Since being delivered to the International Space Station (ISS) by Space Shuttle Endeavour in 2011, the Alpha Magnetic Spectrometer (AMS-02) has recorded more than 200 billion cosmic-ray events with energies extending into the multi-TeV range. Although never designed to be serviceable, a major intervention to the 7.5 tonne detector in 2019/2020, during which astronauts replaced a failing cooling system, extended the lifetime of AMS significantly (CERN Courier March/April 2020 p9). Now, the international collaboration is preparing a new mission to upgrade the detector itself, by adding an additional tracker layer and associated thermal radiators. If all goes to plan, the upgrade will allow physicists to gather key data relating to a mysterious excess of cosmic rays at high energies.
Precise dataset
The increasingly precise AMS-02 dataset reveals numerous unexplained features in cosmic-ray spectra (CERN Courier December 2016 p26). In particular, a high-energy excess in the relative positron flux does not follow the single power-law behaviour expected from standard cosmic-ray interactions with the interstellar medium. While known astrophysical sources such as pulsars cannot yet be ruled out, the spectrum fits well to dark-matter models. If the excess events are indeed due to the annihilation of dark-matter particles, a smoking gun would be a high-energy cut-off in the spectrum. By increasing the AMS acceptance by 300%, the addition of a new tracker layer is the only way that the experiment can gather the necessary data to test this hypothesis before the scheduled decommissioning of the ISS in 2030.
“By 2030 AMS will extend the energy range of the positron flux measurement from 1.4 to 2 TeV and reduce the error by a factor of two compared to current data,” says AMS spokesperson Sam Ting of MIT. “This will allow us to measure the anisotropy accurately to permit a separation between dark matter and pulsars at 99.93% confidence.”
Led by MIT, and assembled and tested at CERN/ESA with NASA support, AMS is a unique particle-physics experiment in space. It consists of a transition radiation detector to identify electrons and positrons, a permanent magnet together with nine silicon-tracker layers to measure momentum and identify different particle species, two banks of time-of-flight counters, veto counters, a ring-image Cherenkov counter and an electromagnetic calorimeter.
The additional tracker layer, 2.6 m in diameter, 30 cm thick and weighing 250 kg, will be installed on the top-most part of the detector. The tracking sensors will populate the opposite faces of an ultralight carbon plane specifically developed for AMS to fulfil thermoelastic stability requirements, surrounded by an octagonal carbon frame that also provides the main structural interface during launch. The powering and readout electronics for the new layer will generate additional heat that is rejected to space by radiators at its periphery. Two new radiators will therefore be integrated into the detector prior to the installation of the layer, while a third, much larger power-distribution radiator (PDS) will also be installed to recuperate the performance of one of the AMS main radiators, which has suffered degradation and radiation damage after 13 years in low-Earth orbit. In January, a prototype of the PDS, manufactured and supported by aerospace company AIDC in Taiwan, was delivered to CERN for tests.
First steps for the upgrade took place in 2021, and the US Department of Energy together with NASA approved the mission in March 2023. The testing of components and construction of prototypes at institutes around the world is proceeding quickly in view of a planned launch in February 2026. The silicon strips, 8 m2 of which will cover both faces of the layer, were produced by Hamamatsu and are being assembled into “ladders” of different lengths at IHEP in Beijing. These are then shipped to INFN Perugia in Italy, where they are joined together to form a quarter plane. Once fully characterised, the eight quarters will be installed at CERN on both faces of the mechanical plane and integrated with electronics, thermal hardware and the necessary brackets. Crucial for the new tracker layer to survive the harsh launch environment and to maintain, once in orbit, the sensor within five microns relative to ground measurements, are the large carbon plane and the shielding cupolas, developed at CERN, as well as the NASA brackets that will attach the layer module to AMS. This hardware represents a major R&D programme in its own right.
By 2030 AMS will extend the energy range of the positron flux measurement from 1.4 to 2 TeV and reduce the error by a factor of two
Following the first qualification model in late 2023, consisting of a quarter of the entire assembled layer, AMS engineers are now working towards a full-size model that will take the system closer to flight. The main tests to simulate the environment that the layer will experience during launch and once in orbit are vibrational and thermal-vacuum, to be performed in Italy (INFN PG) and in Germany (IABG), while the sensors’ position in the layer will be fully mapped at CERN and then tested with beams from the SPS, explains AMS chief engineer Corrado Gargiulo of CERN: “Everything is going very, very fast. This is a requirement, otherwise we arrive too late at the ISS for the upgrade to make sense.”
The new module is being designed to fit snuggly into the nose of a SpaceX Dragon rocket. Once safely delivered to the ISS, a robotic arm will dispatch the module to AMS where astronauts will, through a series of extravehicular activities (EVAs), perform the final mounting. Training for the delicate EVAs is well underway at NASA’s Johnson Space Center. Nearby, at the Neutral Buoyancy Laboratory, the astronauts are trained in a large swimming pool on how to attach the different components under the watchful eyes of safety and NASA divers, among them Gargiulo (see “Space choreography” images). As with the EVAs required to replace the cooling system, a number of custom-built tools and detailed procedures have to be developed and tested.
“If the previous ones were considered high-risk surgery, the EVAs for the new upgrade are unprecedented for the several different locations where astronauts will be required to work in much tighter and less accessible spaces,” explains Ken Bollweg, NASA manager of AMS, who is leading the operations aspect.
Since its inception a decade ago, the Future Circular Collider (FCC) collaboration has evolved in scope and scale – especially since the completion of the conceptual design report in 2018, when directed efforts were made to broaden the project’s reach and attract new partners. Such endeavours are crucial considering the ambitious nature of the FCC project and the immense global collaboration required to bring it to fruition.
Today, the collaboration brings together more than 130 institutes from 31 countries. Contributions from members span a broad spectrum encompassing theoretical and experimental particle physics, applied science, engineering, computing and technology. Ongoing collaborations with research centres internationally are pushing the performance of key technologies such as superconducting radio-frequency cavities and klystrons, as well as magnets based on novel high-temperature superconductors (see “Advancing hardware“). Increased global collaboration is a prerequisite for success, and links with high-tech industry will be essential to further advance the implementation of the FCC.
The proposed four-interaction point layout for the FCC is not only designed to offer the broadest physics coverage, but makes it a future collider commensurate with the size and aspirations of the current high-energy physics community. The attractiveness of the FCC is also reflected in the composition of participants at annual conferences, which shows a good balance between early-career and more senior researchers, geographical diversity, and gender. The latter currently stands at a 70:30 male-to-female ratio, which has been increasing during the course of the feasibility study.
Global working group
The FCC feasibility study has established a global working group with a mandate to engage countries with mature communities, a long-standing participation in CERN’s programmes, and the potential to contribute substantially to the project’s long-term scientific objectives. In addition, an informal forum of national contacts allows exchanges between physicists from different countries and the development of collaborations inside FCC. Each interested country has one or two national contacts who have the opportunity to report regularly on the development of their FCC activities.
Drawing parallels with the LHC and HL-LHC successes, CERN’s unique experience with large-scale scientific collaborations has been invaluable in shaping the cohesive and productive environment of the FCC collaboration. It is imperative to recognise the dedication of existing members while addressing the need for new contributors to bolster the collaboration. As the FCC considers the next stage of its scientific journey, potential partners are invited to bring their unique skills and perspectives.
First discussions on the governance and financial considerations for the FCC project are taking place in the CERN Council. The models aim to provide a structure for both the construction and operation phases, and assume compatibility with the CERN Convention, while also taking into account the United Nations’ sustainable development goals. In parallel, the organisational structure of the FCC experiment collaborations is being discussed. Given the inherently cooperative and distributed nature of these collaborations, a relatively lightweight structure will be put forth, based on openness, equality at the level of participating institutes and a wide consultation within the collaboration for key decisions.
Since 2021, the FCC has implemented a robust organisational structure, acting under the authority of the CERN Council, that facilitates efficient communication and coordination among its members. Looking ahead, the path to the governance model required for the FCC project and operation phases is both exciting and challenging. Importantly, it requires the long-term engagement and support of participants from CERN’s member and associate member states, and from the non-member states, whose community at CERN has been growing with the LHC, particularly from institutes located in North America and the Asia-Pacific regions. As the project evolves further, it is crucial to refine and adapt the collaboration model to ensure the efficient allocation of resources and sustained momentum.
The FCC offers a multitude of R&D opportunities, and the collaborative spirit that defines it promises to shape the future of particle physics. As we go forward, the FCC collaboration beckons individuals and institutions to contribute to the next chapter in our exploration of the fundamental laws and building blocks of the universe.
CERN has been burrowing beneath the French–Swiss border for half a century. Its first major underground project was the 7 km-circumference Super Proton Synchrotron (SPS), constructed at a depth of around 40 m by a single tunnel boring machine (TBM). This was followed by the Large Electron–Positron (LEP) collider at an average depth of around 100 m, for which the 27 km tunnel was constructed between 1983 and 1989 using three TBMs and a more traditional “drill and blast” method for the sector closest to the Jura mountain range. With a circumference of 90.7 km, weaving through the molasse and limestone beneath Lake Geneva and around Mont Salève, the proposed Future Circular Collider (FCC) would constitute the largest tunnel ever constructed at CERN and be considered a major global civil-engineering project in its own right.
Should the FCC be approved, civil engineering will be the first major on-site activity to take place. The mid-term review of the FCC feasibility study schedules ground-breaking for the first shafts to begin in 2033, after which it would take between six and eight years for each underground sector to be made available for the installation of the technical infrastructure, the machine and the experiments.
Evolving engineering
Since the completion of the FCC conceptual design report in 2018, several significant changes have been made to the civil engineering. These include a 7 km reduction in the overall circumference of the main tunnel, a reduction in the number of surface sites from 12 to eight, and a reduction in the number of permanent shafts from 18 to 12 (two at each of the four experiment sites, one at each of the four technical sites). A temporary shaft will also be required for the construction of the transfer tunnel connecting the injection system to the FCC tunnel, although it may be possible to re-use an existing but unused shaft for this purpose. Additional underground civil engineering for the RF systems will also be required. The diameter of the main tunnel (5.5 m) and its inclination (0.5%) remain unchanged, resulting in a tunnel depth that varies between approximately 50 m – where it passes under the Rhône River – and 500 m – where it passes beneath the Borne plateau on the eastern side of the study site.
The tunnel would be constructed using up to eight TBMs, which are able to excavate and install the tunnel lining in a single-pass operation. Desktop studies show that the geology that would be encountered during most of the underground construction would be favourable, since the molasse rock is usually watertight and can be easily supported using a range of standard rock-support measures. The main beam tunnel will, however, need to pass through about 4.4 km of limestone, which may require the drill-and-blast method to be utilised. These geological assumptions need to be confirmed via a major in situ site investigation campaign planned for 2024–2025.
Two sizes of experiment cavern complexes are envisaged, serving both the lepton and hadron FCC stages. One includes a cavern to house the largest planned FCC-hh detector with dimensions of 35 × 35 × 66 m (similar to the existing ATLAS cavern) and the other a 25 × 25 × 66 m cavern to house the smaller FCC-ee detectors (similar to the CMS cavern). A second cavern 25 m wide and up to 100 m long would be required at each experiment area to house various technical infrastructure, while a 50 m-thick rock pillar between the detector and the service caverns would provide electromagnetic shielding from the detector as well as the overall structural stability of the cavern complex. Numerous smaller caverns and interconnecting tunnels and galleries will be required to link the main structures, and these are expected to be excavated using road-header machines or rock breakers.
Conceptual layouts for two of the eight new surface sites have been prepared under a collaboration agreement with Fermilab, and studies of the experiment site at Ferney-Voltaire in France and the technical site at Choulex/Presinge in Switzerland have been undertaken. The requirements for other surface sites will be developed into preliminary designs in the second half of the feasibility study. In addition, several locations have been investigated for a new high-energy linac, which has been proposed as an alternative to using the SPS as a pre-injector for the FCC, with the most promising site located close to the existing CERN Prévessin site.
Feasibility campaign
As an essential step in demonstrating the feasibility of underground civil-engineering works for the FCC, CERN has been working with international consultants and the University of Geneva to develop a 3D geological model using information gathered from previous borehole and geophysical investigations. To improve this understanding, a targeted campaign of subsurface investigations using a combination of geophysical analyses and deep-borehole drilling has been planned in the areas of highest geological uncertainty. The campaign, which is currently being tendered with specialist companies, will commence in 2024 and continue into 2025 to ensure that the results are available before the end of the feasibility study. About 30 boreholes will be drilled and used in conjunction with 80 km of seismic lines to investigate the location of the molasse rock, in particular under Lake Geneva, the Rhône river and in those areas where limestone formations may be close to the planned tunnel horizon.
On the surface, there is scope for staging the construction of buildings. All the buildings that are only required for the FCC-hh phase would be postponed, but the land areas needed for them would be reserved and included in the overall site perimeter. Buried networks, roads and technical galleries would be designed and constructed such that they can be extended later to accommodate the FCC-hh structures.
With a total of around 15 million tonnes of rock and soilto be excavated, sustainability is a major focus of the FCC civil-engineering studies. To this end, in the framework of the European Union co-funded FCC Innovation Study, CERN and the University of Leoben launched an international challenge-based competition, “Mining the Future”, in 2021 to identify credible and innovative ways to reuse the molasse. The results of the competition include the use of limestone for concrete production and stabilisation of constructions within the project, the re-use of excavated materials to back-fill quarries and mines, the transformation of sterile molasse into fertile soil for agriculture and forestry, the production of bricks from compressed molasse, and the development of novel construction materials with molasse ingredients for use in the project as far as technically suitable. The next step is the implementation of a pilot “Open Sky Laboratory” permitting the demonstration of the separation techniques of the winning consortium (led by BG engineering), and collaboration with CERN’s host states and other stakeholders to identify suitable locations for its use. In addition, the FCC feasibility study is working towards a full assessment to minimise the carbon footprint during construction.
The civil-engineering plans for the FCC project have been presented several times to the global tunnelling community, most recently at the 2023 World Tunnel Congress in Athens. The scale and technical complexity of the project is creating a great deal of interest from designers and contractors, and has even triggered a dedicated visit to CERN from the executive committee of the International Tunnelling Association, which reinforces the significant progress that has been made.
It’s exactly 10 years since 350 physicists and engineers met at the University of Geneva to kick-off the Future Circular Collider (FCC) study. A response to the 2013 European strategy for particle physics, the study initially examined options for an energy-frontier collider in a new 80–100 km-circumference tunnel. By late 2018 a conceptual design report (CDR) integrating the physics, detector, accelerator and infrastructure of a staged lepton (FCC-ee) and hadron (FCC-hh) collider was published. Two years of lengthy deliberations later, the 2020 European strategy recommended that the community investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an e+e– Higgs and electroweak factory as a possible first stage.
Studies show that the FCC would deliver benefits that outweigh its costs
After three years of work, mobilising the expertise of physicists and engineers from around the world, a mid-term report of the FCC feasibility study was completed in December 2023. Numerous technical documents and a 700-page overview of the results demonstrate significant progress across all project deliverables, including physics opportunities, the placement and implementation of the ring, civil engineering, technical infrastructure, accelerators, detectors and cost. No technical showstoppers have been identified, and the results were received positively by the CERN Council during a special session on 2 February. Here and in the some related articles, the Courier gathers the key take-aways.
A collider for the times
The scientific backdrop to the FCC is the existence of a 125 GeV Higgs boson together with no sign yet of new elementary particles at the TeV scale – transformational discoveries by the LHC that call for a broad and versatile exploration tool with unprecedented precision, sensitivity and energy reach (see “FCC: the physics case“). An unfathomable amount of work has led to an optimal placement of the FCC ring, surface sites and project implementation with CERN’s host states (see “Where and how“). The 90.7 km FCC tunnel, constituting a major global civil-engineering project in its own right, is well understood (see “Tunnelling to the future“). Assuming a decision to advance to the next stage is taken by the CERN Council after the next European strategy process, a preparatory phase (involving project authorisation, preparation of civil-engineering works, technical design for the collider, injectors and the detectors, further consolidation of physics cases and detector development) would take place from 2026 to 2032. Construction could then take place in 2033–2040, with the installation phase and transition to operation between 2038 and the mid-2040s.
The multi-energy lepton collider FCC-ee, which would produce huge quantities of Z, W and Higgs bosons, and ultimately top-quark pairs, over a period of about 15 years, builds on the remarkable success of LEP, which was instrumental in confirming the Standard Model and in guiding physicists to the discoveries of the top quark and the Higgs boson. Once thought to be the final word on circular e+e– colliders, advances in accelerator technology since LEP (such as top-up injection at B factories and synchrotron-radiation light sources, developments in superconducting RF, and novel beam-focusing techniques) offer collision rates more than two orders of magnitude larger. Boosting the FCC-ee luminosity further, a key outcome of the mid-term report is a new ring-layout that enables four interaction points.
Ideal springboard
The mid-term report confirms that FCC-ee is both a mature design for a Higgs, electroweak and top factory, and an ideal springboard for an energy-frontier collider, FCC-hh, for which it would provide a significant part of the infrastructure. Since the revised FCC-ee placement studies, the overall layout of FCC-hh has changed radically compared to the initial concept phase, with three key benefits: an optimal size of the experiment caverns, with the option of sharing detector components between the lepton and the hadron machines; a reduction in the number of surface sites; and a shorter tunnel for the transfer lines from the injector to the collider ring. The new layout is compatible with an injection scheme that delivers beams to the FCC-hh ring from the LHC or from an upgrade of the SPS.
The mid-term report addresses the challenging R&D for the high-field FCC-hh magnets. A key deliverable of the feasibility study is a summary of R&D plans based on Nb3Sn, high-temperature superconductors (HTS) and hybrid technologies. While Nb3Sn magnets are considered relatively low-risk, HTS technology would enable the most aspirational goals to be reached. Due to the sizable gap in technology readiness between the two options, however, the study team advises against an early decision. Instead, an adapted “phase-gate” process is proposed with regular review, steering and decision points every five years, and coordinated with the CERN high-field magnet programme. Taking into account the time needed to construct and operate FCC-ee and, in parallel, to develop the high-field dipole magnet technology, it is estimated that FCC-hh could begin physics operations in the early 2070s.
The cost of an FCC-ee with four interaction points is estimated to be CHF 15 billion, around a third of which is taken up by the tunnel. The reliability of the FCC-ee cost estimate will be improved following further development of the various accelerator systems and equipment required, along with the subsurface investigations starting in 2024. The final feasibility-study report will also address risk-management and the personnel resources required from project development to construction.
Power consumption is another topic of interest. The FCC-ee will be the largest particle accelerator ever built, with its RF, magnet and cryogenic systems drawing the main loads. The total CERN energy consumption throughout the FCC-ee scientific programme is estimated to vary between 2.0 and 2.8 TWh/year depending on the energy mode, to be compared with about 1.6 TWh/year during the High-Luminosity LHC era. The figures are hoped to be lowered as R&D (for example, to improve the performance of superconducting cavities and the efficiency of power sources) advances. The FCC study team is also working with regional authorities to identify ways in which part of this energy may be re-used for heating in local industries and public infrastructures.
Electrical power would be provided from the French electricity grid, and the system is designed such that no new sub-stations will need to be constructed between the different FCC-ee energy stages. Studies carried out in conjunction with McKinsey and Accenture indicate that by the time the FCC comes into operation, a low carbon footprint can be achieved with an energy mix that contains a large fraction of energy from renewable sources.
Return on investment
Beyond the creation of new knowledge, studies undertaken within the European Union co-funded FCC Innovation Study show that the FCC would deliver benefits that outweigh its cost. Impacts on industry from high-tech developments, the sustained training of early-stage researchers and engineers, the development of open and free software, the creation of spin-off companies, cultural goods and other factors lead to an estimated benefit/cost ratio of 1.66. The FCC project is linked to the creation of around 800,000 person-years of jobs, states the mid-term report, and the FCC-ee scientific programme is estimated to generate an overall local economic impact of more than €4 billion.
The digested mid-term report in summary: the FCC integrated programme is an ideal match for the uncharted physics territory ahead; its placement at CERN is geologically and territorially feasible; no technical showstoppers have been identified; the FCC would return more to society than it costs. Accelerator, detector, engineering and physics studies by the global FCC collaboration are continuing across more than 150 institutes in more than 30 countries, while new partners are sought to work on various R&D (see “The people factor” ). The final report of the FCC feasibility study is due in early 2025.
The CERN vacuum group has been actively designing components for the FCC-ee vacuum system. Among them are 3D-printed synchrotron-radiation absorbers (SRAs), cold-sprayed copper “bosses”, which could be machined to obtain weld- and flange-free beam position monitor button electrodes (pictured), and plasma-sprayed thin titanium tracks to be used as a radiation-hard bake-out heating system. In parallel, a collaboration with a spin-off company from the University of Calabria is dealing with the implementation of shape-memory alloy flanges. The design of a 2 m-long vacuum chamber extrusion with one SRA is almost finalised, and a soon-to-be-built prototype will be tested at the KARA light source at KIT. We have also begun studying a vacuum chamber with a smaller inner diameter compared to the FCC-ee baseline, including its impact on the machine–detector interface and the booster. The length of the vacuum sectors has been optimised, and their integration in the overall tunnel design is under study. The vacuum group is also looking forward to prototyping the required NEG-coating set-up, as the vacuum chambers could be up to 12 m long and coating them in a vertical position, as is usually done, would be difficult, especially for industry when moving to mass production.
A key goal of R&D for the FCC superconducting radio-frequency (SRF) system, conducted by the CERN SY–RF, TE–VSC and EN–MME groups, is to optimise Nb/Cu technology for the fabrication of the cavities. Achieving high SRF performance in thin-film-coated cavities requires minimising substrate defects. Previous experiences show that imperfections located around electron beam welds in areas subjected to high magnetic field areas can constrain the quality factor of Nb/Cu cavities. To surpass the current limitations of Nb-coated cavities, a seamless configuration along with higher substrate quality and shape conformity is a promising alternative. Instead of traditional shaping methods such as deep-drawing or spinning, the ongoing use of techniques such as hydroforming and machining directly from the bulk material shows high potential for valuable results without altering the substrate. Moreover, it ensures effectiveness, repeatability and precision in the final shape of the cavity. Based on the impressive RF performance obtained from seamless Nb-coated 1.3 GHz cavities manufactured at CERN from bulk copper, the CERN teams are confident that such spectacular results will be repeated with a 400 MHz cavity (pictured) that is being machined as a preliminary prototype for the FCC RF study.
At the end of 2023, the first demonstrator of a high-temperature superconductor (HTS) sextupole designed for the FCC-ee arcs was fabricated at CERN (pictured). Built using novel technology from a CERN spin-off company, the magnet adopts a “canted cosine theta” design and is the first such device to use HTS rare-earth barium copper oxide (ReBCO) tape as its conductor – something that was long considered technically challenging. The main advantage of such a magnet is that ohmic losses (a significant source of electric power consumption for a normal-conducting accelerator) are reduced to zero, whereas refrigeration losses are much reduced compared to low-temperature-superconductor devices. Other advantages include increased performance due to the possibility of “nesting” magnets together, which is not possible for normal-conducting magnets that use iron to shape their magnetic fields. The increase in performance is such that up to 40% of the cost of the system can be recovered from the lower required RF voltage and therefore a smaller number of accelerating RF cavities. The magnet is the fruit of a CERN–PSI collaboration called FCCee-HTS4, funded through the CHART consortium in Switzerland. Future plans include the winding of the magnet at CERN, followed by tests for magnetic performance and quality.
Designed to meet strict alignment requirements in the FCC-ee interaction points, the Machine Detector Interface (MDI) alignment system is a key element of the feasibility study. Challenging conditions – including extremely low temperatures, elevated radiation levels and limited space – hinder the deployment of standard survey equipment and sensors in this important region. New and exotic techniques and sensing systems have therefore been studied. The main solution, called in-lined multiplexed and distributed frequency scanning interferometry (IMD–FSI), uses an interferometer with a wavelength-sweeping laser source to measure multiple lengths along a single optical fibre, simultaneously and independently. A network of fibres can then be installed in a helical pattern to monitor the shape of components inside the MDI, such as the support of the screening solenoid. A prototype IMD–FSI system (pictured) has proved extremely promising, and the next step is a full fibre network implementation on a cylinder. This system could also be implemented in other regions of the collider, for example to monitor sensitive tunnel sections or other civil-engineering structures such as towers, dams and bridges.
Designing a next-generation collider with a performance that meets the scientific demands of the particle-physics community is one thing. Ensuring its territorial compatibility, technical feasibility and cost control is quite another. A core element of the FCC feasibility study is therefore the placement of the ring and the necessary surface sites, for which an iterative approach in collaboration with CERN’s host states, France and Switzerland, has been adopted from the outset.
Territorial compatibility requires numerous natural, technical, urban and cultural constraints to be identified and considered. The goal is to limit the consumption of land, keep the quantity of excavated materials to a minimum and re-use as much as possible, minimise the consumption of resources such as electricity and water, avoid visibility, noise and dust nuisances, and create synergies with future neighbours where possible. Following eight years of intense study, one configuration was identified out of some 100 variants as being particularly suitable. This scenario has a circumference of about 90.7 km, eight surface sites and permits the installation of up to four experiments.
During 2023 this reference scenario was reviewed with different regional stakeholders and now serves as the baseline for further design and optimisation activities. These include geophysical and geotechnical investigations to set the optimum depth of the tunnel, links to high voltage grids, access to water for cooling purposes, connections to major rail and road infrastructures, landscape integration and the development of sustainable mitigation measures.
Drill down
Working out how to place a 90.7 km-circumference research infrastructure in a densely populated region requires several dozens of criteria to be met. While initial investigations concerned observations at the square-kilometre level, the focus gradually moved to thousands of square metres and individual land-plot levels. Initial cartographic and database research has progressively been replaced with analysis in the field, working meetings with public administration services and eventually individuals with expert local knowledge. In addition to the scientific and technical requirements, the FCC implementation scenario takes into accountthe project-implementation risks, cost impacts, access to resources (electricity, water, land), transport requirements, and estimates of the urban and demographic evolution. The study also analyses socio-economic benefits for the region.
The reference layout with only eight surface sites requires less than 50 ha of land use on the surface and constitutes a significant reduction in footprint with respect to the initial scenario drawn up in 2014. All sites are situated close to road infrastructure, with less than 5 km of new roads required, and several of the eight sites are located in the vicinity of 400 kV grid lines. The layout of the FCC is integrated geographically with the existing CERN accelerator complex, with beam transfer possible from either the LHC or via the SPS tunnel.
Throughout all studies, CERN has been accompanied by the services of the Swiss and French authorities at different levels
The feasibility study, carried out with relevant consultancy companies, confirms the technical feasibility of all eight surface sites and the underground works. Working meetings with all the municipalities affected in France and Switzerland have not revealed any showstoppers so far, even if decisions by municipalities and the host states are yet to be taken. Next steps include the detailed integration of the surface sites in the environment.
Timescales are critical to be able to continue with such studies. By the end of the feasibility study in 2025, all land plots that are required by the project need to be communicated to the host states. In addition, a formal environmental evaluation phase in both France and Switzerland is necessary for the authorisation procedures. These activities rely on an agreement between CERN and the host states on the steps to be made by each stakeholder, including the associated legal and regulatory conditions.
Throughout all studies, CERN has been accompanied by the services of the Swiss and French authorities at different levels. This dialogue concerns the more detailed expression of the needs and constraints of the local actors and the identification of potential co-development topics and compensatory measures. The findings are gradually being integrated into a process of project optimisation of the reference scenario to further improve its added value for the territory while keeping the science value high and the project implementation risks low.
Results from the LHC so far have transformed the particle-physics landscape. The discovery of the Higgs boson with a mass of 125 GeV – in agreement with the prediction from earlier precision measurements at LEP and other colliders – has completed the long-predicted matrix of particles and interactions of the Standard Model (SM) and cleared the decks for a new phase of exploration. On the other hand, the lack of evidence for an anticipated supporting cast of particles beyond the SM (BSM) gives no clear guidance as to what form this exploration may take. For the first time since the Fermi theory almost a century ago, particle physicists are voyaging into completely uncharted territory, where our only compass is the certitude that the SM in isolation cannot account for all observations. This absence of theoretical guidance calls for a powerful experimental programme to push the frontiers of the unknown as far as possible.
The absence of LHC signals for new phenomena in the TeV range requires physicists to think differently about the open questions in fundamental physics. These include the abundance of matter over antimatter, the nature of dark matter, the quark and lepton flavour puzzle in general, and the non-zero nature of neutrino masses in particular. Solutions could be at even higher energies, at the price of either an unnatural value of the electroweak scale or an ingenious but still elusive structure. Radically new physics scenarios have been devised, often involving light and very-weakly coupled structures. Neither the mass scale (from meV to ZeV) of this new physics nor the intensity of its couplings (from 1 to 10–12 or less) to the SM are known, calling for a versatile exploration tool.
By providing considerable advances in sensitivity, precision and, eventually, energy far above the TeV scale, the integrated Future Circular Collider (FCC) programme is the perfect vehicle with which to navigate this new landscape. Its first stage FCC-ee, an e+e– collider operating at centre-of-mass energies ranging from below the Z pole (90 GeV) to beyond the top-quark pair-production threshold (365 GeV), would map the properties of the Higgs and electroweak gauge bosons and the top quark with precisions that are orders of magnitude better than today, acquiring sensitivity to the processes that led to the formation of the Brout–Englert–Higgs field a fraction of a nanosecond after the Big Bang. A comprehensive campaign of precision electroweak, QCD, flavour, tau, Higgs and top-quark measurements sensitive to tiny deviations from the predicted SM behaviour would probe energy scales far beyond the direct kinematic reach, while a subsequent pp collider (FCC-hh) would improve – by about an order of magnitude – the direct discovery reach for new particles. Both machines are strongly motivated in their own rights. Together, they offer the furthest physics reach of all proposed future colliders, and put the fundamental scalar sector of the universe centre-stage.
A scalar odyssey
The power of FCC-ee to probe the Higgs boson and other SM particles at much higher resolution would allow physicists to peer further into the cloud of quantum fluctuations surrounding them. The combination of results from previous lepton and hadron colliders at CERN and elsewhere has shown that electroweak symmetry breaking is consistent with its SM parameterisation, but its origin (and the origin of the Higgs boson itself) demands a deeper explanation. The FCC is uniquely placed to address this mystery via a combination of per-mil-level Higgs-boson and parts-per-millon gauge-boson measurements, along with direct high-energy exploration, to comprehensively probe symmetry-based explanations for an electroweak hierarchy. In particular, measurements of the Higgs boson’s self-coupling at the FCC would test whether the electroweak phase transition was first- or second-order, revealing whether it could have potentially played a role in setting the out-of-equilibrium condition necessary for creating the matter–antimatter asymmetry.
While the Brout–Englert–Higgs mechanism nicely explains the pattern of gauge-boson masses, the peculiar structure of quark and lepton masses (as well as the quark mixing angles) is ad hoc within the SM and could be the low-energy imprint of some new dynamics. The FCC will probe such potential new symmetries and forces, in particular via detailed studies of b and τ decays and of b → τ transitions, and significantly extend knowledge of flavour physics. A deeper understanding of approximate conservation laws such as baryon- and lepton-number conservation (or the absence thereof in the case of Majorana neutrinos) would test the limits of lepton-flavour universality and violation, for example, and could reveal new selection rules governing the fundamental laws. Measuring the first- and second-generation Yukawa couplings will also be crucial to complete our understanding, with a potential FCC-ee run at the s-channel Higgs resonance offering the best sensitivity to the electron Yukawa coupling. Stepping back, the FCC would sharpen understanding of the SM as a low-energy effective field theory approximation of a deeper, richer theory by extending the reach of direct and indirect exploration by about one order of magnitude.
The unprecedented statistics from FCC-ee also make it uniquely sensitive to exploring weakly coupled dark sectors and other candidates for new physics beyond the SM (such as heavy axions, dark photons and long-lived particles). Decades of searches across different experiments have pushed the mass of the initially favoured dark-matter candidate (weakly interacting massive particles, WIMPs) progressively beyond the reach of the highest energy e+e– colliders. As a consequence, hidden sectors consisting of new particles that interact almost imperceptibly with the SM are rapidly gaining popularity as an alternative that could hold the answer not only to this problem but to a variety of others, such as the origin of neutrino masses. If dark matter is a doublet or a triplet WIMP, FCC-hh would cover the entire parameter space up to the upper mass limit for thermal relic. The FCC could also host a range of complementary detector facilities to extend its capabilities for neutrino physics, long-lived particles and forward physics.
For the first time since the Fermi theory almost a century ago, particle physicists are voyaging into completely uncharted territory
Completing this brief, high-level summary of the FCC physics reach are the origins of exotic astrophysical and cosmological signals, such as stochastic gravitational waves from cosmological phase transitions or astrophysical signatures of high-energy gamma rays. These phenomena, which include a modified electroweak phase transition, confining new physics in a dark sector, or annihilating TeV-scale WIMPs, could arise due to new physics which is directly accessible only to an energy-frontier facility.
Precision rules
Back in 2011, the original incarnation of a circular e+e– collider to follow the LHC (dubbed LEP3) was to create a high-luminosity Higgs factory operating at 240 GeV in the LEP/LHC tunnel, providing similar precision to that at a linear collider running at the same centre-of-mass energy for a much smaller price tag. Choosing to build a larger 80–100 km version not only allows the tunnel and infrastructure to be reused for a 100 TeV hadron collider, but extends the FCC-ee scientific reach significantly beyond the study of the Higgs boson alone. The unparalleled control of the centre-of-mass energy via the use of resonant depolarisation and the unrivalled luminosity of an FCC-ee with four interaction points would produce around 6 × 1012 Z bosons, 2.4 × 108 W pairs (offering ppm precision on the Z and W masses and widths), 2 × 106 Higgs bosons and 2 × 106 top-quark pairs (impossible to produce with e+e– collisions in the LEP/LHC tunnel) in as little as 16 years.
From the Fermi interaction to the discovery of the W and Z, and from electroweak measurements to the discovery of the top quark and the Higgs boson, greater precision has operated as a route to discoveries. Any deviation from the SM predictions, interpreted as the manifestation of new contact interactions, will point to a new energy scale that will be explored directly in a later stage. One of the findings of the FCC feasibility study is the richness of the FCC-ee Z-pole run, which promises comprehensive measurements of the Z lineshape and many electroweak observables with a 50-fold increase in precision, as well as direct and uniquely precise determinations of the electromagnetic and strong coupling constants. The comparison between these data and commensurately precise SM predictions would severely constrain the existence of new physics via virtual loops or mixing, corresponding to a factor-of-seven increase in energy scale – a jump similar to that from the LHC to FCC-hh. The Z-pole run also enables otherwise unreachable flavour (b, τ) physics, studies of QCD and hadronisation, searches for rare or forbidden decays, and exploration of the dark sector.
After the Z-pole run, the W boson provides a further precision tool at FCC-ee. Its mass is one of the most precisely measured parameters that can be calculated in the SM and is thus of utmost importance. In the planned WW-threshold run, current knowledge can be improved by more than an order of magnitude to test the SM as well as a plethora of new-physics models at a higher quantum level. Together, the very-high-luminosity Z and W runs will determine the gauge-boson sector with the sharpest precision ever.
Going to its highest energy, FCC-ee would explore physics associated with the heaviest known particle, the top quark, whose mass plays a fundamental role in the prediction of SM processes and for the cosmological fate of the vacuum. An improvement in precision by more than an order of magnitude will go hand in hand with a significant improvement in the strong coupling constant, and is crucial for precision exploration beyond the SM.
High-energy synergies
A later FCC-hh stage would complement and substantially extend the FCC-ee physics reach in nearly all areas. Compared to the LHC, it would increase the energy for direct exploration by a factor of seven, with the potential to observe new particles with masses up to 40 TeV (see “Direct exploration” figure). The day FCC-hh directly finds a signal for beyond-SM physics, the precision measurements from FCC-ee will be essential to pinpoint its microscopic origin. Indirectly, FCC-hh will be sensitive to energies of around 100 TeV, for example in the tails of Drell–Yan distributions. The large production of SM particles, including the Higgs boson, at large transverse momentum allows measurements to be performed in kinematic regions with optimal signal-to-background ratio and reduced experimental systematic uncertainties, testing the existence of effective contact interactions in ways that are complementary to what is accessible at lepton colliders. Dedicated FCC-hh experiments, for instance with forward detectors, would enrich further the new-physics opportunities and hunt for long-lived and millicharged particles.
Further increasing the synergies between FCC-ee and FCC-hh is the importance of operating four detectors (instead of two as in the conceptual design study), which has led to an optimised ring layout with a new four-fold periodicity. With four interaction points, FCC-ee provides a net gain in integrated luminosity for a given physics outcome. It also allows for a range of detector solutions to cover all physics opportunities, strengthens the robustness of systematic-uncertainty estimates and discovery claims, and opens several key physics targets that are tantalisingly close (but missed) with only two detectors. The latter include the first 5σ observation of the Higgs-boson self-coupling, and the opportunity to access the Higgs-boson coupling to electrons – one of FCC-ee’s toughest physics challenges.
No physics case for FCC would be complete without a thorough assessment of the corresponding detector challenges. A key deliverable of the feasibility study is a complete set of specifications ensuring that calorimeters, tracking and vertex detectors, muon detectors, luminometers and particle-identification devices meet the physics requirements. In the context of a Higgs factory operating at the ZH production threshold and above, these requirements have already been studied extensively for proposed linear colliders. However, the different experimental environment and the huge statistics of FCC-ee demand that they are revisited. The exquisite statistical uncertainties anticipated on key electroweak measurements at the Z peak and at the WW threshold call for a superb control of the systematic uncertainties, which will put considerable demands on the acceptance, construction quality and stability of the detectors. In addition, the specific discovery potential for very weakly coupled particles must be kept in mind.
The software and computing demands of FCC are an integral element of the feasibility study. From the outset, the driving consideration has been to develop a single software “ecosystem” adaptable to any future collider and usable by any future experiment, based on the best software available. Some tools, such as flavour tagging, significantly exceed the performance of algorithms previously used for linear-collider studies, but there is still much work neededto bring the software to the level required by the FCC-ee. This includes the need for more accurate simulations of beam-related quantities, the machine-detector interface and the detectors themselves. In addition, various reconstruction and analysis tools for use by all collaborators need to be developed and implemented, reaping the benefits from the LHC experience and past linear-collider studies, and computing resources for regular simulated data production need to be evaluated.
Powerful plan
The alignment of stars – that from the initial concept in 2011/2012 of a 100 km-class electron–positron collider in the same tunnel as a future 100 TeV proton–proton collider led to the 2020 update of the European strategy for particle physics endorsing the FCC feasibility study as a top priority for CERN and its international partners – provides the global high-energy physics community with the most powerful exploration tool. FCC-ee offers ideal conditions (luminosity, centre-of-mass energy calibration, multiple experiments and possibly monochromatisation) for the study of the four heaviest particles of the SM with a flurry of opportunities for precision measurements, searches for rare or forbidden processes, and the possible discovery of feebly coupled particles. It is also the perfect springboard for a 100 TeV hadron collider, for which it provides a great part of the infrastructure. Strongly motivated in their own rights, together these two machines offer a uniquely powerful long-term plan for 21st-century particle physics.
To uncover the fundamental laws of the universe and its evolution is a great human endeavour. The most effective way to achieve this goal in particle physics is via powerful, high-energy accelerators. The July 2012 discovery at CERN of the Higgs boson with a mass of 125 GeV opened a door to an unknown part of the universe. The Higgs boson is not only at the heart of the Standard Model (SM) but is also at the centre of many mysteries. These include the large hierarchy between the weak and the Planck scales, the nature of the electroweak phase transition, the origin of mass, the naturalness problem, the stability of the vacuum, and many other related fundamental questions about nature beyond the SM, such as the origin of the matter–antimatter asymmetry and the nature of dark matter.
Precise measurements of the Higgs boson’s properties serve as probes into the underlying fundamental physics principles of the SM and beyond. For this reason, in September 2012 Chinese scientists proposed the Circular Electron–Positron Collider (CEPC) and the Super proton–proton Collider (SppC) as an international, large science project hosted in China to match the grand goals of particle physics, complementary to linear and muon colliders. Around the same time, physicists at CERN proposed the Future Circular Collider (FCC) staged across electron-positron (e+e–) and hadron-hadron operations.
Since then, the global high-energy physics community has reached consensus on the importance of an e+e– Higgs factory as the next collider after the LHC. In Europe, the 2020 update of the European strategy for particle physics concluded that a Higgs factory is the highest priority, while the US Snowmass 2021 community study and subsequent P5 report released in December 2023 also stressed the importance of overseas Higgs factories. CEPC scientists have actively contributed to both exercises. Meanwhile in Japan, which proposed to host an International Linear Collider (ILC) Higgs factory in 2012, a new baseline design to start at a collision energy of 250 GeV instead of 500 GeV was presented in 2017.
In China, both the 464th and 572th Xiangshan Science Conferences in 2013 and 2016 concluded that “CEPC is the best approach and a major historical opportunity for the national development of an accelerator-based high-energy physics programme”. In 2023, CEPC was identified as the top future particle accelerator in the planning study conducted by the Chinese Academy of Sciences (CAS). This followed an April 2022 statement by the International Committee for Future Accelerators (ICFA) that “reconfirms the international consensus on the importance of a Higgs Factory as the highest priority for realising the scientific goals of particle physics” and expressed support for Higgs-factory proposals worldwide. Five years after the publication of a conceptual design report in November 2018, a technical design report (TDR) for the CEPC accelerator – numbering more than 1000 pages and representing the first such report for a Higgs factory based on a circular collider – has now been completed.
CEPC is a circular Higgs factory comprising four accelerators: a 30 GeV linac, a 1.1 GeV damping ring, a booster with an energy up to 180 GeV, and a collider operating at four different energy modes corresponding to ZH production (240 GeV), the Z-pole (91 GeV), the W+W– threshold (160 GeV) and the tt threshold (360 GeV). The machines are connected by 10 transport lines. While the linac and damping ring would be constructed on the surface, the booster and collider would be situated in an underground ring with a circumference of 100 km, reserving space for a later hadron collider, SppC.
CEPC in focus
The CEPC collider features a double-ring structure, with electron and positron beams circulating in opposite directions in separate beam pipes and colliding at two interaction points where large detectors will be installed. The 100 km-circumference full-energy CEPC booster, positioned atop the collider in the same tunnel, functions as a synchrotron featuring a 30 GeV injection energy and an extraction energy equal to the beam collision energy. To maintain constant luminosity, top-up injection will be employed. The 1.8 km-long linac, which serves as an injector to the booster, accelerates both electrons and positrons using S- and C-band radio-frequency systems, equipped with a damping ring to reduce positron emittance. As an alternative option, polarisation schemes are also under study.
A follow-up to CEPC is SppC, a proton–proton collider with a centre-of-mass energy of up to 125 TeV. The tunnel, primarily consisting of hard rock that will be excavated using either a tunnel boring machine or drill-and-blast methods, allows the SppC to be installed without removing the CEPC. This unique layout opens exciting long-term possibilities for electron–proton and electron–ion physics in addition to the CEPC’s e+e– and the SppC’s proton–proton and ion–ion physics operations. Furthermore, the CEPC would be configured to operate as a high-energy synchrotron-radiation light source with two gamma-ray beamlines, extending the usable synchrotron-radiation spectrum to unprecedented energy (from 100 keV to more than 100 MeV) and brightness ranges. The 30 GeV injection linac can also produce a high-energy X-ray free electron laser by adding an undulator.
Dedicated goals
The CEPC operation plan and physics goals follow a “10-2-1-5” scheme, dedicating 10 years as a Higgs factory, two years as a Z factory, one year as a W factory and possibly an additional five years’ operation at the ttthreshold. The four collision modes (corresponding to H, Z, WW and tt production) have a baseline synchrotron radiation power of 30 MW per beam. Luminosity upgrades are also considered by increasing the synchrotron-radiation power per beam up to 50 MW, reaching a luminosity of 8 × 1034 at 240 GeV. With the upgraded luminosity plan, 4.3 million Higgs bosons, 4.1 trillion Z bosons, 210 million W bosons and 0.6 million ttpairs would be produced in the two CEPC detectors.
After the completion of the CEPC conceptual design report, the accelerator entered a five-year-long TDR study during which the design was further optimised. The resulting report, released on 25 December 2023, emphasises the optimal luminosity, coverage of H, Z, W and ttenergies, and the full spectrum of technology R&D, civil-engineering designs, industrial and international collaborations and participation.
Smaller emittances at the interaction points have been adopted to increase the luminosities, dynamic apertures including various errors for four energies match the design goals, beam–beam and collective effects have been verified, and the machine–detector interface has been optimized with a 20 cm-diameter central beryllium pipe at the interaction points. The booster has adopted a theoretical minimum emittance-like lattice design with an injection energy raised to 30 GeV and output energy of up to 180 GeV.
CEPC accelerator R&D has been conducted in synergy with the fourth-generation 6 GeV High Energy Photon Source project at IHEP in Beijing. These R&D activities cover the collider and booster magnets, superconducting quadrupoles for the insertion regions, NEG-coated vacuum chambers, superconducting cryomodules, cryogenic systems, continuous-wavelength high-efficiency klystrons, magnet power supplies, mechanics, S-band and C-band linac and positron source, damping ring, instrumentation and feedbacks, control system, survey and alignment, radiation protection and environmental aspects.
CEPC is the best approach and a major historical opportunity for the national development of an accelerator-based high-energy physics programme
As three examples, firstly the CEPC booster 1.3 GHz 8 × 9-cell cavity cryomodule has been shown to reach a quality factor/accelerating gradient of 3.4 × 1010/23 MVm–1, surpassing the booster specification (see “CEPC technologies”, left image). Secondly, the CEPC 650 MHz one-cell cavity reached 2.3 × 1010/41.6 MVm–1 at 2 K with electrical-polishing treatment and 6.3 × 1010/31 MVm–1 with medium-temperature treatment. Thirdly, three collider-ring 650 MHz, 800 kW continuous-wavelength high-efficiency klystrons have been developed at IHEP, where the second klystron (see “CEPC technologies”, right image) has reached an efficiency of 77.2% at 849 kW in pulsed mode compared with the design value of 77% at 800 kW in CW mode. The third klystron is a multibeam klystron with a design goal of 80.5%, and its electron source is currently undergoing tests. The achievements of the CEPC accelerator TDR are also a result of strong industrial participation and contributions, via the CEPC industrial promotion consortium.
High-energy ambitions
As part of future strategic technology R&D in high-energy physics and beyond, the CEPC team has proposed an alternative beam-driven plasma injector with beam energies from 10 to 30 GeV. To develop and demonstrate the necessary plasma technologies, such as positron acceleration, staged acceleration and high beam qualities for future linear colliders, IHEP initiated a plasma acceleration experimental programme in September 2023 using the injector linac of BEPCII (a 1.89 GeV e+e– collider with a luminosity of 1033 cm–2s–1) and experimental facilities funded by CAS to the tune of RMB 0.12 billion ($17 million).
The SppC, in conjunction with the CEPC, would not only provide unprecedented precision on Higgs-boson measurements but explore a significantly larger region of the new-physics landscape, propelling our understanding of the physical world to new heights. A future hadron collider is both more costly than a Higgs factory and more technically challenging. Critical issues such as high-field (20 T or higher) superconducting magnets, synchrotron radiation in a cryogenic environment and a sophisticated beam-collimation system for quench protection must be adequately addressed before construction can begin.
High-field magnets based on high-temperature iron-based superconductors are proposed as the key development path for the SppC. This technology has a much higher magnetic field potential (>30 T) and lower cost than the NbTi/Nb3Sn technologies used nowadays, and significant progress has been made, together with industry, during the past eight years. In 2016 more than 100 m of iron-based “7-core” tape was fabricated, reaching a current density of 450 A/mm2 at 10 T and 4.2 K in 2022.
The SppC is expected to achieve a peak luminosity of 1035 cm–2s–1 per interaction point and an integrated luminosity of approximately 30 ab–1, assuming two interaction points and a runtime of 20–30 years. To further reduce the energy consumption of SppC and CEPC (which has a total power consumption of 262 MW at the ZH energy with a synchrotron-radiation power of 30 MW per beam), various countermeasures are under study.
From 2019 to 2022, CEPC accelerator activities were guided by an International Accelerator Review Committee. In June and September 2023, the CEPC accelerator international TDR and cost review were carried out at Hong Kong University of Science and Technology, while the civil-engineering cost was reviewed by a domestic committee in June 2023. The total CEPC cost is estimated at RMB 36.4 billion ($5.15 billion), with accelerator, infrastructure and experiments taking up RMB 19 billion, 10.1 billion and 4 billion, respectively. Among all the CEPC candidate sites, three – Qinhuangdao, Huzhou and Changsha – have been studied in the TDR.
At the end of October 2023, the CEPC international advisory committee supported the conclusion of the TDR review that the accelerator team is well prepared to enter an engineering design report (EDR) phase. The following month, CEPC–SppC proposals were presented at the ICFA Seminar at DESY, declaring the completion of the CEPC accelerator TDR.
Concerning the technology and status of the CEPC detectors, a full spectrum R&D programme has been carried out, spanning the pixel vertex detector, silicon tracker, time projection chamber and drift chamber, time-of-flight detector, calorimeters, high-temperature superconducting solenoid and mechanical design, among others. This R&D also benefits from past experiences with BESIII (in particular concerning the drift chamber and superconducting magnet) and from the High-Luminosity LHC upgrades for ATLAS and CMS (such as the silicon-strip detector and high-granularity calorimeter). The CEPC detector TDR reference design began in January 2024 and will be completed in mid-2025 within the EDR phase (2024–2027).
EDR and schedule
The aim is to present the CEPC proposal (including accelerator, detector and engineering) for selection by the Chinese government around 2025, with construction to start in around 2027 and to be completed around 2035. A preliminary accelerator EDR plan has been established and is to be reviewed by the International Accelerator Review Committee in 2024.
The SppC, in conjunction with the CEPC, would propel our understanding of the physical world to new heights
Concerning CEPC development towards construction, CAS is planning for China’s 15th “five-year plan” for large science projects, for which a steering committee chaired by the CAS president was established in 2022. High-energy physics and nuclear physics, one of eight fields in the plan, has selected nine proposals that have been reviewed in an open and international way. CEPC is ranked first, with the smallest uncertainties by every committee (including domestic committees and an international advisory committee). A final report has been submitted to CAS for consideration.
CEPC has always been envisioned as an international big-science project, and participation is warmly welcomed both in scientific and industrial ways. The CEPC accelerator TDR represents the efforts of thousands of domestic and overseas scientists and engineers. Such a facility would play an important role in future plans of the worldwide high-energy physics community, deepening our understanding of matter, energy and the universe to an unprecedented degree while facilitating extensive research and collaboration to explore the frontiersof technology.
The physics landscape has changed. We have not seen signs of new particles above the Higgs-boson mass. Typical limits are now well above 1 TeV based on LHC data, which means we need to look for the new physics that we anticipate at higher energies. The consensus during the recent US Snowmass process was that we should aim for 10 TeV in the centre-of-mass. A muon collider has the feature that its expected wall-plug power scales very favourably as you go to the multi-TeV scale. While significant technology development is required to establish the overall feasibility, performance and cost of such a machine, our current performance estimates make it a very interesting candidate. This motivates an active R&D and design programme to validate this approach.
Why was the US Muon Accelerator Program (MAP) discontinued a decade ago?
MAP was approved in early 2011 to assess the feasibility of the technologies required. By 2014, the community had just discovered the Higgs boson and was focused on pursuing a Higgs factory. Mature concepts based on superconducting (ILC) and normal-conducting (CLIC) linear-collider technologies were at hand, and these approaches envisioned subsequent energy upgrades that would enable the exploration of a new-particle spectrum extending into the TeV scale. Because of the relatively low mass of the Higgs, work was also going into a large circular collider design that would represent minimal technical risk. A muon collider, a concept with much lower overall maturity level and with significantly different operating characteristics, did not appear to provide a timely path to realising the Higgs factory.
The other application of interest involving muon–accelerator technologies was the neutrino factory. However, the field concluded that a long-baseline neutrino experiment based on the “superbeam source” represented the best path forward. In a constrained budget environment, the concepts being pursued by MAP didn’t have sufficient priority and support to continue.
What do we know so far about the feasibility of a muon collider?
As the MAP effort concluded, several key R&D and design efforts were nearing completion and were subsequently published. These included demonstrations of normal-conducting RF cavities in multi-Tesla magnetic fields operating with >50 MV/m accelerating gradients, simulated 6D cooling-channel designs capable of achieving the necessary emittance cooling for collider applications, and a measurement of the cooling process at the international Muon Ionization Cooling Experiment (MICE). While MICE only characterised the performance of a partial cooling cell, the precise measurements provided by its tracking detector system confirmed that the muons behaved consistently with the cooling process as described in the simulation codes that were employed to design the cooling channel for a high-brightness muon source.
Any future collider operating at the energy frontier will have to be supported by a global development team
Another key advance was detailed simulations of the performance of a muon-collider detector in the lead-up to the last European strategy update. These efforts, utilising the beam-induced background samples prepared by MAP, demonstrated that useful physics results could be obtained with reasonable assumptions about the performance of the individual elements of the detector.
How are things going with the International Muon Collider Collaboration (IMCC)?
The IMCC, led by CERN with European funding support from the MuCol project, presently coordinates global activities towards R&D and design. The collaboration’s input has been crucial in developing the technically limited timeline towards a multi-TeV muon collider as outlined in the accelerator R&D roadmap commissioned by the European Laboratory Directors Group. The IMCC is making excellent progress towards a reference design for the muon-collider complex as well as defining a cooling demonstrator. An interim report is currently being prepared. However, current funding levels for the effort correspond to roughly half of the estimated levels required to achieve the technically limited timeline. With the strong support for pursuing an energy-frontier muon collider in the US, it is hoped that a fully global effort will be able to support the effort at levels that much more closely match the requirements of a technically limited timeline.
How does the IMCC relate to the P5 recommendations for reinvigorated muon collider R&D at Fermilab?
Any future collider operating at the energy frontier will have to be supported by a global development team, and the issue of where such a machine can be sited will depend on a complex set of circumstances that we certainly can’t predict now. The fundamental goal is to identify the technology and one or more sites where it can be deployed so that we are able to continue our exploration of the fundamental building blocks and processes in the universe for all humankind. Thus, the current IMCC activities are fully aligned with the aspiration expressed by P5 to explore the option for conducting muon collider R&D in the US and exploring the possibility of Fermilab as a host site for a future machine.
What are the key accelerator challenges to be overcome?
While there are a number of challenging subsystems to engineer, the most novel aspect of the machine remains the ionisation cooling channel. Demonstration of the beam operations of a cooling module at high beam intensity will be necessary to give us confidence that the technology is robust enough for high-energy physics applications. In addition to this absolutely unique subsystem of the muon collider, we require detailed end-to-end simulations of the overall machine performance, detailed engineering conceptual designs for all key components, and successful engineering demonstrations of suitable-scale prototypes for several critical systems. These include the target, the fast-ramping magnet system for the high-energy accelerator stages, the large-aperture collider ring magnets that must be adequately shielded against the decay products of the muon beams, and detector subsystems that can robustly operate in an environment with the beam-induced backgrounds from the muon decays.
And the detector challenges?
Tremendous progress in detector technology has resulted from the design and operation of the LHC detectors. Further progress in obtaining precision physics measurements in very high-occupancy environments as we prepare for the HL-LHC provides confidence for the detector requirements of a muon collider, which will have to deal with similar hit rates. While the details of the occupancy in the detectors for these two types of machine are not identical, the concepts being implemented for better time and spatial segmentation appear quite effective for both.
A particular feature of the muon collider detector is the “shielding nozzle” that was first introduced in MAP to protect the innermost detector elements. These nozzles impact the overall physics performance by limiting the near-axis coverage. However, with detailed detector performance studies underway, we are now in a position to carry out detailed detector and shielding studies to optimise these elements for overall physics performance.
How is the vast neutrino flux being addressed?
The very high-energy muon beams in a collider result in a narrow cone of neutrinos being produced in the forward direction as they circulate around the collider ring. When the beams are moving through dipoles, the constant change in transverse direction helps to dilute this flux, but any straight sections in the ring effectively act as a high-energy neutrino source that shines in a specific direction. The tremendous flux of neutrinos from a straight section of a TeV-scale collider are expected to create ionising radiation wherever they exit Earth’s surface. Thus, there are a set of mitigation strategies incorporated into the design effort to make sure that there are absolutely no risks. This includes minimising the number of straight sections, incorporating magnet-movers that allow the vertical trajectories of the beams to be changed slowly throughout the collider, and ensuring that the beams do not exit in populated areas.
What does the timeline for a 10 TeV muon collider look like?
We need to deliver a complete end-to-end reference design in time for the next European strategy update and for the US interim panel review that was recommended in the P5 report. A conceptual design report (CDR) for a demonstrator facility then has to be completed such that construction could begin by around 2030. Over the course of the next decade, the engineering design concepts for each subsystem have to be prepared and prototyping R&D has to be carried out, while also producing a CDR for the high-energy facility, including detailed performance simulations. By the late 2030s, the demonstrator facility and prototyping programme would enable detailed technical specifications for all key systems. Upgrades to the demonstrator facility could be necessary to further clarify performance and technical specifications. The final steps would be to complete a technical design that incorporates results from the demonstrator programme and to develop site-specific plans for the labs that would like to be considered as potential hosts for the facility. The start of 10 TeV collider operations would then be guided by a physics-driven plan, including potential intermediate stages, but likely at least a decade after construction approval.
The current schedule puts physics operations of a high-energy muon collider about five years earlier than an FCC-ee. Is this realistic?
I would characterise these two timelines as being of different types. The FCC-ee timeline is based on an integrated plan for CERN, while the 3 TeV muon collider is explicitly a technically limited plan which assumes that a sufficient funding profile can be provided, and that there are no external constraints that could impact deployment. In other words, the muon-collider timeline remains an aspiration, whereas the FCC-ee timeline attempts to build-in actual deployment constraints.
What is the estimated cost of a 10 TeV muon collider?
At present, the cost estimates rely on broad extrapolations from existing collider systems. While these extrapolations suggest that a multi-TeV muon collider may well be one of the most cost-effective routes to the energy frontier, the uncertainties remain large. To deliver a “realistic” cost estimate, we will require a complete end-to-end reference design, engineering conceptual designs for all of the unique systems required, detailed cost estimates for the engineering conceptual designs and extrapolated cost estimates for the remaining “standard” accelerator systems. With the present technically limited schedule as prepared by the IMCC, this would suggest that a detailed and realistic cost estimate could be available around the end of this decade.
How does a high-energy muon collider fit into the global picture?
There are multiple ways this can fit. At present, we need to acknowledge that the R&D for the magnets for a high-energy proton–proton machine, such as those being pursued in Europe and China, still require an extensive R&D programme. This is likely a multi-decade effort in and of itself, and is commensurate with the timescales needed to carry out muon-collider R&D and design work. Having more than one technology option on the table to achieve our ultimate physics goals is a necessity. Furthermore, the complementarity between lepton- and hadron-collider paths may be needed to support our overarching scientific goals.
A detailed and realistic cost estimate could be available around the end of this decade
From a somewhat different point of view, the potential applications of a high-intensity muon source extend beyond colliders. The technology offers improved performance and new opportunities for other scientific goals such as a high-performance source for future neutrino and charged lepton flavour violation experiments, materials science and active interrogation of complex structures, among others. Clarifying the broader context for the technology is currently being pursued within the IMCC effort.
The 2020 European strategy for particle physics justifiably singled out the Higgs boson as the most mysterious element of the Standard Model. Uncovering the particle’s true nature and answering the numerous questions raised by its interactions with other particles is set forth as the highest priority of the field. And this, the strategy concluded, requires the next dream machines: an e+e– Higgs factory and, in the longer term, a 100 TeV hadron collider. Getting there will be no easy feat, and thus several intermediate steps, necessary for bringing this programme to fruition, have been set in motion.
Firstly, the European Committee for Future Accelerators (ECFA) was called upon by the CERN Council to formulate a global detector R&D roadmap for both short- and long-term experimental endeavours. A painstaking consultation process across the entire range of detector technologies – from gas, liquid and solid-state detectors to particle-
identification systems, calorimetry and blue-sky R&D – culminated in a 250-page document and the creation of detector R&D collaborations to focus on the most relevant topics. In parallel, the European Laboratory Directors Group has compiled an accelerator R&D roadmap spanning activities such as high-field magnets, high-gradient accelerating elements, plasma-wakefield acceleration, energy- recovery linacs, and more.
With the accelerator and detector development in the best of hands, what remains is to converge on the next machine: namely the e+e– collider that takes us as close as we can to a full understanding of the Higgs boson and the electroweak and top-quark sectors. Thankfully, we already know a lot about the reach of such “HET” factories from previous studies, in particular those carried out during the previous strategy update. To encourage further work en route to the next strategy update, ECFA has put together a HET-factory study group that brings together both the linear and circular e+e– detector communities. The goal is to solidify our understanding of the requirements that the physics places on the experiments and on the associated beams. A common software framework with more realistic detector simulation and a parallel study of detector structures are the other working areas in the study group. Good progress is visible, and the third and last major workshop on the HET-factory study will take place in October 2024.
Major players
The other major players in the global high-energy physics scene completed their corresponding strategy processes either several years ago (Japan with the ILC and China with the CEPC) or recently (US with the P5 process). All eyes are now turned to Europe as we enter the final stretch towards the next update of the European strategy. With the Future Circular Collider feasibility study due to be completed next year, all the elements needed for a fully informed decision on the future of European – and global – particle physics will soon be in place.
The entire field, and especially the younger generations, are most eagerly awaiting this decision
The next strategy process will build on the excellent work that took place in the context of the previous one, which culminated with a large community gathering in Granada. Taking into account the updated information, it is both expected and highly desirable that the process converges quickly, with a definitive recommendation on both the next e+e– collider and the longer-term prospects. The entire field, and especially the younger generations, are most eagerly awaiting this decision. Today, in parallel with maximally exploiting the physics potential of the LHC, our most important duty is to ensure that current PhD candidates find themselves at the centre of future discoveries a few decades from now.
Is all this possible for Europe? Absolutely! CERN has an unparalleled track record on the world stage with the ISR, SppS and LEP legacies, as well as the tremendous success of the LHC. These have not only provided some of the greatest advances in our understanding of the fundamental elements of nature, but also serve as guarantors of CERN’s ability to continue advancing the energy frontier, keeping Europe at the leading edge of scientific knowledge. All that is currently needed is the final direction – and the start signal. Quo vadis European particle physics? Towards the next discovery frontier, to further unravel the mysteries of the fascinating universe we have come to inhabit.
