^−2s^−1 scaling up with energy to deliver several ab^−1 per year . The collider ring might be ~6 km for 3 TeV and maybe ~10–14 km for 10 TeV . The acceleration sequence could involve a linac or rapid pulsed synchrotron . The key hardware elements are: a high-power proton driver to create muons; a target and front-end channel to capture pions and muons; a sequence of ionization cooling channels – arguably the most challenging piece – to compress the muon beam emittance within its short lifetime; and strong focusing and RF systems to accelerate the muons. The final collider ring needs very strong magnets to focus the muons at collision to maximize luminosity, and perhaps “crab waist” crossing schemes to mitigate beam-beam. All these pieces are under active research, with a conceptual design in progress.

Estimated Timeline & Feasibility: Until recently, muon colliders were considered futuristic, but they have gained momentum. The 2020 update of the European Strategy for Particle Physics recommended R&D into a muon collider as a potential long-term option, and the U.S. Snowmass process also showed strong interest. The IMCC led by CERN, with participation from the US and others, has laid out a technically limited timeline for a multi-TeV muon collider: aiming to complete a first end-to-end conceptual design by ~2026 , build a muon collider demonstrator in the early 2030s, and potentially realize a first ~3 TeV collider by the late 2030s or 2040s . This optimistic schedule would have a 10 TeV collider operating perhaps in the 2050s . However, it’s emphasized that this is a technically limited schedule assuming fast progress and full funding, whereas a more realistic schedule might be slower . The demonstrator mentioned would be a smaller-scale machine to test muon cooling and acceleration . Feasibility-wise, many components have been validated separately: the Muon Ionization Cooling Experiment in the UK demonstrated the principles of muon cooling in 2017–2019 ; studies of target systems and capture have shown multi-MW target feasibility; and normal-conducting RF cavities have been tested in magnetic fields relevant for cooling channels . A full end-to-end demonstration has never been done – that’s the goal of the next decade of R&D. If that succeeds, building a 3 TeV muon collider could become a concrete proposal. Muon colliders have the advantage that much of the ring hardware can reuse tech from hadron colliders , and the size is relatively small for the energy achieved. The feasibility concern is mainly about the front-end: producing enough muons and cooling them quickly. The IMCC is working on an updated design based on innovations like parametric ionization cooling and new magnet configurations. Staged upgrades are natural: one could first build a 3 TeV collider, then later upgrade the same ring with more cooling and maybe more accelerator length to 10 TeV. Or build a 10 TeV ring after demonstrating at 3 TeV. The strategy is yet to be decided. There is also synergy with neutrino factory R&D .

Scientific Goals & Physics Potential: A muon collider offers an extraordinary physics reach because it combines the advantages of leptons with ultra-high energy. A 10 TeV muon collider would surpass even a 100 TeV pp collider in some areas of BSM discovery reach . For example, it could produce heavy neutral Higgs bosons or supersymmetric particles that have electroweak charges up to near 5 TeV in mass . It could also potentially produce vector boson scattering resonances if the electroweak symmetry breaking sector has surprises at multi-TeV scale . Moreover, being a lepton collider, a muon collider can directly measure the Higgs boson’s properties in s-channel production: uniquely, it could produce the 125 GeV Higgs via µ^+µ^− → H resonance. With energy resolution good enough, it could scan the Higgs pole and measure the Higgs width extremely precisely, or even discover a hypothetical new scalar that couples to muons. At high energy , it can probe multi-Higgs interactions with much higher rates than e^+?e⁻ colliders and without the huge backgrounds of a pp collider. It’s also ideal for exploring any lepton-specific new physics: e.g., if there is a new Z′ boson that couples to the second or third lepton generation, a muon collider would be the best way to produce it if within reach. In terms of precision, a muon collider at 3 TeV still retains some of the capability to do precision measurements – for instance, it can study the production of W and Z bosons at high energy for signs of anomalous couplings or non-standard behavior. The physics potential is still being fully evaluated . Importantly, if no new particles are found by HL-LHC or other planned machines, a muon collider at 10 TeV provides one of the best shots at an energy jump that could reveal something completely new – effectively leaping into an energy regime previously thought only accessible to a very large hadron collider, but with the cleanliness of a lepton machine. In summary, the muon collider’s goals are to provide an energy frontier lepton platform: it would both act as a discovery machine for new heavy particles up to ~ half its energy, and a precision machine for any new phenomena or Higgs interactions at those energies, free from the QCD background of hadron colliders. This dual role is why it’s generating excitement as a long-term option.

Cost & International Collaboration: Because a muon collider is still in the R&D phase, cost estimates are highly uncertain. Early rough estimates extrapolate from existing collider components and suggest it could be competitive in cost per TeV with hadron colliders , but these are speculative . The current strategy is to refine a cost projection by 2030 after a reference design is established . Certainly, building a muon collider will require a global effort. The IMCC includes CERN, which might host a muon collider at a future date , and significant interest from the US – the U.S. P5 panel in 2023 recommended a muon collider R&D program and mentioned exploring Fermilab as a potential host site . This suggests a global coordination where maybe one of the major labs in Europe or the US would eventually host the facility, with worldwide contributions. Since the muon collider doesn’t exist as a formal project yet, there’s no committed funding – but we can anticipate that the cost would necessitate shared investment. In terms of cost-benefit, a muon collider’s selling point is energy for cost: you achieve 10 TeV collisions in a facility that might be comparable in size to LHC or a bit larger, rather than needing a 100 km tunnel. If the technology works out, it might be the most cost-effective route to the energy frontier in the latter half of this century . That argument will solidify once a detailed design and cost breakdown is available . Also, a muon collider would drive innovation in several tech areas , which could attract funding from high-tech R&D budgets. Politically, the notion of a cutting-edge, unique facility might rally international support, but conversely, some stakeholders might view it as high risk until proven.

Technical Challenges & Risks: The muon collider probably has the highest technical risk of any proposal discussed, because it requires integrating technologies that have never been combined at scale under tight timing constraints. The foremost challenge is muon cooling: to get luminosity, the muon beams must be shrunk to a tiny size in phase space. Ionization cooling – passing muons through absorbers to reduce momentum then reaccelerating longitudinally – is theoretically workable and was partially demonstrated by MICE , but an actual cooling channel might involve tens of meters of tightly packed solenoids, RF cavities, and absorbers. Demonstrating a section of this channel with the needed performance is crucial. Without sufficient cooling, luminosity will be low. Another challenge is creating enough muons: a ~4 MW, 8 GeV proton source on target is considered, but target technology must survive extreme thermal shock. Next, accelerating muons rapidly: one may use a linac or recirculating linear accelerators or fast-cycling synchrotrons. Fast-ramping magnets would be needed for synchrotrons – a technology that exists only up to certain energies . Pushing those to higher energy is R&D. The collider ring itself, while smaller than FCC-hh, needs magnets that can handle significant radiation from muon decays and possibly have open midplanes or special shielding for neutrino radiation. As noted, neutrino radiation is a unique problem: as muons decay, the decay neutrinos can exit the earth and produce a radiation footprint at ground level. At TeV energies with tens of bunches at high rep-rate, calculations show this could be non-negligible . The mitigation proposed includes carefully controlling the beam trajectory to distribute neutrino directions and avoid any one spot getting a high dose , and choosing a site with sufficient depth and away from population. This will be a factor in site selection and design. Detector backgrounds from muon decays in the ring are also significant: each muon can decay in the collider ring near the collision point and produce energetic electrons that spray into the detectors. Detector designs therefore include heavy shielding and advanced timing to distinguish collision events from decay backgrounds. Risks: If the cooling cannot reach design goals, luminosity might fall short by an order of magnitude, making the physics output less compelling. If the engineering of the rapid acceleration fails, the muons might decay before collision. There’s also integration risk: even if each subsystem works, they must work in concert on an unforgiving time schedule . This leaves little room for error or delays in the acceleration chain. However, the effort is ongoing to retire these risks one by one. An interim design report is in preparation by IMCC, and test facilities are being considered . The community acknowledges that a muon collider timeline now is aspirational, and achieving it requires a globally coordinated R&D program at a level of commitment comparable to what was invested in LHC development or the SSC magnets in the past . In short, the muon collider promises game-changing capabilities but faces an uphill battle in R&D, with no guarantees until key demonstrations are successful.

3. Comparative Analysis

To compare these collider proposals, we summarize key parameters and considerations in Table 1 and Table 2 below. Table 1 outlines the technical specifications and timelines of each proposal, while Table 2 compares their scientific focus, estimated costs, and key challenges. This highlights the different trade-offs: Higgs factory vs. energy frontier, linear vs. circular, near-term achievable vs. requiring significant R&D breakthroughs, etc., positioning each project in the global context.

Table 1.Key Parameters and Timeline of Proposed Next-Generation Colliders

Table 2.Comparison of Focus, Collaboration, Cost, and Challenges

• , ILC TDR , CLIC Implementation Plan - Project Implementation Plan]), CEPC CDR , CERN/ICFA statements for muon collider , etc.)*

From the comparison, it’s evident that no single project optimally covers all physics — hence the emphasis on complementarity. Higgs factories focus on guaranteed precision gains, using well-established technologies at moderate cost, and are achievable on the 15–20 year horizon. Proton colliders chase the energy frontier; they promise the highest discovery potential but come with high costs, long timelines, and significant technology pushes . CLIC sits somewhat in between, offering an energy ramp beyond other lepton colliders, but requiring novel acceleration techniques and high power. The Muon Collider is the bold, long-term play that could leapfrog both in energy and maintain precision, but it’s currently at the feasibility R&D stage and not yet a guaranteed option.

In the global HEP landscape, there is a degree of regional specialization but also overlap: Europe has positioned FCC as its strategic direction , China is advancing CEPC/SppC, Japan is considering the ILC, and the U.S. is prioritizing neutrino physics and its Electron-Ion Collider for the 2020s while contributing to global collider efforts. This means strategic choices in the next few years could determine whether multiple colliders go forward in parallel or a single global collider is chosen. For instance, if Japan green-lights ILC soon, that could satisfy the Higgs factory role and perhaps alleviate pressure on Europe/China’s plans; conversely, if Japan does not proceed, the focus shifts to either FCC-ee or CEPC as the Higgs factory. Likewise, if China builds CEPC in the 2030s, by the time CERN is ready for FCC-ee in the 2040s, some physics might already be done — but FCC-ee could then extend to even higher precision or to the top-quark domain which CEPC would also cover. These considerations show that coordination and timing are critical: the community ideally wants continuous progress .

Another comparative point is the risk vs. reward. Projects like ILC and CEPC are lower-risk technically and scientifically , whereas FCC-hh or a Muon Collider carry higher risk but also the highest potential reward . Funding agencies and strategy panels must weigh this balance. Table 2’s last column underscores how some challenges are common to both FCC-hh and SppC — a natural area for collaboration rather than redundant R&D. Indeed, CERN and Chinese institutes are already sharing R&D results on high-field magnets and other technologies, recognizing the benefit of not duplicating efforts. Similarly, whether a linear collider is built in Japan or at CERN , the underlying SRF or X-band technologies and detector development have been a collaborative international effort under the linear collider community.

In terms of scientific impact, a Higgs factory is often seen as guaranteed Nobel-level physics , whereas a 100 TeV collider could be transformative if new particles exist in that range, but even if not, it will push our knowledge of QCD and the high-energy behavior of the SM to new heights. The muon collider could, in principle, provide both precision and energy reach, but it needs validation. Hence, many strategies envision a two-step: do a Higgs factory first while R&D continues on advanced concepts , then later in the century do the big leap to a ~100 TeV hadron collider or a muon collider. The comparative tables reflect that likely sequencing.

Finally, each project’s position is influenced by sociopolitical factors: CERN has a track record and existing infrastructure, making FCC or CLIC a natural continuation ; China sees CEPC/SppC as a chance to lead and is moving rapidly on it; Japan’s window for ILC may close if delayed too long, as others move on. In the next section, we provide strategic recommendations taking these factors into account.

4. Strategic Recommendations

Given the extensive analysis of the proposals, a few strategic insights emerge on the most promising initiatives and how to maximize the scientific return while managing risks and resources:

• Prioritize a Higgs/Electroweak Factory in the Near Term: The case for a high-precision lepton collider is very strong and broadly supported . Such a machine guarantees rich scientific output by measuring the Higgs boson’s properties and electroweak parameters with an order-of-magnitude improvement in precision. It also serves as a stepping stone for training the next generation of physicists and developing technologies for bigger projects. Recommendation: The international community should coalesce around one Higgs factory project to begin construction by the early 2030s. If Japan commits to the ILC, that should become a fully international project rather than competing efforts. If ILC is not promptly approved, focus should shift to either CEPC or FCC-ee – whichever is more shovel-ready – to ensure a Higgs factory is realized. Notably, CEPC’s cost-effectiveness and schedule might make it feasible to have results by the 2030s; a later FCC-ee could still happen, but duplication of two similar machines should be avoided or staged. One scenario is ILC in Japan in 2030s, followed by an FCC-ee in the 2040s which complements ILC’s Higgs program. Alternatively, CEPC in China in 2030s and then perhaps an ILC at 500 GeV or a muon collider later. The strategy should be decided within the next 1–2 years as these projects are at decision points.

• Maintain Momentum on High-Energy Collider R&D : Even as a Higgs factory gets built, R&D towards the next energy frontier collider must continue, given the long lead time. This includes the high-field magnet development for a hadron collider and the advanced concepts like the muon collider or plasma acceleration. The FCC-hh and SppC can be viewed not as rivals but as parallel developments – collaborations like the US DOE’s magnet development program, CERN’s FCC magnet R&D, and China’s high-Tc superconducting magnet research should pool expertise. A technologically driven schedule is wise: for example, aim to demonstrate robust 16 T magnets by ~2030 , which would pave the way for an informed decision on a 100 TeV-class collider by the mid-2030s. If magnet progress is fast and the Higgs factory is in Asia, Europe might choose to proceed with FCC-hh in the 2040s as a solely hadron machine . Conversely, if the muon collider R&D yields a breakthrough , the community might then pivot: a Muon Collider could be seen as the more cost-effective way to reach multi-10 TeV scale without a 100 km tunnel . It would be prudent for strategy groups to keep the muon collider option open. Specifically, support the IMCC’s roadmap with modest funds now – this has high leverage, since a positive result by 2030 could completely change long-term plans. In short, invest in R&D to “buy down” risk for the advanced colliders: high-field magnets, muon cooling, high-gradient RF, etc., so that by the time a decision is needed, these projects have credible designs and cost estimates.

• Foster Global Collaboration and Avoid Redundancy: The sheer scale and cost of these projects mean no single nation can do it alone . International collaboration is not just beneficial, it’s increasingly essential. Politically, this requires careful diplomacy: CERN engaging with China’s Institute of High Energy Physics, Japan’s KEK, the U.S. DOE, etc., to carve out roles. One positive example to emulate is the Linear Collider Collaboration, which unified designs for ILC and CLIC in many aspects and shared R&D. Another is the CERN-CEPC dialogue on tunneling and magnet R&D. Strategic coordination could look like: if CEPC goes ahead, CERN might contribute and in return China might contribute to FCC-hh later. Similarly, if ILC goes ahead in Japan, CERN and China could both participate, and Japan could later join FCC or a muon collider. Such interweaving of contributions can mitigate the “competition” and turn it into synergy. It will be important to align schedules to maximize complementarity: e.g., don’t run two Higgs factories at the same time doing the same measurements; instead, plan one after the other or with different focuses . A global planning forum might be needed to coordinate these timelines beyond what the regional strategy updates do. The 2020 update of the European Strategy explicitly mentions the need for a global context for future colliders , and the recent ICFA statement supports a Higgs factory as a global priority – these are encouraging signs.

• Leverage Computational and AI Advances: Future colliders will produce unprecedented volumes of data and will require exquisite control systems – an area where modern computing and AI can play a transformative role. Recommendation: Incorporate cutting-edge machine learning and AI from the design phase through operation. For example, AI-driven accelerator controls can optimize machine tuning and stability in real-time. SLAC experiments have shown ML can dramatically speed up beam tuning and maintain stability . Applying this to future colliders could improve luminosity uptime and reduce operational costs. Similarly, AI in detectors will be essential: high-luminosity hadron colliders at 100 TeV might have event rates where only AI-based trigger algorithms can handle the complexity. It’s advisable that each project establish a joint task force with computer scientists to integrate AI solutions – this could include anomaly detection for accelerator components and efficient analysis frameworks to handle exabyte-scale datasets. The computing models of LHC will not scale to 100× more data without new paradigms; distributed computing, cloud resources, and intelligent data reduction will be needed. By investing in these areas now, the community can ensure that when these colliders turn on, they can reach their potential without being bottlenecked by data or control issues. Additionally, simulations for collider design can shorten design cycles and optimize performance beyond human-tuned designs .

• Address Environmental and Societal Factors Early: Large colliders inevitably raise environmental questions and require strong public and political support. It’s strategically important to frame these projects not just as discovery machines for esoteric physics, but as drivers of innovation and societal benefit. For instance, high-field magnet R&D contributes to medical MRI advances, accelerator technology has spinoffs in industry and medicine, and big science projects often inspire and train a high-tech workforce. The FCC feasibility study explicitly is looking at sustainability . Following that lead, all projects should incorporate green design principles – perhaps using renewable energy sources for powering the collider, recycling waste heat , and minimizing the carbon footprint of construction. Politically, cost is a dominant factor: the memory of the SSC cancellation in the US looms as a caution . Thus, proposals need robust cost control plans and staging that delivers physics results stepwise to keep funders invested. Building in stage gates can reduce risk. It’s also important to start outreach now – engaging the public with the excitement of a 100 TeV collider or a muon collider, explaining why we push these frontiers. Success of LHC has shown that big discoveries do capture the public imagination, but keeping that support means being honest about the uncertainties and emphasizing the knowledge gains and technological progress that colliders bring . Given global political winds, international projects can be challenging – but physics has a long history of transcending national rivalries for common goals . Continued diplomacy and perhaps new frameworks may be required.

In choosing among initiatives, feasibility and scientific return must be balanced. The most promising near-term initiative is clearly a Higgs factory, due to its guaranteed impact and relative feasibility. The most promising long-term is either a 100 TeV-class hadron collider or a multi-TeV muon collider – at this moment, it is wise to pursue R&D on both paths until it becomes clearer which can deliver the needed performance for acceptable cost. The community should also remain flexible: for example, if a surprising discovery at LHC or a Higgs factory occurs , plans might shift to a collider optimized for that mass scale.

5. Conclusion and Summary

In conclusion, the international physics community is charting an ambitious course for the post-LHC era, with multiple collider proposals that address both the high-energy frontier and the high-precision frontier. The motivations are compelling: unresolved mysteries like dark matter, the matter–antimatter asymmetry, neutrino masses, and the hierarchy problem demand new experimental probes. The LHC, while immensely successful , has shown us the limits of the current machine – to go further, we need either significantly more energy or much finer measurement capability .

We reviewed the major collider projects under consideration:

• The Future Circular Collider at CERN, a dual machine program that offers a long-term roadmap through the end of the 21st century . FCC-ee would provide an exquisite tool for precision studies of the Higgs and electroweak sector, while FCC-hh at 100 TeV would explore uncharted territory at the energy frontier, with potential to discover new particles up to tens of TeV . The FCC project is technically feasible but will require unprecedented funding and coordination – a commitment by CERN and global partners on the scale of ~$20B over decades. Key challenges like 16 T magnets and 100 km tunneling are the focus of ongoing R&D .

• The International Linear Collider in Japan, which could be realized on a shorter timescale if approved, leveraging proven superconducting technology to deliver ~250–500 GeV collisions . It promises a clean Higgs factory and has detailed design documents . Its realization now hinges more on political will and cost-sharing negotiations than on technical unknowns. An operational ILC in the 2030s would greatly enhance our knowledge of the Higgs and might reveal subtle BSM effects .

• The Compact Linear Collider, CERN’s high-gradient linear collider concept, which provides a scalable path to multi-TeV e^+?e⁻ collisions . It pushes accelerator innovation with its two-beam scheme . CLIC could achieve higher energies than ILC , directly probing new physics in the TeV range, though it requires solving challenging engineering problems in RF power generation and alignment. Its first stage is within reach if chosen, and it stands as a compelling option should a linear collider become the favored direction in Europe - Project Implementation Plan]).

• China’s CEPC and SppC proposal, which parallels the FCC strategy on a slightly smaller scale. CEPC would be a valuable Higgs/Z factory with potential start in the 2030s . If realized, it would make China a central player in particle physics, and its cost estimate is relatively moderate . The follow-up SppC aims at the energy frontier ~75–100 TeV , which, if successful, could compete with or complement FCC-hh. The CEPC/SppC timeline is ambitious, and executing it will test China’s scientific infrastructure and international collaboration model. Already, CEPC has spurred R&D advances that benefit the field .

• The re-emerging Muon Collider concept, which represents a bold leap in collider innovation. Though in early stages, it holds the promise of combining the strengths of hadron and lepton colliders – offering multi-TeV collision energy in a cleaner environment than pp collisions . The next decade will be critical to prove whether the muon collider’s unique challenges can be tamed. If they can, a 10 TeV muon collider by ~2050 could be the ultimate tool to delve into phenomena at the highest energies with precision measurements, effectively carrying the torch of discovery into a regime no other machine could reach .

Comparatively, we see a likely scenario of at least two complementary colliders: one lepton collider for precision in the near-to-medium term, and one proton or muon collider for the energy frontier later on. The strategic coordination of these efforts will determine how effectively we can push the frontiers of knowledge. With finite resources, the community may have to choose – for example, ILC vs CLIC vs FCC-ee for the Higgs factory role, and later 100 TeV hadron vs muon collider for the energy frontier. These decisions will be informed by the outcomes of feasibility studies and R&D milestones .

In making these choices, global collaboration is paramount. The future collider programs will likely be the largest scientific projects ever attempted, and they transcend what any single region can do. Encouragingly, there is an international consensus forming: the idea that a Higgs factory is “the highest priority” next step , and strong support for exploring novel concepts like muon colliders in parallel . The next steps will involve turning these plans into reality – securing funding, finalizing sites, and ramping up construction. As we do so, the integration of new technologies and careful management of cost and risk will determine success.

In summary, the next-generation colliders being considered – FCC, ILC, CLIC, CEPC/SppC, and the Muon Collider – each present a pathway to new discoveries and deeper understanding of fundamental physics. They address the shortcomings of the LHC and promise to keep pushing the boundaries of human knowledge. The technical and financial challenges are enormous, but not insurmountable with worldwide effort. By proceeding judiciously – choosing the right project at the right time, fostering collaboration, and innovating with new technologies – the particle physics community aims to ensure that the coming decades yield a rich harvest of scientific results. The excitement and potential of these projects is high: whether it’s unveiling new particles, conclusively deciphering the Higgs boson’s role in the cosmos, or perhaps discovering something entirely unexpected, future colliders will be at the heart of 21st-century fundamental science. Each step – from precision Higgs studies to explorations of the ultra-high-energy frontier – brings us closer to answering the deep questions about the universe left in the wake of the Standard Model’s triumphs.