In physics, no matter how much theorizing we do — or how precise our calculations and predictions become — there’s one overarching factor that determines whether we make progress or not: the amount and quality of relevant data, experimentally and/or observationally, that we acquire. Theoretically, our Standard Model of reality includes quantum field theory for electromagnetism and the nuclear forces, plus general relativity for gravity as our laws, with the Standard Model of particles plus dark matter and dark energy representing the energy content of our Universe. This is supported by an enormous suite of experimental and observational evidence, but it cannot represent all that’s out there in the Universe.

Theoretically, there are an enormous number of puzzles that remain unsolved within our standard picture. They include:

• the nature of dark matter and dark energy,
• the absence of CP-violation in the strong interactions,
• the insufficient amount of weak CP-violation to explain the observed matter-antimatter asymmetry,
• the nature (and details) of the electroweak phase transition,
• the existence (or non-existence) of new particles at the electroweak scale,
• and just how precisely the particles we know of obey the Standard Model’s predictions, versus whether, where, and by how much they depart from them.

Throughout the 20th and 21st centuries, progress in particle physics has relied on the ability to collect new experimental data at the frontiers, and with the Large Hadron Collider (LHC) at CERN nearing its end-of-life, the creation of a future collider has been anything but a certainty.

However, after two years of work, the CERN Council has just updated the European Strategy for Particle Physics: recommending the construction of a new flagship project, the Future Circular Collider (FCC), an all-new electron-positron collider. Here’s why, of all the options, it’s the best decision for physics, for humanity, and for civilization here in the 21st century.

A series of infrastructure upgrades, some of which have already taken place and others which are still to come later this decade, will transform the LHC into the HL-LHC: the high luminosity LHC. It will be capable of collecting superior amounts of high-quality data, each year, as was taken over the LHC’s first decade of life, from 2008-2018. However, to learn more about the Universe at a fundamental level, a machine fundamentally capable of probing what the HL-LHC cannot will need to be built: a Higgs factory of some variety.

Whenever we build a new experiment, mission, or apparatus, there’s one “metric” we use to evaluate its merits above all others: the amount of discovery potential that it brings along with it. In astronomy and astrophysics, that corresponds to things like the amount of light you can gather, the resolution you can obtain, and the range of wavelength coverage you can make use of. In particle physics, that corresponds to the key/relevant events you generate at the energies you can achieve in your collider, leading to a number of those events you can expect to observe, as well as the precision to which you can measure the outcomes of those events in your detectors.

At any moment in physics history, we have our current understanding of the Universe: theories that enable us to make predictions for what we expect to see — assuming that our current understanding holds — beyond the limits of where we’ve already looked. That sets our expectations for the minimum of what we should find when the new machine, mission, or apparatus is complete: things like the number or faintness or redshifts of new galaxies in astronomy and astrophysics, or the number of exotic particle events of a particular species for a new collider.

It was considerations like this that led to Fermilab’s discovery of the bottom quark in the late 1970s, the Large Electron-Positron (LEP) collider’s precision measurements of the W-and-Z bosons in the late 1980s, Fermilab’s upgraded TeVatron’s discovery of the top quark in the mid-1990s, and the LHC’s discovery of the Higgs boson in the late 2000s and early 2010s. We knew what we were seeking when we built these machines, and they delivered.

This to-scale diagram shows the relative masses of the quarks and leptons, with neutrinos being the lightest particles and the top quark being the heaviest. No explanation, within the Standard Model alone, can account for these mass values. We now know that neutrinos can be no more massive than 0.45 eV/c² apiece, meaning that the difference between a neutrino’s mass and an electron’s mass is more than three times as large as the difference between the electron’s mass and the top quark’s mass. The very heavy W boson, Z boson, and Higgs boson are not shown, but would range between 80 and 126 GeV on this chart: slightly smaller than the top quark.

Credit: Luis Álvarez-Gaumé/CERN Latin American School of HEP, 2019

In another sense, however, they didn’t deliver. Sure: they were able to uncover novel particles — as well as to measure previously unmeasured properties about those particles — that helped us complete and better-understand the Standard Model of elementary particles. The weak gauge bosons, W-and-Z, were only discovered in the 1980s. The top quark’s discovery was only announced in 1995. Neutrino oscillations, and the determination that neutrinos had mass, occurred in 1998 at Super-Kamiokande and independently in 2001 at the Sudbury Neutrino Observatory. And the final particle in the Standard Model, the Higgs boson, was definitively discovered in 2012.

But there was no evidence for anything exotic or unexpected; there were no serendipitous discoveries that would point the way to new physics. In particular, there were no:

• hints of grand unified theories appearing in the data,
• evidence for supersymmetric particles at electroweak scales,
• signals indicating a composite Higgs boson or multiple Higgs bosons,
• accelerator signals hinting at the creation of dark matter,
• measurements supporting the existence of flavor-changing neutral currents,
• or observed departures from the Standard Model’s predicted branching/decay ratios.

These are signals that accelerator physicists knew to look for: signals that would imply the existence of physics beyond the Standard Model. Yet even here in 2026, a full 18 years since the LHC first began colliding protons, we have no good evidence for any of these exotic possibilities.

The observed Higgs decay channels vs. the Standard Model agreement, with the full suite of Run 1 data from ATLAS and CMS included. The agreement is astounding, and yet frustrating at the same time, as no evidence for either a second Higgs boson or for a non-Standard Model Higgs boson has yet arisen.

To many people — and this is both disappointing and unfortunate — it’s evidence that we should give up, stop building colliders, and stop interrogating nature any further. After all, they reason, if all we’ve been seeing is particles and behavior consistent with the Standard Model alone, there’s no reason to think that a future collider of any type is going to reveal something novel, unexpected, and groundbreaking. All we can expect, they’ll argue, is to continue to measure what’s predicted by the Standard Model to better and better precision: an enormous waste of money and resources that could be better spent elsewhere.

That perspective, although common, is defeatist in the worst way: it gives up not only before we even make the attempt, but before we give nature the opportunity to show us how it truly behaves. To understand why, we need to go back in history to a mostly forgotten experiment that began in the 1980s.

LEP, the Large Electron-Positron collider, was the flagship machine at CERN from the 1980s through the early 2000s: before it was removed and the LHC was installed in its place. Instead of colliding protons with either protons or antiprotons, it collided electrons with positrons: a much lower-energy process, owing to the limitations of synchrotron radiation, but a much cleaner one. Protons (and antiprotons) are composite particles, and when you collide them:

• only a fraction of the overall energy goes into the collision itself,
• and dozens or even hundreds of “extra particles” are produced during the collision, creating messy signals that can only be reconstructed with significant efforts and often with some ambiguities remaining.

But with electrons and positrons, the signal is clean, and up to 100% of the collision energy can go into the creation of new particles.

The production of matter/antimatter pairs (left) from two photons, or from any colliding pairs of matter and antimatter, is a completely reversible reaction (right), with matter/antimatter annihilating back to two photons in the majority of cases. This creation-and-annihilation process, which obeys E = mc², is the only known way to create and destroy matter or antimatter. This process is incredibly effective at electron-positron colliders, producing clean creation signatures of exotic particles.

Credit: Dmitri Pogosyan/University of Alberta

LEP didn’t discover the W-and-Z bosons; they had been discovered in a hadron (proton-antiproton) collider known as the Super Proton Synchrotron, also at CERN, earlier: back in 1983. However, discovering a particle isn’t the only interesting thing you can do at an accelerator-collider facility. You can also:

• measure the particle’s properties, such as spin, mass, and charge,
• measure the particle’s branching ratios, where you create large numbers of them and observe the ratios at which they decay into other particles,
• measure the mean lifetime of the particle as well as its width, which corresponds to the uncertainty inherent in its mass,
• and, if you create them in large enough numbers, measure the effects of radiative corrections, which both probe intricate features embedded within the Standard Model while revealing or ruling out various beyond-the-Standard-Model effects and contributions.

LEP’s big success was in creating enormous numbers of W-and-Z bosons, as its nature as an electron-positron collider allowed scientists to “tune” the accelerator to specific energies, maximizing the production rate of these rare particles. Many of the world’s best current limits on these particles still come from LEP-era science, all because, in all the time since, we haven’t had another electron-positron collider machine that has matched LEP’s capabilities to produce W-and-Z bosons and measure their decay outcomes.

This animation shows the reconstructed mass from Higgs candidate events over time, in the decay channel where the Higgs boson decays to two photons, in the ATLAS detector at CERN’s Large Hadron Collider. By mid-2012, the data was sufficient to reveal the presence and properties of the Higgs boson to 5-sigma significance in both the ATLAS and CMS detectors, leading to the official discovery announcement in July of 2012.

In the time since LEP was decommissioned, we’ve come a lot further in particle physics. We’ve discovered the Higgs boson and measured its mass, spin, and charge, and the LHC has produced so many of them that we’ve been able to measure its decay channels (and its branching ratios) down to precisions of about 1%. We’ve also produced more top quarks and additional W-and-Z bosons, probing physics at a deep level in many ways. But the LHC, which is a proton-proton collider, is fundamentally limited in the types of precision experiments it can perform, and in the types of constraints that can be placed on the existing particles from its data. Even with future runs still to come, it will only be able to ultimately measure the Higgs’s properties down to precisions of about 0.2% in total.

What LEP was able to do, back in the 1980s, 1990s, and early 2000s, was to discover what scientists call radiative corrections to the W-and-Z bosons: contributions of higher loop-order terms, even if there isn’t enough energy/mass to produce the particles in those loops (i.e., virtual particle contributions), that play a role in determining the types and frequencies of W-and-Z boson decays. Whereas SPS and Fermilab couldn’t identify even 1st-order corrections from their data, LEP data enabled the measurement of 1st, 2nd, 3rd, 4th, and even 5th loop-order corrections.

For precision data, which is where the contributions of beyond the Standard Model physics would first appear, electron-positron colliders are the ultimate frontier.

The cross-section for producing a Higgs boson (plus other particles) through various production channels at a lepton-antilepton collider, with an electron-positron collider being the leading option. The enormous yellow peak corresponds to producing a Z boson and a Higgs boson together: the most prolific production channel, which peaks at 216.3 GeV of energy: the Higgs boson mass plus the Z boson mass.

Another way of looking at it is that hadron colliders are the engines of discovery: the W-and-Z bosons, top quark, and Higgs bosons were all discovered at hadron colliders. But electron-positron (lepton) colliders are how you precisely examine what you know is there once you’ve already found it. That requires:

• producing enormous numbers of the particle in question,
• with the detector positioned around the collision point,
• in the center-of-momentum (center-of-mass) frame of the collision,
• with the capability of detecting the mass, charge, momentum, and trajectory of each daughter particle (i.e., secondarily produced particle) created from the initial decay.

You want this in as clean of an environment as possible, with as few stray or pollutant particles influencing your detection data, as you can muster.

The four options under consideration by CERN, for its future, were as follows.

1. We could dig a new, larger tunnel, somewhere between 80-100 km long, to build the largest electron-positron collider ever constructed: the Future Circular Collider.
2. We could build a new linear collider, where you don’t have synchrotron radiation but have to instead accelerate your particles to high energies along a linear track in one shot, to collide electrons and positrons.
3. We could rip out the LHC, reinstall LEP, and increase the energy and strength of the bending magnets past the old maximum: an idea floated as LEP3.
4. Or we could not build a collider at all, and abdicate our ambitions to probe nature at a deeper level than the LHC already has.

After two years of analysis, the CERN Council decided on a 91-km circumference incarnation of the first option: the best of them all.

Before there was the LHC, the same tunnel that now houses it was built back in the 1980s to accommodate LEP: the Large Electron-Positron Collider. Operating for more than a decade, it sought to find either the Higgs or the top quark, but couldn’t reach the required energies. With a push to just a few extra percent over LEP’s maximum energy, the proposed LEP3 would be our fastest, cheapest path to a Higgs factory, but would offer less energy reach, would produce fewer overall events, and would offer less long-term value than a future circular collider.

Credit: CERN

Your energy, in an electron-positron machine, determines what you can create. If you can reach the energy of a Z boson (91.2 GeV), you can produce it. If you can double the energy of a W boson, you can produce W+/W- pairs: about 161 GeV of energy is needed. If you can reach the energy of a Higgs boson and Z boson together (about 216.3 GeV), you can begin producing large numbers of Higgs bosons. And if you can reach double the mass of a top quark (around 347 GeV), you can produce large numbers of top-antitop pairs. (LEP, originally, topped out at 209 GeV in energy, illustrating why it was excellent for studying W-and-Z bosons, but couldn’t discover the Higgs.)

For electrons and positrons, modern accelerator technology can gain them about 30 million electron-Volts of energy per meter. Linear colliders don’t have synchrotron radiation (the radiation that results when you accelerate a charged particle by bending it with a magnetic field), but to reach 347 GeV of energy, you’d need an accelerator that was over 11 kilometers long: more than twice as long as each of LIGO’s gravitational wave arms, filled with accelerating cavities and with only one collision point.

LEP3 could only reach the 216.3 GeV threshold, and would produce the fewest number of Higgs events.

But with the large circular collider option, not only would you reach all of the energies needed with multiple collision points, you’d also have the potential for upgradeability: potentially to a hadron collider that could be many times as powerful as the current LHC is.

Deep underground, this tunnel is part of interior workings of the Large Hadron Collider (LHC), where protons pass each other at 299,792,455 m/s while circulating in opposite directions: just 3 m/s shy of the speed of light. Particle accelerators like the LHC consist of sections of accelerating cavities, where electric fields are applied to speed up the particles inside, as well as ring-bending portions, where magnetic fields are applied to direct the fast-moving particles toward either the next accelerating cavity or a collision point. In a linear collider, there is no bending, but each particle only gets boosted once by each meter of accelerating cavity.

It’s important to recognize that we haven’t just found the Higgs boson, we’ve also begun to study it. We’ve determined that it is a spin-0 particle, the only one in the Standard Model with no electric charge and no ability to interact under the strong force. Its parity was measured: in agreement with the Standard Model’s predictions. Its production and decays confirmed that elementary particles acquire mass via the Higgs field. The Higgs’s decays reveal its couplings to bottom quarks, top quarks, charm quarks, and muons: again, consistent with the Standard Model.

But open questions remain. At what levels do Higgs bosons lead to CP-violation in lepton decays? Does the Higgs boson self-interact, and what can we learn about the Higgs field from studying the Higgs boson? Are there any new particles out there, and if so, do they couple to the Higgs? Can the Higgs decay to dark matter particles itself?

With a 91 kilometer radius circular collider, we can go all the way up to 365 GeV in energy, produce millions of Higgs bosons for study, and unprecedentedly large numbers of W bosons, Z bosons, and top quarks as well. The LHC is planned to end its life in the mid-2030s, and with the current plan outlined by the CERN council, the new circular collider, known as FCC-ee (for Future Circular Collider doing electron-positron collisions), could begin science operations as early as the 2040s.

The Future Circular Collider (in blue) would overlap slightly with the current Large Hadron Collider, but requires an additional ring (and tunnel) chosen to be approximately 91 km in circumference: dwarfing the LHC’s current 27 km circumference. Bigger tunnels and stronger magnets are needed for a more energetic hadron collider, with the proton-proton version of the FCC proposing ~16 T magnets, approximately double the LHC’s current magnet strength. However, an electron-positron version would come first: as soon as the 2040s.

The beautiful thing about scientific discovery is that the Universe has always been there, ready to reveal even its most deeply held secrets, as soon as anyone interrogates it in the right way: a way that compels it to reveal its answers to whatever physical questions we know how to pose. Even though the United States has been abandoning many lines of frontier-driven scientific research:

• a space-based gravitational wave observatory (LISA is under the auspices of the European Space Agency),
• the energy frontier of particle physics (Fermilab’s TeVatron permanently shut down in 2011),
• space-based CMB research (NASA’s COBE and WMAP were succeeded by ESA’s Planck, and will be further surpassed by Japan’s LiteBIRD),

fundamental science nevertheless continues for humanity and human civilization.

And that’s as it should be. Science is not solely under the purview of one nation, and it is not merely for the benefits of those who uncover its secrets. Rather, this is central to the endeavor of human knowledge, as anyone with the right resources and opportunities can not only contribute to it, but potentially discover the next great breakthrough that opens the door to what’s presently thought to be impossible. The key is to invest in performing the relevant searches for ourselves. No matter what the outcome is, we will have learned and gained so much. The only certainty is that if we fail to look, we’re guaranteed to discover nothing new at all.

This article CERN to lead particle physics throughout the 21st century is featured on Big Think.

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