In particle physics, the smallest problems often require the biggest solutions.
Along the border of France and Switzerland, around a hundred meters underneath the countryside, protons speed through a 27-kilometer ring—about seven times the length of the Indy 500 circuit—until they crash into protons going in the opposite direction. These particle pileups produce a petabyte of data every second, the most interesting of which is poured into data centers, accessible to thousands of physicists worldwide.
The Large Hadron Collider (LHC), arguably the largest experiment ever engineered, is needed to probe the universe’s smallest constituents. In 2012, two teams at the LHC discovered the elusive Higgs boson, the particle whose existence confirmed 50-year-old theories about the origins of mass. It was a scientific triumph that led to a Nobel Prize and worldwide plaudits.
Since then, experiments at the LHC have focused on better understanding how the newfound Higgs fits into the Standard Model, particle physicists’ best theoretical description of matter and forces—minus gravity. “The Standard Model is beautiful,” says Victoria Martin, an experimental physicist at the University of Edinburgh. “Because it’s so precise, all the little niggles stand out.”
The Large Hadron Collider lives in a 27-kilometer tunnel ring, about 100 meters underneath France and Switzerland. It was used to discover the Higgs boson, but further research may require something larger still. Maximilien Brice/CERN
The minor quibbles physicists have about the Standard Model could be explained by new particles: Dark matter, the invisible material whose gravity shapes the universe, is thought to be made of heretofore undiscovered particles. But such new particles may be out of reach for the LHC, even after it undergoes upgrades that are set to be completed later this decade. To address these lingering questions, particle physicists have been planning its successors. These next-generation colliders will improve on the LHC by smashing protons at higher energies or by making more precise collisions with muons, antimuons, electrons, and positrons. In doing so, they’ll allow researchers to peek into a whole new realm of physics.
Martin herself is particularly interested in learning more about the Higgs, and learning exactly how the particle responsible for mass behaves. One possible find: Properties of the Higgs suggest that the universe might not be stable in the long, long term. [Editor’s note: About 10790 years. Other problems may be more pressing.] “We don’t really know exactly what we’re going to find,” Martin says. “But that’s okay, because it’s science, it’s research.”
There are four main proposals for new colliders, and each one comes with its own slew of engineering challenges. To build them, engineers would need to navigate tricky regional geology, design accelerating cavities, handle the excess heat within the cavities, and develop powerful new magnets to whip the particles through these cavities. But perhaps more daunting are the geopolitical obstacles: coordinating multinational funding commitments and slogging through bureaucratic muck.
Collider projects take years to plan and billions of dollars to finance. The fastest that any of the four machines would come on line is the late 2030s. But now is when physicists and engineers are making key scientific and engineering decisions about what’s coming next.
Supercolliders at a glance
Size (circumference): 27 kilometers
Collision energy: 13,600 giga-electron volts
Colliding particles: protons and ions
Luminosity: 2 × 1034 collisions per square centimeter per second (5 × 1034 for high-luminosity upgrade)
Location: Switzerland–France border
Start date: 2008–
Size (length): 31 km
Collision energy: 500 GeV
Colliding particles: electrons and positrons
Luminosity (at peak energy): 3 × 1034 collisions per cm2 per second
Location: Iwate, Japan
Earliest start date: 2038
Size (circumference): 4.5 km (or 10 km)
Collision energy: 3,000 GeV (or 10,000 GeV)
Colliding particles: muons and antimuons
Luminosity: 2 × 1035 collisions per cm2 per second
Location: possibly Fermilab
Earliest start date: 2045 (or in the mid-2050s)
Size (circumference): 91 km
Collision energy: 240 GeV | 85,000 GeV
Colliding particles: electrons and positrons | protons
Luminosity: 8.5 × 1034 | 30 × 1034 collisions per cm2 per second
Location: Switzerland–France border
Earliest start date: 2046 | 2070
Size (circumference): 100 km
Collision energy: 240 GeV | 100,000 GeV
Colliding particles: electrons and positrons | protons
Luminosity: 8.3 × 1034 | 13 × 1034 collisions per cm2 per second
Location: China
Earliest start date: 2035 | 2060s
Possible supercolliders of the future
The LHC collides protons and other hadrons. Hadrons are like beanbags, full of quarks and gluons, that spray around everywhere upon collision.
Next-generation colliders have two ways to improve on the LHC: They can go to higher energies or higher precision. Higher energies provide more data by producing more particles—potentially new, heavy ones. Higher-precision collisions give physicists cleaner data with a better signal-to-noise ratio because the particle crash produces less debris. Either approach could reveal new physics beyond the Standard Model.
Three of the new colliders would improve on the LHC’s precision by colliding electrons and their antimatter counterparts, positrons, instead of hadrons. These particles are more like individual marbles—much lighter, and not made up of any smaller constituents. Compared with the collisions between messy, beanbag-like hadrons, a collision between electrons and positrons is much cleaner. After taking data for years, some of those colliders could be converted to smash protons as well, though at energies about eight times as high as those of the LHC.
These new colliders range from technically mature to speculative. One such speculative option is to smash muons, electrons’ heavier cousins, which have never been collided before. In 2023, an influential panel of particle physicists recommended that the US pursue development of such a machine, in a so-called ‘muon shot’. If it is built, a muon collider would likely be based at Fermilab, the center of particle physics in the United States.
A muon collider “can bring us outside of the world that we know,” says Daniele Calzolari, a physicist working on muon collider design at CERN, the European Organization for Nuclear Research. “We don’t know exactly how everything will look like, but we believe we can make it work.”
While muon colliders have remained conceptual for more than 50 years, their potential has long excited and intrigued physicists. Muons are heavy compared with electrons, almost as heavy as protons, but they lack the mess of quarks and gluons, so collisions between muons could be both high energy and high precision.
Superconducting radio-frequency cavities are used in particle colliders to apply electric fields to charged particles, speeding them up toward each other until they smash together. Newer methods of making these cavities are seamless, providing more-precise steering and, presumably, better collisions. Reidar Hahn/Fermi
The trouble is that muons decay rapidly—in a mere 2.2 microseconds while at rest—so they have to be cooled, accelerated, and collided before they expire. Preliminary studies suggest a muon collider is possible, but key technologies, like powerful high-field solenoid magnets used for cooling, still need to be developed. In March 2025, Calzolari and his colleagues submitted an internal proposal for a preliminary demonstration of the cooling technology, which they hope will happen before the end of the decade.
The accelerator that could theoretically come on line the soonest, would be the International Linear Collider (ILC) in Iwate, Japan. The ILC would send electrons and positrons down straight tunnels where the particles would collide to produce Higgs bosons that are easier to detect than at the LHC. The collider’s design is technically mature, so if the Japanese government officially approved the project, construction could begin almost immediately. But after multiple delays by the government, the ILC remains in a sort of planning purgatory, looking more and more unlikely.
The Standard Model of particle physics is the current best theory of all the understood matter and forces in our universe (except gravity). The model works extremely well, but scientists also know that it is incomplete. The next generation of supercolliders might give a glimpse at what’s beyond the Standard Model.
So, the two colliders, which are both technically mature, that have perhaps the clearest path to construction are China’s Circular Electron Positron Collider (CEPC) and CERN’s Future Circular Collider (FCC-ee).
CERN’s FCC-ee would be a 91-km ring, designed to initially collide electrons and positrons to study the parameters of particles like the Higgs in fine detail (the “ee” indicates collisions between electrons and positrons). Compared with the LHC’s collisions of protons or heavy ions, those between electrons and positrons “are much cleaner, so you can have a more precise measurement,” says Michael Benedikt, the head of the FCC-ee effort. After about a decade of operation—enough time to gather data and develop the needed magnets—it would be upgraded to collide protons and search for new physics at much higher energies (and then become known as the FCC-hh, for hadrons). The FCC-ee’s feasibility report just concluded, and CERN’s member states are now left deciding whether to pursue the project.
Similarly, China’s CEPC would also be a 100-km ring designed to collide electrons and positrons for the first 18 years or so. And much like the FCC, a proton or other hadron upgrade is in the works after that. Later this year, Chinese researchers plan to submit the CEPC for official approval by the Chinese government as part of the next five-year-plan. As the two colliders (and their proton upgrades) are considered for construction in the next few years, policymakers will be thinking about more than just their potential for discovery.
CEPC and FCC-ee are, in this sense, less abstract physics experiments and more engineering projects with concrete design challenges.
Laying the groundwork
When particles zip around the curve of a collider, they lose energy—much like a car braking on a racetrack. The effect is particularly pronounced for lightweight particles like electrons and positrons. To reduce this energy loss from sharp turns, CEPC and FCC-ee are both planned to have enormous tunnels, which, if built, would be among the longest in the world. The construction cost of such an enormous tunnel would be several billion U.S.dollars, roughly one-third of the total collider price.
Finding a place to bury a 90-km ring is not easy, especially in Switzerland. The proposed path of the FCC-ee has an average depth of 200 meters, with a dip to 500 meters under Lake Geneva, fit snugly between the Jura Mountains to the northwest and the Prealps to the east. The land there was once covered by a sea, which left behind sedimentary rock—a mixture of sandstone and shale known as molasse. “We’ve done so much tunneling at CERN before. We were quite confident about the molasse rock,” says Liam Bromiley, a civil engineer at CERN.
But the FCC-ee’s path also takes it through deposits of limestone, which is permeable and can hold karsts, or cavities, full of water. “If you hit one of those, you could end up flooding the tunnel,” Bromiley says. During the next two years, if the project is green-lit, engineers will drill boreholes into the limestone to determine whether there are karsts that can be avoided.
FCC-ee would be a 91-km ring spanning underneath Switzerland and France, near the current Large Hadron Collider. One of the proposed locations for the CEPC is near the northern port city of Qinhuangdao, where the 100 km circumference collider would be buried underground.Chris Philpot
CEPC, in contrast, has a much looser spatial constraint, and can choose from nearly anywhere in China. Three main sites are being considered: Qinhuangdao (a northern port city), Changsha (a metropolis in central China), and Huzhou (a coastal city near Shanghai). According to Jie Gao, a particle physicist at the Institute of High Energy Physics, in Beijing, the ideal location will have hard rock, like granite, and low seismic activity. Additionally, Gao says, they want a site with good infrastructure to create a “science city” ideal for an international community of physicists.
The colliders’ carbon footprints are also on the minds of physicists. One potential energy-saving measure: redirecting excess heat from operations. “In the past we used to throw it into the atmosphere,” Benedikt says. In recent years, heated water from one of the LHC’s cooling stations has kept part of the commune of Ferney-Voltaire warm during the winters, and Benedikt says the FCC-ee would expand these environmental efforts.
Getting up to speed
If the civil-engineering challenges are met, physicists will rely on a spate of technologies to accelerate, focus, and collide electrons and positrons at CEPC and FCC-ee more precisely and efficiently than they could at the LHC.
When both types of particles are first produced from their sources, they start off at a comparatively low energy, around 4 giga-electron volts. To get them up to speed, electrons and positrons are sent through superconducting radio-frequency (SRF) cavities—gleaming metal bubbles strung together like beads of a necklace, which apply an electric field that pushes the charged particles forward.
Both China’s Circular Electron Positron Collider (CEPC) [bottom] and CERN’s Future Circular Collider (FCC-ee) [top] have preliminary designs of the insides of their tunnels, including the collider itself, associated vacuum and control equipment, and detectors.Chris Philpot
In the past, SRF cavities were welded together, which inherently left imperfections that led to beam instabilities. “You can never obtain a perfect surface along this weld,” Benedikt says. FCC-ee researchers have explored several techniques to create cavities without seams, including hydroforming, which is widely used for the components of high-end sports cars. A metal tube is placed in a pressurized cell and compressed against a die by liquid. The resulting cavity has no seams and is smooth as blown glass.
To improve efficiency, engineers focus on the machines that power the SRF cavities, machines called klystrons. Klystrons have historically had efficiencies that peak around 65 percent, but design advances, such as the machines’ ability to bunch electrons together, are on track to reach efficiencies of 80 percent. “The efficiency of the klystron is becoming very important,” Gao says. Over 10 years of operation, these savings could amount to 1 terawatt hour—about enough electricity to power all of China for an hour.
Another efficiency boost comes from focusing on the tunnel design. As electrons and positrons follow the curve of the ring, they will lose a considerable amount of energy, so SRF cavities will be placed around the ring to boost particle energies. The lost energy will be emitted as potent synchrotron radiation—about 10,000 times as much radiation as is emitted by protons circling the LHC today. “You do not want to send the synchrotron radiation into the detectors,” Benedikt says. To avoid this fate, neither FCC-ee nor CEPC will be perfectly circular. Shaped a bit like a racetrack, both colliders will have about 1.5-km-long straight sections before an interaction point. Other options are also on the table—in the past, researchers have even used repurposed steel from scrapped World War II battleships to shield particle detectors from radiation.
Both CEPC and FCC-ee will be massive data-generating machines. Unlike the LHC, which is regularly stopped to insert new particles, the next-generation colliders will be fed with a continuous stream of particles, allowing it to stay in “collision mode” and take more data.
At a collider, data is a function of ‘luminosity’— the ratio of detected events per square centimeter, per second. The more particle collisions, the “brighter” the collider. Firing particles at each other is a little like trying to get two bullets to collide—they often miss each other, which limits the luminosity. But physicists have a variety of strategies to squeeze more electrons and positrons into smaller areas to achieve more of these unlikely collisions. Compared to the Large Electron-Positron (LEP) collider of the 1990s, the new machines will produce 100,000 times as many Z bosons—particles responsible for radioactive decay. More Z bosons means more data. “The FCC-ee can produce all the data that were accumulated in operation over 10 years of LEP within minutes,” Benedikt says.
Back to protons
While both the FCC-ee and CEPC would start with electrons and positrons, they are designed to eventually collide protons. These upgrades are called FCC-hh and Super proton-proton Collider (SPPC). Using protons, FCC-hh and SPPC would reach a collision energy of 100,000 GeV, roughly an order of magnitude higher than the LHC’s 13,600 GeV. Though the collisions would be messy, their high energy would allow physicists to “explore fully new territory,” Benedikt says. While there’s no guarantee, physicists hope that territory teems with discoveries-in-waiting, such as dark-matter particles, or strange new collisions where the Higgs recursively interacts with itself many times.
One pro of protons is that they are over 1,800 times as heavy as electrons, so they emit far less radiation as they follow the curve of the collider ring. But this extra heft comes with a substantial cost: Bending protons’ paths requires even stronger superconducting magnets.
Magnet development has been the downfall of colliders before. In the early 1980s, a planned collider named Isabelle was scrapped because magnet technology was not far enough along. The LHC’s magnets are made from a strong alloy of niobium-titanium, wound together into a coil that produces magnetic fields when subjected to a current. These coils can produce field strengths over 8 teslas. The strength of the magnet pushes its two halves apart with a force of nearly 600 tons per meter. “If you have an abrupt movement of the turns in the coil by as little as 10 micrometers,” the entire magnet can fail, says Bernhard Auchmann, an expert on magnets at CERN.
It Is Unlikely That Any Collider—Whether Based in China, at Cern, the United States, or Japan—Will Be Able To Go It Alone.
Future magnets for FCC-hh and SPPC will need to have at least twice the magnetic field strength, about 16 to 20 T, pushing the limits of materials and physics. Auchmann points to three possible paths forward. The most straightforward option might be “niobium three tin” (Nb3Sn). Substituting tin for titanium allows the metal to host magnetic fields up to 16 T but makes it quite brittle, so you can’t “clamp the hell out of it,” Auchmann says. One possible solution involves placing (Nb3Sn) into a protective steel endoskeleton that prevents it from crushing itself.
Then there are high-temperature superconductors. Some magnets made with rare earth metals can exceed 20 T, but they too are fragile and require similar steel supports. Currently, these materials are expensive, but demand from fusion startups, which also require these types of magnets, may push the price down, Auchmann says.
Finally, there is a class of iron-based high-temperature superconductors that is being championed by physicists in China, thanks to the low price of iron and manufacturing-process improvements. “It’s cheap,” Gao says. “This technology is very promising.” Over the next decade or so, physicists will work on each of these materials, and hope to settle on one direction for next-generation magnets.
Time and money
While FCC-ee and CEPC (as well as their proton upgrades) share many of the same technical specifications, they differ dramatically in two critical factors: timelines and politics.
Construction for CEPC could begin in two years; the FCC-ee would need to wait about another decade. The difference comes down largely to the fact that CERN has a planned upgrade to the LHC—enabling it to collect 10 times as much data—which will consume resources until nearly 2040. China, by contrast, is investing heavily in basic research and has the funds immediately at hand.
The abstruse physics that happens at colliders is never as far from political realities on Earth as it seems. Japan’s ILC is in limbo because of budget issues. The muon collider is subject to the whims of the highly divided 119th U.S. Congress. Last year, a representative for Germany criticized the FCC-ee for being unaffordable, and CERN continues to struggle with the politics of including Russian scientists. Tensions between China and the United States are similarly on the rise following the Trump administration’s tariffs.
How physicists plan to tackle these practical problems remains to be seen. But it is unlikely that any collider—whether based in China, at CERN, the United States, or Japan—will be able to go it alone. In addition to the tens of billions of dollars for construction and operation of the new facility, the physics expertise needed to run it and perform complex experiments at scale must be global. “By definition, it’s an international project,” Gao says. “The door is wide open.”
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