On the Swiss–French border, at the headquarters of the European laboratory CERN, a battle is under way for the future of particle physics. CERN’s leaders want to build the biggest machine on the planet here: an enormous particle accelerator that would open in 2070 and would dwarf the Large Hadron Collider (LHC), the lab’s current flagship facility.

Everything about the plan is unprecedented. The Future Circular Collider (FCC), as it’s called, would sit in a tunnel 91 kilometres in circumference, more than three times the size of the LHC’s. Its cost is likely to be at least US$30 billion and it would smash protons together at energies eight times greater than those in the LHC. It is hoped that expanding this energy frontier will reveal never-before-seen particles that could solve some pressing issues regarding the standard model — the current best theory of the Universe’s fundamental particles and fields — and shed light on some of physics’ greatest mysteries, such as the nature of dark matter.

The technologies to reach such energies aren’t ready yet. So the plan is to dig the tunnel and insert a simpler machine that, starting around 2045, would collide electrons and their antiparticles, called positrons (see ‘CERN’s plan for a mega-collider’). This interim collider would produce and study copious numbers of elementary particles known as Higgs bosons to understand their pivotal role in nature. Later, this ‘Higgs factory’ would be dismantled.

The two-stage FCC plan is backed by many physicists. It is spearheaded by CERN’s director-general (DG), Fabiola Gianotti, and supported by Mark Thomson, who is due to replace her in January 2026. “If approved, the FCC would become the most powerful instrument ever built to study the laws of nature at the most fundamental level,” Gianotti said in a statement to Nature.

But many others are unhappy with it, Nature has found. Interviews with more than two dozen researchers show that many are critical of the FCC strategy, because it will take so long to come to fruition and because sinking resources into it could close off alternative ideas.

“The issue is whether the community is willing to sacrifice the next 50 years to get a toy which may or may not be the way for [fixing] the standard model,” says Halina Abramowicz, a particle physicist at Tel Aviv University in Israel. Critics also say that CERN’s leadership has decided to back the FCC without adequately consulting the community.

In such a giant and political project, which involves financial contributions from many of CERN’s member countries and the opinions of tens of thousands of researchers, disagreements are inevitable. (When the LHC was built, Germany threatened to leave CERN if its budget-cut demands weren’t met.) But the discontent has reached an unprecedented pitch, says Ruben Saakyan, a particle physicist at University College London, who chairs the UK Particle Physics Advisory Panel. “The community was never split like this before,” he says.

It’s also unclear whether CERN’s member states will pay for the project. Germany has already said that it won’t raise its budget contributions. And projects elsewhere might undercut the case for the FCC: in particular, China is deciding whether to approve a similar machine.

The next year could be decisive for the European mega-collider plan. By December, a strategy working group will submit its conclusions on the idea to the CERN Council, the organization’s governing body. At stake is not only the ambitious experiment itself, but also the working lives of generations of physicists — and Europe’s role in particle physics for the rest of the century.

Decades of circular colliders

CERN emerged after the Second World War as part of a deliberate effort to pursue science for peace, and it has been a key centre for particle-physics research ever since. With an annual budget of almost 1.5 billion Swiss francs (US$1.7 billion) set by an international convention, and funding from 24 member states as well as non-member countries such as the United States and Japan, it is a beacon for international scientific cooperation.

For nearly two decades, it has hosted the LHC, the world’s largest and most powerful collider. The LHC itself replaced a previous electron–positron collider in the same tunnel, called LEP, that was built in the 1980s. But CERN hosts many other experiments and technology programmes, including work on antimatter, cosmic rays, alternative accelerator technologies, advanced magnets and isotopes for medical applications.

It was at the LHC that, in 2012, Gianotti announced the discovery of the Higgs boson. This is perhaps CERN’s crowning discovery: not just another particle, but the linchpin of the standard model. The discovery of the Higgs was the first direct evidence of a field that permeates the Universe, the Higgs field. The varying interactions of other fundamental particles with this field explains why they have different masses.

The LHC has not managed to top that moment. The Higgs boson was shaken out by smashing protons at high energies, but the collider has so far failed to deliver further, much-anticipated discoveries, such as the nature of dark matter. With the LHC’s life scheduled to end in 2040, thoughts of its successor have been brewing since the 2010s.

The standard model can’t explain dark matter or the unknown particles that determine the nature of the Higgs field, among other major questions in particle physics. But it is not clear from theorists’ models whether smashing higher-energy protons would turn up new, extremely massive particles that might provide answers.

Still, many researchers think that it is worthwhile. “Exploration of the energy frontier will enable us to deepen our understanding of physics at the shortest distances, which we know is intimately connected to the physics of the Universe on the largest scales,” Gianotti says. “It’s like an open ocean,” says particle physicist Pierluigi Campana, who is based near Rome and chairs the International Committee for Future Accelerators. He compares the quest for the energy frontier to that of the first explorers who took their canoes across the Pacific Ocean and settled its many islands.

The two-stage FCC concept was first presented in 2019. The idea is that the initial-stage ‘Higgs factory’ might reveal some deviations from standard-model predictions, which could hint at whether new particles exist and how massive they might be. This question is linked to a central mystery about the standard model: how the Higgs boson ‘breaks the symmetry’ between two of the three fundamental forces in the standard model: the electromagnetic force and the weak nuclear force. At the high energies that existed straight after the Big Bang, these two forces were unified.

Then, once research has produced breakthroughs in the necessary technology, such as how to produce sufficiently high-strength superconducting magnets that steer and focus beams of particles, the second-stage FCC could be built to discover those particles — if they are within its reach. (Some physicists say that new particles could include the constituents of dark matter, but many theorists now think that such particles are likely to be much lighter, not heavier, than the range already searched by the LHC.)

Costly collider

Although most particle physicists agree that both FCC machines would be good to have, the costs are daunting. No full costing is yet available; CERN documents have suggested the first phase alone might cost $17 billion. However, estimates by Vladimir Shiltsev, an accelerator physicist at Northern Illinois University in DeKalb, and his collaborators suggest that is a minimum value and that the two phases together would cost at least $30 billion, and probably much more (T. Roser et al. J. Instrum. 18, P05018; 2023).

Researchers have proposed several other possible designs for future colliders. For decades, a leading proposal for a Higgs factory was not a circular collider but a straight one, called the International Linear Collider. It was studied in detail with the intent of placing it in Japan, but that country did not finalize its approval. Advocates of a linear Higgs factory modelled on the International Linear Collider say it would do all the Higgs studies of the circular version, but be cheaper and faster. Jenny List, a physicist at the German Electron Synchrotron (DESY) in Hamburg, says that a machine with a 21–33-kilometre tunnel could cost less than half as much as the first stage of the FCC. It could also study how two Higgs particles interact with each other. That research would not be directly accessible at the FCC, and could be crucial to understanding the nature of the Higgs field, says Michael Peskin, a theoretical physicist at the SLAC National Laboratory in Menlo Park, California. “We know how to build it; it has a reasonable cost, and it really can be running at the time the LHC ends, if we can get our act together,” he says.

CERN’s $17-billion supercollider in question as top funder criticizes cost

The linear and circular options each have their strengths and weaknesses, physicists say. Proponents of the FCC plan say a linear tunnel would be a dead end once it has served its purpose as a Higgs factory. But List counters that a linear collider can be upgraded by lengthening the tunnel later on. And it could host a future linear accelerator based on one of several advanced technologies that are being developed, such as the US-led Cool Copper Collider. This is a new concept for linear accelerators that could drastically reduce electricity consumption compared with machines of similar power.

“There is no reason in the world to build a circular Higgs factory” as opposed to a linear one, says Abramowicz, pointing in particular to its expected high electricity bill. And some researchers suggest that it would be better to explore a number of options than to lock future generations of scientists into an expensive path to 2070 and beyond, when it’s unclear whether the FCC would be the right tool for answering physicists’ questions. “I would find it very unfair to impose a physics programme on my grandchildren,” says Jochen Schieck, a physicist at the Austrian Academy of Sciences in Vienna, who is a member of the CERN Council.

For many physicists, one persuasive argument for the FCC is that it can continue to support the large community of 15,000 researchers and support staff that has grown around the LHC experiments. That, says Abramowicz, is the real reason why many are behind the circular collider idea: it could produce collisions at four independent ‘interaction points’, each with a massive detector producing data that could involve a collaboration of thousands of physicists. A linear collider can conduct only one experiment at a time, so it would support fewer physicists.

Reach higher energies sooner

The thought that the giant proton collider wouldn’t be ready until 2070 also worries some researchers, because it means they won’t see the new energy frontier in their working lifetimes. Some say that CERN should make an all-out effort in research and development for advanced accelerator technologies that could enable facilities to reach higher energies sooner. This would include the magnet research necessary for the FCC, but would also take in new — but unproven — ideas, such as colliding beams of muons, particles that are heavier cousins of electrons.

New CERN chief pledges to forge ahead with $17-billion supercollider

Some researchers, including John Womersley, a former chief executive of the UK Science and Technology Facilities Council, and Tulika Bose, an LHC physicist at the University of Wisconsin–Madison, want to see higher-energy machines developed as quickly as possible.

Womersley has suggested cutting short the LHC’s running time, to 2035, and using the allocated funding to develop technologies for the FCC’s second stage. Bose suggests skipping the Higgs factory altogether.

A spokesperson for CERN says that the upcoming data from the upgraded LHC will already give early-career researchers “a fantastic, exciting and instructive position to be in”, and that if all goes according to plan, there will be only a few years between the conclusion of that programme and the start of an electron–positron collider in the mid-2040s.

How CERN pushed forward its plan

A criticism of the current FCC plan is that CERN didn’t listen sufficiently to the community before formulating it, and that the financial and human resources it has put into the feasibility study have dwarfed investment in other programmes, such as advanced accelerator research.

Some of the disagreement is about how to read a pivotal document released in 2020 after a symposium in Bad Honnef, Germany (see go.nature.com/4hrjmqp). Held by a working group appointed by the CERN Council and chaired by Abramowicz, its aim was to update the strategy for European particle physics and CERN’s future. At that meeting, researchers who were present say, a representative from Germany’s government privately told physicists (including Gianotti) that Germany couldn’t afford to contribute to a massive new accelerator — views that would become public in 2024.

What emerged in the document, some say, was an unclear compromise between those who wanted endorsement of a two-stage FCC plan and alternative scenarios. The document listed a Higgs factory as ‘highest priority’ (without ruling out a linear collider), and then stated but didn’t rank other priorities. These included investigating the feasibility of a future hadron collider at CERN with the possibility of a Higgs factory as a first stage, and ramping up efforts to develop technologies for future accelerators.

US particle physicists want to build a muon collider — Europe should pitch in

Some researchers who took part in the strategy process, including Schieck and Siegfried Bethke, a physicist at the Max Planck Institute for Physics in Garching, Germany, who is a former member of the CERN Council, say that this document was carefully written to leave the door open for alternative Higgs factory designs and to avoid making a two-stage FCC the top priority — calling only for its feasibility to be investigated. It did not back the precise option that CERN’s leadership has pursued, the two-stage plan that reaches fruition as far away as 2070. CERN could have put more effort into exploring the linear collider option and more resources into advanced accelerator technologies, they say.