Mark Thomson, physicist: ‘We are going to open a new window onto the universe that we haven’t had before’

Election season has come to a close at Europe’s CERN, the largest particle physics laboratory in the world. On Wednesday, representatives from its 24 member states selected British physicist Mark Thomson as its new director-general, entrusted with leading the institution into its next chapter. The gigantic facility employs 2,500 workers and has a network of nearly 20,000 collaborators around the world. Its top priority is to finalize plans to build the biggest particle accelerator on the planet, a circular superconducting proton accelerator ring measuring 62 miles across. With it, humanity will attempt to enter unknown territory.

Thomson, 58, was the first to publicly announce his candidacy to replace Italian former director-general Fabiola Gianotti in January 2026. His two competitors were also men: Paris Sphicas, a veteran Greek CERN researcher, and Robbert Dijkgraaf, a theoretical physicist and former minister of education, culture and science in the Dutch government.

Thomson, the son of a shopkeeper father and housewife mother, says that when he was 13, he was already reading books on particle physics and yearning to know about “how the universe works” in greater depth. His search for the most essential components of matter led him to become a professor of particle physics at the University of Cambridge and to work in the world’s great temples of the discipline: the Large Hadron Collider in Geneva, at the headquarters of CERN, and the gigantic underground neutrino detector DUNE, which is being built 4,500 feet underneath the earth’s surface in the United States. Currently, the Brit is the executive chair of the Science and Technology Facilities Council in the United Kingdom, a vast organization of thousands of scientists with a budget of over $1 billion, comparable to that of CERN.

In this interview with EL PAÍS that took place during Thomson’s brief visit to the British embassy in Madrid — before his appointment — he defends the importance of Europe continuing to be a world leader in the field over the next 70 years, despite recent gains made by China. To achieve that, he warns, Europe will have to seek out accords that would allow it to carry out one of the largest engineering projects in history, which will cost $16 billion and take more than 15 years to build.

Question. What is the big question that CERN is currently trying to answer?

Answer. The first is really understanding what the Higgs boson is. This is the particle we discovered in 2012 at CERN. It’s very strange, completely unique. It’s not just another particle. It doesn’t look like anything we’ve ever seen before. And it plays a really unique role in determining how the universe works. What it does is, if you go into the deepest space, into the vacuum where there’s nothing, the presence of the Higgs boson is always there. The Higgs field, we call it. And if it wasn’t for that Higgs field, then all of the particles that we know would have no mass. The universe would look very … well, we wouldn’t be here. So that’s fascinating. Now we really want to understand how it works, exactly what it is doing. It is possible that there is more than one Higgs bonson and we want to understand its interaction with the rest of the matter that we know with very, very, very high precision. That would provide a new window on the universe that we haven’t had before.

Q. The Higgs acts upon conventional matter. Is the rest of it totally unknown?

A. That is the second question to answer. So about 5% of the universe, in terms of its mass, is the stuff we see, the visible universe. And then another 25% is what we call dark matter. We know it’s out there. We can kind of see its presence in the way it affects galaxies and the way galaxies move, but we really don’t know what it is. So that’s a very large part of the universe that we notice, but that we haven’t seen at all. Then there’s dark energy, which is the remainder, and who knows what that is, I’m not even going to go there.

Q. Three big questions to answer, then.

A. The other question that I find really, really fascinating is that every particle, elementary and fundamental — there aren’t very many of them — like electrons in atoms, they have a heavier copy named muon and an even heavier copy called the tau lepton. We have no idea why there are three copies. They all have different masses, so they interact differently with the Higgs. So, this is really about understanding why there are this particular number of particles in the universe and why they have these very strange patterns of mass. And we don’t really have any real insights at the moment as to why that’s the case. Maybe it’s linked to the Higgs boson, maybe not. That’s called the flavor puzzle. We’ve measured all these things, but we don’t understand the origin of their differences.

Q. You’ve said you love neutrinos, why is that?

A. When I started my PhD back in Oxford in the mid-1980s, I worked on an underground experiment in the United States which was actually looking for proton decay. In proton decay, there is the interaction of these ghostly particles called neutrinos. They’re all over the place, millions of them going through you every second and they almost never do anything. At that time, we were starting to see in some of the experiments hints that neutrinos were doing something we didn’t really understand at that time called neutrino oscillations. I’ve always had huge interest in, what are these particles? Thirty years ago, we thought their mass was zero, because their mass was so small. Now we know they’re not zero, but they’re much smaller than any other particle. So, there’s another puzzle there, why is a neutrino mass so, so light?

Q. There are some physicists who say that without neutrinos, we wouldn’t be here.

A. This is the other thing about neutrinos that is potentially really, really important. This is the focus of the current generation of big neutrino experiements, like DUNE, where I worked years ago. Another reason why humans are here is because we are all made of matter. We believe that at the start of the Big Bang, matter and anti-matter were created equally. What should happen is that they come together and meet each other and then annihilate and turn into light. So, the universe should be a very boring place, no matter the amount of matter left. And what we know has happened somewhere in the history of the universe very early on for every, let’s say, billion anti-matter particles, there were 1,000,000,001 matter particles. This subtle difference is what we see today in the universe. What’s the origin of that? I think our best bet, which may not be true, is something to do with neutrinos. That requires two things: that neutrinos and anti-particles are subtly different, and also that neutrinos are different types of particles, compatible with the other particles and that’s kind of a technical thing.

Q. Where do you want to take CERN over the next five years if you are selected?

A. I wanted to be very open and transparent about the fact that I wanted to be considered for this role and that I have the experience to be able to do the job. CERN is unique in the world and we are very lucky to have it in the heart of Europe. It’s maybe the unique area where Europe is right at the lead of science. We’ve got this unique scientific infrastructure built just over 70 years of deep European collaboration. We really want to track out its future.

Q. What needs to be done to achieve that?

A. We are currently running the Large Hadron Collider, which is the most powerful collider ever built. We’re making it a more powerful machine that’s putting in some more advanced magnet technology near where the experiments are. This will be an incredible opportunity as a discovery machine — you don’t know what you’re going to discover, and you might not discover anything, but that’s the whole nature of discovery. The new LHC will give us a new frontier to see the universe as well as other really exciting measurements, including some really subtle property of the Higgs boson that I think we’ll be able to measure at the high limit LHC. So that sets out CERN’s future until 2040.

Q. And after that?

A. The second challenge for CERN is, what’s the long-term future of the Sun? These big projects take 20 years or more from start to actual operation. Now is the time to start to take that decision of what comes next. At the moment, the prime candidate for what comes next is something called the Future Circular Collider. Hopefully they will drop the “future” at some point.

Q. What kind of accelerator will it be?

A. The Large Hadron Collider is a 17-mile ring. This will be a 57-mile ring, which enables us to accelerate particles to higher and higher energies. The initial phase of this machine will collide electrons and anti-electrons, and it will essentially be a factory for making Higgs bosons. This is the place where we can study this unique, completely new type of matter and really understand it. The challenge, of course, of these big projects is not just time but cost, which is estimated at around $16 million. That will require all of the member states of CERN to come together and have a real consensus behind the machine. And that would be my prime goal, parallel with deliver the LHC. It sounds like a very large amount of money, but it will be shared among the 25 member states over a period of 15 years or so. The way you make breakthroughs is you have to do stuff that’s never been done before. And building this accelerator will push forward technology, which will be an engine for the economy and innovation.

Q. Do you think that the leadership of Europe particle physics is in peril?

A. China has some ambitions to build something quite similar to the plans that we’re putting forward at CERN. I would say that we have the advantage of 70 years of heritage of building accelerators and turning them into colliders. It’s absolutely critical to success. I don’t think we should underestimate the difficulty of starting from — I won’t say starting from scratch, because technology in China is very strong, but actually it’s very hard to reproduce that experience.

Q. The LHC consumes as much power as a small city. The accelerator would require much more electricity and to excavate an underground tunnel between France and Switzerland. Can a project like that be sustainable?

A. This is a really important question, and it depends on the perspective you come at it from. A lot of the power comes from the French grid, and a lot of that is nuclear. So, in one sense, depending on your perspective on nuclear, in terms of carbon impact, it’s not that high. The thing that probably drives the carbon footprint of the huge circular collider is the construction, the concrete, the excavation. You can look at that as a negative or huge opportunity for scientists and engineers at CERN to say what we can do to make it more sustainable.

Q. Do you feel that other European countries will look at your candidacy from a different point of view because of Brexit?

A. I hope it won’t come into it. The United Kingdom is the second-largest contributor to CERN’s budget. The leadership of this laboratory is really very critical and it should be about the person. It’s not a role in which you’re representing your nation, you’re representing CERN and the member states of CERN.

Q. Has leaving the European Union hurt science in the United Kingdom?

A. Yes. Now that we’re back in Horizon Europe [the European Union scientific funding program], I think we’re starting to rebuild those relationships, but there’s no question they were damaged and that has not been good.

Q. CERN has decided to end its collaboration with Russia and Belarus due to the invasion of Ukraine. Would you support taking similar action against Israel, a member country, given its invasion of Lebanon and military operations in Gaza?

A. That’s a very political question and it really comes down to the member states. Scientific collaboration is really important. We saw the value of those links with Russian scientists. But the invasion of Ukraine was an act of aggression against one of the associate members of CERN. I think the CERN Council took a very difficult decision.

Q. Will you feel comfortable as the director-general of an organization like CERN, taking into account Israel’s membership?

A. The prime directive of CERN is science for peace. I think the director-general supports that particular direction, then it’s really up to the members to decide. It’s not something I would be uncomfortable with, because I really believe in the ethos of scientific collaboration.

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