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Plans move ahead for even larger particle collider at CERN

Image: CERN

In two decades, Geneva may be encircled by the biggest scientific instrument ever built, albeit several dozen metres below the surface. Plans advanced a couple of weeks ago at CERN, the European Organization for Nuclear Research, for an enormous particle accelerator that would help physicists delve further into the workings of the universe.

Researchers are looking to build a new collider 100 km in circumference, dubbed the Future Circular Collider (FCC), that is considerably larger than the 27 km Large Hadron Collider (LHC), currently the world’s largest and most powerful particle-smashing machine. If built, the deep-underground tunnel housing the FCC would be nearly twice as long as the 57-km-long Gotthard Base Tunnel in the Swiss Alps. Don’t expect joy rides, though – unless you are an electron or a proton.

“The planned accelerator would be an increase in ambition of the research programme that we had in the past 50 or 60 years,” says Dr Patrick Janot, a CERN physicist who is co-leading the physics side – both experiment and theory – of the project. CERN has operated particle accelerators since the 1950s, shortly after it was established. Its first accelerator was the Synchrocyclotron, which ran between 1957 and 1990, was a relatively tiny machine; it fit within a single room and can now be visited as part of CERN’s guided tours.

Since then, CERN built and operated several other accelerators, including the Large Electron-Positron Collider (LEP, 1989–2000), and the LHC, which has been running since 2010. These machines are necessary for CERN’s primary goal: to understand nature and the fundamental laws of the universe. By colliding particles at very high energies, scientists can produce never-before-seen particles and observe novel phenomena, testing existing theories of physics and coming up with new ones.

Explorations at the energy frontier. “We have a theory – which was confirmed by LEP and the LHC – called the Standard Model of particle physics,” he explains. The Standard Model explains the behaviour of all known particles in the universe and three of the four forces through which they interact. “But we know from observation of phenomena in the universe that the Standard Model is not complete.” For example, while there has been observational evidence of dark matter in galaxies, we have not yet seen the elementary particle associated with it. Other mysteries include the absence of antimatter in a universe dominated by matter.

The LHC has made important strides in humanity’s understanding of the universe, not least through the discovery of the Higgs boson (one of the elementary particles in the Standard Model) in 2012 and the many precision measurements of phenomena predicted by the theories. It has not, however, shown any major discrepancies between theory and experiment that would point scientists in the direction of new ideas. Physicists hope to find new particles and phenomena by pushing at the energy frontier, by constructing a bigger machine than the record-holding LHC.

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A representation of the Standard Model of particle physics, showing all of the known kinds of particles in the universe. Image: CERN

A 100 km gamble? Just because scientists hope to see something new does not mean nature has to oblige. “In fundamental research, there is never certainty,” Janot elaborates. “You look for the unknown. And the best that you can do is go further, go deeper, be smarter, and this is what we’re trying to do. But there is no guaranteed outcome in basic science. The FCC would be the most complete and the most thorough way to find an answer of all the projects that exist now, but it still doesn’t guarantee it will definitely be found.”

Learning from the history of LEP and the LHC, the current plans are to build two successive accelerators in the same tunnel: the LHC was constructed within the same tunnel that was once occupied by LEP. So, first, a machine is envisioned to be built that collides electrons with its antimatter counterpart, the positron. This electron-positron machine will operate at four different stages of increasing energy to allow the scientists to study specific phenomena at each energy, associated with the four heaviest particles we know of, including the Higgs boson. The measurements would allow scientists to identify exactly how the Standard Model differs from reality by making precise measurements of known phenomena. “Study these with the greatest precision, and then you have a complete overview of what the effects of new physics could be in different directions,” Janot says.

The plan is to then repurpose the same tunnel to build a proton-proton collider that will operate at an energy of 100 teraelectronvolts (TeV); for comparison the LHC currently collides particles at 13 TeV. This huge leap in energy is expected to open up an entire new territory for the intrepid scientists to explore. Since all previous accelerators have led to crucial discoveries in particle physics, there is an almost-universal and tantalising hope of discovering new particles in this energy regime.

Encircling Geneva, you say? The current plans envision an accelerator that goes around the Salève on one side before going underneath Lake Geneva to touch the slopes of the Jura and the Vuache. The depth at which the accelerator will be constructed is yet to be determined, with various geological surveys being performed to determine the optimum site. The tunnel poses an enormous civil-engineering challenge and will account for most of the initial costs of the project.

When LEP was built, the 27-km-circumference tunnel that needed to be excavated to house it was the biggest civil-engineering project in Europe at the time. The bigger 100 km ring will prove to be even more difficult, and CERN is collaborating with experts in tunnelling to assess the various options for both the tunnel itself and the vertical shafts needed to access the tunnel. And of course, they have to take into account existing and planned constructions on the surface.

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An artist’s interpretation of the FCC’s tunnel. Image: CERN

New magnets and green power supplies. The tunnelling challenges are only one technological hurdle. A second challenge relates to the energy consumed by the machine. “Until now we were spending energy with a small efficiency to get what we wanted,” Janot says. “We are trying to be very careful and develop power sources that are very efficient, so you can recover nearly 100 per cent of the energy put into the source. With LEP, half the power was lost.”

A further challenge has to do with the magnets needed to constrain the particle beams to their designated trajectories. To accelerate particles to high energies around a circular racetrack, you have to bend the bunches of particles around the curves of the machine. The higher the energy of the particle beams, the higher the magnetic field necessary to bend them. This is mainly crucial for the hadron-collider phase of the project.

“We have been blocked for decades at a magnetic field of 14 tesla,” explains Janot. This is around 350,000 times the magnetic field of the earth itself. Magnets going up to 16 tesla are needed for the future hadron collider. “There has been progress with new superconducting materials needed to efficiently power the electromagnets. We are confident that we will manage, but it might not be before my retirement.”

Colliding until the turn of the century. Physicists are used to research programmes with timelines of decades. It took a hundred years after Einstein’s prediction of gravitational waves for scientists to observe them. Black holes, predicted a century ago, were imaged for the first time in 2019. And the Higgs boson itself was discovered nearly five decades after the theorists predicted its existence.

The FCC is no different, and CERN is cautiously taking its first steps in this direction. “Two weeks ago,” Janot notes, “the CERN council approved the feasibility study of the FCC with a focus on the first stage, which is the tunnel and the electron-positron machine.” The objective is to produce a report by 2025 detailing both the physics cases for the machine and providing an overview of the costs associated with it. At the moment, the tunnel alone is expected to cost around five billion euros, with a similar amount spent on the accelerator. It is important to note that these costs are akin to the costs of building new roads: they will be spread out over a long duration.

“My wish, even though I will be retired by then,” Janot laughs, “is that the building of the tunnel will start in 2030 and that the commissioning of the electron-positron machine will start in the early 2040s. In the interim, there is ample work to do to keep us busy for these twenty years.” This machine will then operate for 15 or 20 years and then be replaced over 10 years by the hadron collider. The hadron-collision programme is expected to last 30 or 40 years, taking the project well into 2110 – 90 years from now!

The FCC collaboration has more than 30 participating countries from Europe, the Americas, Asia and Oceania. It serves another important goal of CERN, which is to unite the world in the pursuit of science for peace. “This is an important project not just for CERN and for Europe but for the world because it is a global project.”

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