Future Circular Collider: Top Approach

Interview with Marumi Kado on the future of particle physics and the origin of everything

For many people, particle physics is intangible and abstract. The best-known particle is probably the Higgs particle, which is often referred to as the God particle. In 2012, it was lured out of hiding by researchers at CERN's Large Hadron Collider. What has happened since then? Marumi Kado explains in simple terms what particle physics is really about, where it stands today in its mission to understand the world, and which big questions humanity still has to answer. Could a future ring-shaped machine with a circumference of 100 kilometres help to solve the mystery?

Interview: Tobias Beuchert

People have little connection to particle physics in their everyday lives. Why should they be interested in it?

Modern physics has changed everything. We started to understand the universe as a whole and how the infinitely large scales connect to the infinitely small. But should people care? Should they know more about all that? This is a philosophical question, way beyond my competence. I don't have a good answer! Also, at the moment there are other issues that impact everyday life way more than particle physics.

But isn't the general public caring already? When we discovered the Higgs particle, I went to the announcing conference in Melbourne. This "god particle" was all over the news. Everybody was talking about it, even an officer at the airport asked me about it. Every time, I give public talks, I feel a lot of enthusiasm.

And, because things are so fascinating, people have questions. Particle physics can be abstruse, abstract and hard to grasp. I am happy that we have science communicators, who have the expertise to translate between scientists and the public. And scientists should definitely give people the opportunity to take part in their great scientific achievements.

Could you give us a brief introduction to your work and modern particle physics?

There is a lot we know already. We are made of atoms, atoms are made of electrons and nuclei. Nuclei are made of protons and neutrons, which are in turn made of quarks and gluons. To investigate nature at these smallest scales, you need to concentrate a lot of energy and rip apart matter into its smallest ingredients.

What we do, can be compared to what happened in the universe shortly after the Big Bang. It was extremely hot, energetic and only filled with particles. We do that by accelerating protons inside a 27 kilometer long circular tube and let them collide from two exact opposite directions at the Large Hadron Collider (LHC) at CERN, which is the European Organization for Nuclear Research. Their kinetic energy of a proton beam is comparable to that of an ICE train traveling at approximately 150 kilometers per hour. The beams are made of 3000 bunches of protons, each bunch filled with 100 billion protons, driving towards each other at nearly the speed of light. Losing the beam in the machine would be a complete disaster.

That way, we reach collision energies of nearly 14 Teraelectronvolts. At these energies the ingredients of protons, quarks and gluons, collide, which results in an explosion of all kind of particles into all directions. Some are generated just in that moment and only exist at these high energies.

But I guess people have onl heard of one famous particle, right?

The Higgs! This discovery is outstanding for many reasons. But we have discovered close to a hundred other new particles at the LHC like tetaquarks, pentaquarks and other exotic particles, all extremely important to understand the fundamental forces of nature. All these particles, however, did not make the headlines. Particle physics not just about discovering new particles. It goes way deeper!

Okay, this you have to explain more.

Particles are only the tip of the iceberg. Arguably an important one, because it gives us a direct access to the fundamental paradigms of physical laws. What we are interested in are the so-called fields, out of which particles are generated. Imagine the water surface of a lake. It is like smashing into the calm surface of a lake - the splashing waterdrops are like the Higgs particles, excitations of the Higgs field. And by finding the Higgs particle, we found evidence for the Higgs field. The properties of these fields are the unseen bottom of the iceberg so to say. One field that many know is the electromagnetic field, the corresponding particles are the photons, so light essentially.

How does that relate to the Higgs particle? What is so special about it anyway?

The Higgs field is there everywhere all the time, but the Higgs particle you can only find at very high energies at the LHC. The Higgs field is very special and different from other known fundamental fields: when it interacts with other particles, it gives particles their mass. And because it supposedly permeates the entire universe, it connects the largest known distance scales, about 100 Billion light years, with the smallest scales down to 10^-19 meters.

In the end, the LHC, a collider ring, 175 meters below the ground, worth several billion Euros, was built to look for the Higgs particle?

Yes, and much more. The Higgs was a major and one could say expected discovery. But of course, you only know for sure, when you see it. Back then, when the LHC was not yet approved, a justification for the LHC was the so-called No-Lose Theorem: either we find the Higgs and with it new physics, or we find something different. This "theorem" was based on robust arguments. It is the strength of the theory behind the particle zoo, the so-called Standard Model, that the existence of the Higgs could be predicted. But we had to enter an energy regime that was completely unexplored, where we didn't know what nature really had to offer.

"Seeing nothing else" while having systematically searched for any possible deviations from the Standard Model of particle physics has resulted in a shift in paradigm.

The Standard Model is the model that describes the particle nature of matter and the fundamental forces best at the moment - except of gravity, which is explained by general relativity.

What are the biggest unsolved Questions?

It is absolutely fascinating that when you look around, the table, the trees, the walls! These are made of molecules, which are composed of atoms. The various atoms only differ by the number of electrons, protons and neutrons in their nuclei. But each atom is composed of exactly the same ingredients: protons, neutrons and electrons. Everything seems to be made of just a small number of identical particles! This is what we call reductionism. Why are things simpler at smaller scales? The more extreme we go with particle accelerators, the smaller the distance scales we explore by ripping apart matter into its ingredients, the more simplicity we see. Why? Does nature follow this trend towards even higher energies and smaller distance scales that we haven't been able to probe with the LHC yet? Or will nature, in contrast, behave more complex?

And there are other pressing puzzles in our understanding of fundamental physics: why are the mass of the Higgs particle or the cosmological constant that drives the acceleration of the universe so much smaller than what would be expected? The cosmological constant would more naturally be up to 120 orders of magnitudes larger than what is observed. What is the nature of the Higgs field anyway? What is dark matter?

Yes, what is dark matter?

Noone knows. Is it a fundamental field like the Higgs field with an associated particle? At LHC, we are searching for it. But maybe its mass is too large for the LHC to be able to measure it. Colleagues here at the Max Planck Institute for Physics also work on other explanations such as primordial black holes, a large number of small black holes distributed in space. Solving the puzzle of dark matter will require a very broad range of experiments, well beyond colliders.

What else?

Another unsolved question is, how to explain the tiny bit of matter exceeding antimatter in the early universe, which lead to our existence.

Which explains, why our world is made of matter without any sign for regions of antimatter in the universe?

Yes: In the early Universe, particles and antiparticles existed at nearly identical amounts. Luckily for us, there was just a tiny bit more matter than antimatter, just 1 out of a billion more. So when matter and antimatter annihilated or canceled each other out, only matter was left - enough to create what we see around us today. We don't know why that was and the Higgs could play a role in the answer.

And then there is THE open question, the search for the "theory of everything", right?

Indeed. How can gravity be described at microscopic quantum scales, how can the fundamental forces that are contained in the Standard Model be unified with the theory of gravity? To paraphrase a prominent theorist "with the discovery of the Higgs it is the first time in history that we have a theory that can be extrapolated to exponentially high energies". So in some sense it is the closest we have ever been to a complete theory. But we also know that it cannot be the full story at much larger energy scales, when gravity becomes much stronger. How to describe gravity at much smaller distance scales is perhaps the biggest of the big questions in fundamental physics. But it is not the only one.

Can the LHC still offer answers to these open questions?

Definitely yes! Of course, now that nearly the highest possible collision energy has been reached, a rapid new discovery is very unlikely. But there is still room for discoveries, which will take time and much more data. When you collide protons, it's not only about the energy of the collision, but statistics matter. Having more collisions and more data will allow us to improve the precision of measurements of the particles inside the Standard Model. To do so, we will upgrade the LHC in the coming years. All this will contribute to make progress on answering the fundamental questions.

This is by far the highest priority in my department at the Max-Planck-Institute for Physics. The ATLAS detector at LHC is a detector around the region where protons collide, as large as several buildings. It has different layers detecting different types of particles that are created in collisions. We are working on almost all layers of the detector. And we are working on the data acquisition and analysis. We also use AI algorithms to reconstruct and identify such particles.

What is the true benefit of AI in particle physics?

AI is clearly extremely fashionable and I use it a lot, but it has not happened that something new was discovered because we used AI, something that would not have been discovered otherwise yet. Some examples might come in the future. Our safeguard is to use AI only in controlled environments, for example using trustable control samples.

What will come after the LHC?

We still have the LHC and I am in awe what the machine is able to produce. It is our duty to extract as much as possible from it. To make decisive progress on the still open questions mentioned above, we will likely have to go to higher energies and excite any yet unknown fields that may be hidden out there.

The Future Circular Collider (FCC) is currently the best approach. It would reach 100 Teraelectronvolts, an energy seven-times higher than what the LHC can do. Instead of 27 kilometers, the FCC-collider-ring would have a circumference of nearly 100 kilometers at which it has to maintain extreme precision of high energy beams. It is a major challenge! But this project is technically feasible as a study from October 2025 found.

Could there be a revolution just like at LHC?

In fact, making a prediction like the Higgs particle is extremely difficult. But there could be a revolution in many other ways. The first would be to find something completely new and unexpected. Something that when seen would tell us what other pieces are missing to solve our outstanding puzzles. The more precise measurements of the FCC could find deviations from the Standard Model of particle physics. Maybe we even find something that was already predicted such as supersymmetry, an extension to the Standard Model of particle physics that could also explain dark matter.

What if you see nothing else?

What if Magellan had not found the passage? We would know that no passage exists up to certain latitudes. Maybe we will only discover a desert at higher energies, this would be also extremely valuable and confirm the need for a change in paradigm. Certainly Magellan and history teaches us that great discoveries do not just happen by serendipity they also need perseverance.

So is the Future Circular Collider still worthwhile, even if there is no crystal-clear prediction, as there was with the Higgs at the LHC?

Of course! The pursuit of a great scientific goal is undoubtedly worth it from both a scientific and philosophical point of view. And it would be a great benefit to society as well, the LHC has demonstrated this through the innumerable byproducts for society.

The Internet, or World Wide Web, was only invented in 1989 at CERN. The original idea was to enable researchers around the world to exchange information automatically.

We also should not be scared of the timescales, the LHC including the large electron-positron collider is a program that took 70 years. The FCC would take just as long. Of course many of us would not see the results of the FCC. But I am incredibly grateful that others have dedicated their lives to make the LHC happen and I hope the new generation will have a machine that they can work with.

The main issue is financial feasibility, this machine is expensive. But we have an amazing asset in Europe: CERN. It is extremely well organized and is leading the field worldwide. It enables collaborations at intergovernmental level and is securing funding for such large projects.

Are you worried that the FCC could not be realized?

I am worried, but I also see good reasons to be optimistic. Nothing is granted and the financial effort is substantial. That said, the current CERN management has been extreme active and creative in finding suitable funding schemes. Such a formidable flagship project would ensure Europe's leadership in the field.

In the end, science is political…

Everything that matters to humankind is political, yes. Politicians have an incredibly difficult job to tell what the interest of society in the future should be, while there are also other pressing topics. I am not in the position to decide. If I were, I would support any research infrastructure of that kind.

Marumi Kado is a Director at the Max Planck Institute for Physics