CERN’s new chief on the gamble that could fix our picture of reality

When Mark Thomson was 13, he read a book about the European Organization for Nuclear Research, better known as CERN, a particle physics lab whose remit was to interrogate the fabric of reality. The book left him both fascinated by how the universe worked and frustrated by its lack of detail. More than 40 years later, Thomson is CERN’s director general, taking charge just as it shuts down the Large Hadron Collider (LHC) for upgrades and decides where to place its next multibillion-pound bet.

The goal of this gamble is to answer big, lingering questions we still have. In a sense, particle physics hasn’t changed since Thomson was a boy: it is dazzling in its outline, but maddening in the details it cannot yet supply. The field’s crown jewel, the standard model, describes the particles and forces that make up the visible universe with extraordinary precision. And in 2012, the discovery of the Higgs boson seemed to be the masterstroke that completed its picture of reality. But for all its success, the standard model says nothing about dark matter, an invisible substance thought to make up most of the cosmos, and it offers no deeper explanation for the masses of the particles it catalogues. It also cannot account for why the universe contains matter at all after the big bang.

With the LHC set to undergo major upgrades that will sharpen its search for rare phenomena, Thomson spoke to New Scientist reporter Alex Wilkins at CERN in Geneva, Switzerland, about what answers the LHC may still yield, and why its physicists are going all in on a £13 billion collider as its successor.

Alex Wilkins: How much has changed since you first read about CERN as a teenager?

Mark Thomson: When I first read about CERN, we had three main fundamental forces, plus gravity. We knew about electromagnetism and knew about the particle that conveyed it [known as the photon], but we had never seen the particles associated with the weak force [known as the W and Z bosons]. Those were discovered at CERN in 1983. We also didn’t know that fundamental particles called neutrinos had mass. Just over 25 years ago, we thought these particles were massless. And the real massive discovery, obviously, was the discovery of the Higgs boson in 2012.

The Higgs boson is radically different to any other particle we know. It has no spin and no electric charge, only mass, which potentially connects it to several outstanding issues within the standard model of particle physics. It is one of a kind, at least as far as we know. It also has the very strange property that, in some sense, the quantum field associated with the Higgs boson is present everywhere in the universe. It is this property that gives all other particles their masses. Without the Higgs field, all known particles would be massless. Consequently, the Higgs field determines many properties of the universe, for example, the mass of the electron and, consequently, the size of atoms. There are also deep questions about the nature of the Higgs boson, such as whether it is a fundamental particle and is unique, or if there are other Higgs bosons.

How much is left to find?

We’ve definitely not found everything. If you take a look back at the really big, game-changing discoveries – like neutrino mass, the Higgs boson, the discovery of gravitational waves, the discovery of dark energy – these things come along every five to 10 years. You don’t get these game-changing discoveries all the time, and you shouldn’t expect to. We’re now at a point in time where we understand the universe really well, but we also understand there are so many questions that we don’t understand, but we can start to answer.

We know there’s dark matter out there. At some point, we will discover what it is. We don’t know when, but we will discover what it is.

We also know that the particles that make up the universe have a very strange pattern of masses. It looks semi-random, and we don’t really understand whether there’s something fundamental hidden in that pattern, but we know it has something to do with the Higgs boson. I would really love to know why particles’ masses have that pattern.

We also don’t know why there’s any matter left in the universe after the big bang. In principle, in the big bang, you produce matter and anti-matter. At some point, they come together, and they annihilate and we get energy. That’s not what happened [because we observe matter in the universe]. So, there are all these really big questions out there, and at some point, we need answers to them.

The LHC will soon be shutting down for the high-luminosity upgrade. Can you tell us what will be done?

In the summer, on 29 June at 6am, we will switch off the LHC for four years. We’re replacing about 1.2 kilometres of the 27-kilometre ring with this very advanced technology. When the particles come around [the collider], we bend them towards each other. If you make the bunches of protons smaller and smaller, you get many, many more collisions. You concentrate everything in the same place. That’s what these super-high-field magnets are doing.

We have this incredible superconducting cable that powers these magnets. Installing this is a massive task. It’s by far the biggest thing that CERN has done for the last 20 years – and, at the same time, the big experimental collaborations, ATLAS and the Compact Muon Solenoid, which we sometimes call the general-purpose detectors, are upgrading their giant detectors. These are, again, the biggest projects that the experiments have done since building the detectors themselves.

The High-Luminosity LHC will produce an enormous number of Higgs bosons, which is necessary to measure, for the first time, key properties such as how it interacts with itself.

Particle physicists have now begun to look at the future beyond the LHC and to think about constructing an even more powerful collider, like the Future Circular Collider (FCC). Why?

Occasionally, I write down my 10 big questions in particle physics, and half of them have something to do with the Higgs boson. Like, does the Higgs boson interact with dark matter? We know [dark matter] is there, but we don’t know the answer to that question. Why does the Higgs boson have the properties that it does? Is the Higgs boson on its own, or are there multiple Higgs bosons?

The only way you can really start to address those questions is to make what we’re calling a Higgs factory, producing many Higgs bosons in much cleaner environments, so we can then look at the properties of the Higgs boson. If we see deviations from the properties we expect, we might then learn something about the unknown universe.

From what we know today, there’s a grouping of interesting physics at what we call the electroweak scale, which corresponds to energies that we think existed about a 100th of a nanosecond after the big bang. At this time, elementary particles cease to be massless. The Higgs mechanism gives the W, Z and Higgs bosons clustered around the electroweak scale their mass. The top quark also has a similar mass. However, all the other fundamental particles have much smaller masses – which is perhaps more surprising.

Last year, across Europe, each individual scientific community came together and asked themselves the question: what should we do next at CERN? There was a massive consensus that the FCC is by far the best machine to do the science. That’s because there’s a huge gap in scientific sensitivity between this particular machine [and] the other things you could do. It’s very unusual, even in a specific scientific community, to get such strong agreement. So we’re convinced it’s the best machine to do the science that we feel we need to do to continue our exploration of the universe.

Will there ever be a particle accelerator big enough? Is FCC the end of the line?

One of the advantages of something like the FCC is that it starts off being an electron-positron collider, but potentially in 30 or 40 years’ time, our successors could say what we need now is a Hadron Collider like the LHC, but a bigger one. We would then have the tunnel. So, it paves the way to doing the next phase of our exploration.

Now, we’re very focused on FCC; that’s what we’re going out to our member states to sell, but it does set the path, potentially, very, very long term. It would enable you to explore [physics at an energy] scale up to 100 times what we’re looking at now. So, we’re really exploring this whole electroweak scale, Higgs [and W and Z bosons]. If you don’t see anything there, then I think you start asking that question [of whether we need a larger collider], but we’re exploring the place where we think it feels like it’s the right place to see something completely new. I’m quite optimistic that at some stage in the next 10-plus years, we are going to break the standard model that we have. We are going to find a chink in its armour. It might not be where we expect it to be, but what we’re doing with all of our big scientific problem projects is we’re looking in the right place, or looking in the right places. We’re asking the universe the right questions.

It’s a very expensive project – £13 billion. Is it really the best use of money?

In any scientific ecosystem, you’re going to have a range of experiments. You’re going to do some small things at one end, but you have to have these really big bets and do the really ambitious science. The FCC is the one that’s right out at the end. I don’t believe it’s stopping other science from happening. Medical research, for example, will continue regardless of the FCC.

We are seeing economic challenges across Europe. From the perspective of the FCC, we’re not asking for money now. We’d actually be investing the money in the early 2030s, so we don’t know what the economy will be like at that moment, but I do think it’s our duty as CERN not just to make the scientific case, but to actually make that wider economic case. That case is there.

Sometimes the technologies we develop here at CERN, or in big science, do change the world, but they change the world 20 years down the line. If you didn’t do the investment early on, there are things you just would not end up with. The World Wide Web was developed 500 metres away from where we are now, in a small office inside CERN, as a way of sharing data between physicists. That really has changed the way the world works. The accelerator technologies we develop are not only useful for things like advanced cancer therapy, which uses proton accelerators, for example, but they’re also used in other fields of science. So, the economic benefits are huge, but sometimes they’re very long-term.

CERN and Mont Blanc, dark and frozen matter: Switzerland and France

Prepare to have your mind blown by CERN, Europe’s particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.