Giant collider to reveal the secrets of the universe

The 27km-long Large Hadron Collider (LHC) at CERN in Geneva is the world’s largest and most powerful particle accelerator. Inaugurated in 2008, the LHC was responsible, among other fundamental discoveries, for confirming in 2012, the existence of the Higgs boson, a particle that had been theoretical for 50 years.*

However impressive, the LHC is reaching the end of its life. Scientists estimate (using statistical models) that its potential for discovery will reach its limits around 2040-2045. The next step then is “the FCC, the Future Circular Collider”, says Yann Léchevin, member of the team working on the FCC’s feasibility study. This spring, CERN unveiled new details of this study, which is still in progress, confirming that the physicists are aiming for something much bigger than the LHC.

If built as currently planned, the FCC ring will stretch out over 91km, more than three times the length of the LHC. It will encircle Geneva and pass under Lake Geneva. Therefore, it will need to be buried deeper than the LHC tube. “Between 150 and 450 m deep”explains Yann Léchevin, “the study has yet to determine this.

Eight sites with vertical shafts will provide access to the ring for construction work, experiment preparation and housing for electricity, cooling, air, and communication ducts. “Four experimental caverns, 66m long and 35m high, will be excavated. They will house the detectors that will observe the particle collisions.” There will also be several technical caverns which will be longer, but lower. The FCC will be linked to CERN’s existing accelerator complex. “The beams that will be injected into the FCC will be prepared at CERN.”

But, in fact, how will an accelerator three times as long improve the observation of particles that are billions of times smaller than a grain of sand? To give them even more speed? “No, the LHC already propels them at the maximum speed – the speed of light. A larger accelerator will enhance luminosity, and therefore the quality of observation.”

To put it simply, Yann Léchevin compares an accelerator to a roundabout in which we would study the passage of vehicles. If the radius of curve and equipment of the LHC enable us to observe the behaviour of motorbikes, that of the FCC (larger and equipped with more powerful electromagnets) would enable to observe the behaviour of trains! “The speed of passage remains the same, but the bundles of beams and energies are much larger.” Phenomena become much more visible and sharper. “You can zoom in more and see new things.”

The improvement in observation quality by the FCC is promised in two stages.

First, an electron-positron accelerator (the FCC-ee). Nicknamed the “Higgs factory”. By 2048, it will provide a better understanding of these famous bosons and the “weak force” (one of the four fundamental forces). “Thanks to this accelerator, we will also be able to better define the field of the second stage, which will be studied by the FCC-hh, a proton-proton accelerator.”

This second accelerator, scheduled to be operational in 2065, is the Holy Grail aim. This “LHC on steroids” should enable collisions with an energy of 100 trillion electron volts, pulverising the 13.5 trillion of the current accelerator.

In case you have problems understanding the central importance of this gain in electron volts – and therefore that of the FCC – the enthusiastic physicists at CERN are ready to sum it up for you: “The FCC is the only machine capable of taking us a great leap forward in the study of matter.”

Researchers hope to be able to confirm the existence of particles that have never been observed before; to finally understand what dark energy and dark matter are (which make up 95% of the universe but remain a total mystery); to understand why there is so much more matter than its opposite, antimatter. Their enthusiasm is quite understandable.

All that remains is to build the machine.

Unlike the LHC, which is grafted onto the existing infrastructure of another CERN accelerator, here they have to start from scratch. Hence the cost, estimated at 17 billion euros for the first stage alone.

As far as form is concerned, stresses Yann Léchevin, “we’re sticking with what we know already. The FCC design is not very different from that of the LHC. The size of the caverns and accesses is comparable. So is the diameter of the tunnel, and it remains modest: 5.50 metres, which is much narrower than the road tunnels through the Alps.”

Only the length of the FCC is unprecedented. The 91km that – incidentally - pass under a city, the largest lake in Western Europe, and two rivers. All in an area known for its rather complex geology.

The feasibility study is now trying to tackle the issue.

The aim is to verify that the chosen route (from among the hundred or so being considered) is feasible. “We have to check the geological strata, the absence of water, the stability of the subsoil, and so on."

The geophysical maps drawn up will make it possible to estimate the cost of the work more reliably. And to avoid unpleasant surprises during the works. With eight tunnel boring machines planned, there is little margin for error. “More than the size of the project, its precision will be a real civil engineering challenge!”

The geological study will also determine the location of the eight access shafts. This is important because they will be the only visible parts of the accelerator. The visual and environmental impact has already been taken into account in the study (it has influenced the number of shafts and the choice of route), but it will be necessary to further reduce it at various locations.

As you can imagine, a scientific infrastructure such as the Future Circular Collider goes beyond CERN and its 23 member states. “We’re talking about creating a world-class machine”, explains Yann Léchevin. “The FCC is part of the European Strategy for Particle Physics, which in turn is linked to the international scientific community as a whole.“