CERN has taken a major step towards building a 100-kilometre circular super-collider to push the frontier of high-energy physics.

The decision was unanimously endorsed by the CERN Council on 19 June, following the plan’s approval by an independent panel in March. Europe’s preeminent particle-physics organization will need global help to fund the project, which is expected to cost at least €21 billion and would be a follow-up to the lab’s famed Large Hadron Collider. The new machine would collide electrons with their antimatter partners, positrons, by the middle of the century. The design—to be built in an underground tunnel near CERN’s location in Geneva, Switzerland—will enable physicists to study the properties of the Higgs boson and, later, to host an even more powerful machine that will collide protons well into the second half of the century.

The approval is not yet a final go-ahead. But it means CERN can now put substantial effort into designing a collider and researching its feasibility, while pushing to the backburner research and development efforts for alternative designs for LHC follow-ups, such as a linear eletron-positron collider or one that would accelerate muons. “I think it’s a historic day for CERN and particle physics, in Europe and beyond,” CERN director-general Fabiola Gianotti told the council after the vote.

This is “clearly a branching point” for the lab, says former CERN director-general Chris Llewellyn-Smith. Until today, several other options were on the table for a next-generation collider, but the CERN Council has now made an unambiguous, unanimous statement. “This is a major step, to get the countries of Europe to say ‘Yes, this is what we would like to happen’,” says Llewellyn-Smith, who is a physicist at the University of Oxford, UK.

Two stages

The decision comes in a document approved today, called European Strategy for Particle Physics Update. It outlines on two stages of development. First, CERN would build an electron-positron collider with collision energies tuned to maximize the production of Higgs bosons and understand their properties in detail.

Later in the century, the first machine would be dismantled and replaced by a proton-proton smasher. That would reach collision energies of 100 teraelectronvolts (TeV), compared with the 16 TeV of the LHC, which also collides protons and is currently the most powerful accelerator in the world. Its goal would be to search for new particles or forces of nature and to extend or replace the current standard model of particle physics. Much of the technology that the final machine will require has yet to be developed, and will be the subject of intensive study in coming decades.

“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach,” said Gianotti.

“I think certainly this is the right direction to pursue,” says Yifang Wang, who heads the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences in Beijing. CERN’s proposed new machine is similar in concept to a proposal that Wang has spearheaded for a Chinese colllider, in the wake of the LHC’s discovery of the Higgs boson in 2012. Like CERN’s now-official strategy, Wang’s proposal also included the possibility to host a proton collider in a second stage, following the LHC’s model (the 27-kilometre LHC ring occupies the tunnel that housed CERN’s Large Electron-Positron Collider in the 1990s). CERN’s decision “is confirmation that our choice was the right one”, Wang says.

While fully endorsing a CERN circular collider, the strategy also calls for the organization to explore participation in a separate International Linear Collider, an older idea that has been kept alive by physicists in Japan. Hitoshi Yamamoto, a physicist at Tohoku University in Sendai, Japan, says the endorsement is encouraging. “I believe that the conditions for ILC to move to the next step in Japan and also globally are now firmly in place.”

Funding tour

CERN’s strategy envisions 2038 as the beginning of construction for the new, 100-kilometer tunnel and the electron-positron collider. Until then, the lab will continue to operate an upgraded version of its current collider called High Luminosity LHC, which is currently under construction.

But before CERN can start building its new machine, it will have to seek new funding beyond the regular budget it receives from member states. Llewellyn-Smith says that countries outside of Europe including the United States, China and Japan might need to join CERN to form a new, global organization. “Almost certainly it will need a new structure,” he says.

The costly plan has detractors—even in the physics community. Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany, has emerged as a critic of pursuing ever higher energies when the scientific payback—apart from measuring the properties of known particles—is far from guaranteed. “I still think it’s not a good idea,” Hossenfelder says. “We’re talking about tens of billions. I just think there is not enough scientific potential in doing that kind of study right now.”

The new collider will be in uncharted territory, says Tara Shears, a physicist at the University of Liverpool, UK. While the LHC had a clear target to look for the Higgs boson as well as theorists’ well-motivated reasons to believe that there could be new particles in the range of masses it could explore, the situation now is different. “We don’t have an equivalent, rock-solid prediction now—and that makes knowing where and how to look for answers more challenging and higher risk.”

Still, she says, “We do know that the only way to find answers is by experiment and the only place to find them is where we haven’t been able to look yet.”

In closing the meeting, which most members attended remotely, CERN Council president Ursula Bassler said, “The big task now is in font of us, putting this strategy into reality.” She then popped a bottle of champagne before ending the teleconference.

This article is reproduced with permission and was first published on June 19 2020.