European plans for a new supercharged particle accelerator

What could the FCC teach us about the universe?
09 February 2024

Interview with 

Sarah Williams, University of Cambridge


A view down one of the tunnels of the Large Hadron Collider.


It’s been ten years since the enigmatic particle referred to as the Higgs Boson was discovered at the Large Hadron Collider or LHC, which is based in a circular 27 kilometre long tunnel  in Switzerland. By smashing together protons at close to the speed of light, it helped to confirm key theories conceived half a century ago about the particles that make up the Universe around us, but important gaps still remain. Now the team at CERN are envisaging an even bigger particle accelerator ten times as powerful and three times the size of the LHC. And it’s got a price tag to match. Sarah Williams is from the department of physics at the University of Cambridge...

Sarah - In a nutshell, what we would like to do is build a very large circular tunnel that would be around 90 kilometres in length that we would use for our next generation particle collider. The aim is that this would actually be kind of two colliders in one. We would first use the tunnel to house a collider that would collide electrons with their antiparticles, the positron, and this would be what we call the Higgs factory. So this would allow us to make very precise measurements of the Higgs Boson. This is the particle that we discovered at the LHC over 10 years ago now. But actually, although we discovered it, there's still lots we can learn about its properties and how it behaves with respect to other elementary particles. What we'd then do is upgrade this detector. We'd install a whole new accelerator, and we'd then use that to collide protons. So this would kind of be a bit like a super LHC. We'd be raising the energy and also allowing a lot more collisions. And in doing that we'd be really pushing our discovery reach for new particles. We could discover to the highest possible level based on current technologies. We'd be searching for particles that can explain dark matter. We'd also be aiming to discover processes in our current model that we haven't been able to observe before. The very particular measurements about the Higgs, including how it interacts with itself, could have some really big implications for our understanding of the history of the universe and cosmology.

Chris - This proposal would see something being created that's about three times bigger than what you've got already at CERN with the LHC. Why do you need a bigger accelerator to do these things?

Sarah - That's a really good question, and for anyone wanting an idea of scale, the length of this tunnel is around half the length of the M25 motorway. The reason we have to get a much bigger ring is that particles lose energy if you accelerate them around in a circle. So what that really means is to go to higher energy collisions, particularly for protons which are heavier than electrons. We need a really high ring. So the reason that we're scaling up the size of the ring is we want to scale the energy of the collisions, and that of course gives us the potential to discover more particles.

Chris - And how much energy can you harness and therefore endow these particles with when they're in this accelerator?

Sarah - Let's take the proton collider to begin with. The LHC at the moment is colliding protons at a centre of mass energy of 14 tera electron volts. So that actually seems quite big, but actually you have to remember that this is confined on a very small scale. So the energy in colliding protons in the LHC is actually roughly that of a flying mosquito. For the FCC, we'd be wanting to go to a hundred TV, so that's around a factor of 10 increase in energy. And what that means is it would also scale up the mass reach for the heaviest particles we could possibly discover.

Chris - What do you think is missing then? Where are the gaps that you want to probe with this? Because many physicists thought that with the discovery of the Higgs Boson, as you say 10 years ago, that we almost had a complete picture there. So where are the gaps?

Sarah - Probably that's a problem in the way we were advertising the discovery of the Higgs. Many people said that when we discovered the Higgs, we'd completed the standard model. The standard model is our model of what we think the basic building blocks of the universe are and how they interact with each other. The problem with that is that we know the standard model can't be a complete theory of everything. There are a few obvious gaps. One of them that people might have heard about is dark matter. This is something we know exists in the universe. We can see how it impacts the behaviour of galaxies and things based on its gravitational interactions. But we don't have a candidate in the standard model for what dark matter could be. So that's an obvious target for the future. Other things we'd like to target are, as I said earlier, very precise measurements of properties of the Higgs Boson, that could have important implications for how we understand why we live in a matter dominated universe today. Anyone that's read His Dark Materials might have come across the idea of antimatter. And of course the fact that we don't see antimatter everywhere in the universe tells us that there must be some small difference between matter and antimatter. That means that we live in a matter dominated universe today. And explaining how that happened and what these differences are is also something we're trying to do.

Chris - Not all scientists are supportive, though. The spend is quite big. It's getting close to $20 billion. The original LHC cost about $3 billion. So this is a big uptick in spend. People like Sir David King, the government's former chief scientist have said this is irresponsible at a time when we have effectively bigger fish to fry with that sort of money. What sort of a difference could this make if we do go down this path and should we go down this path? Why do we need to do this?

Sarah - I think there are a few things I'd like to say to that, and it's worth me declaring at this stage that I really do think this project is the best way forward for our field. Going to the price tag, I think it's worth realising that this will deliver half a century of science for our field, and it's spread out over a large number of cooperating countries over a long period of time. When we were building the International Space Station, the cost of the European contribution to that which was spread over 10 years, came out around 8 billion euros. And this is roughly the same magnitude of the price tag we're looking at for the tunnel for the FCC, Future Circular Collider. And at the time it was argued that this was around one Euro cup of coffee per person every year for the 30 years of which they'd been operating. In terms of whether or not the science is worth it, whilst there are other challenges going on in the world, we shouldn't cut down the importance of doing fundamental science. There are lots of really exciting spinoffs that come from technologies. I quite like the fact that some of the magnet technologies that are being considered for the FCC, these technologies are called high temperature superconductors, and these could also have applications in fusion technology, which is of course very relevant for trying to solve the energy crisis. In terms of the questions we could answer, we are really talking about pushing our understanding of the fundamental particles in the universe to a factor of 10 higher in precision, and raising the energy reach of what we could discover. And I think that's worth doing.


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