It is a truth universally acknowledged that a physics laboratory with a world-leading scientific facility must have a plan for an even better machine to succeed it. So it is with the European particle physics laboratory, CERN, near Geneva, which is home to the world’s biggest atom smasher, the 27-kilometer-long Large Hadron Collider (LHC). Today, CERN’s governing council announced it will launch a technical and financial feasibility study to build an even bigger collider 80 to 100 kilometers long (actually two of them in succession) that could ultimately reach an energy seven times higher than the LHC. The first machine wouldn’t be built before 2040.
There is “some pride of the member states of CERN [that it is] the leading particle physics laboratory, and I think there is interest in CERN staying there,” says Ursula Bassler, a physicist and president of the CERN council, the panel of representatives from the 23 nations that support the lab. However, CERN Director-General Fabiola Gianotti emphasizes that no commitment has been made to build a new mammoth collider, which could cost $20 billion. “There is no recommendation for the implementation of any project,” she says. “This is coming in a few years.”
Physicists have been debating what collider to build next since well before the LHC started to take data in 2010. In the early 2000s, discussions centered on a 30-kilometer-long, straight-shot, linear collider that would smash electrons into positrons. Such a machine would complement the circular LHC, which smashes countercirculating beams of protons. The two types of machines have different strengths. A proton collider can generally reach higher energies and discover heavier new particles. But protons are made of other particles called quarks, so they make messy collisions. In contrast, electrons and positrons are indivisible fundamental particles, so they make cleaner collisions. Historically, physicists often have found new particles at proton colliders and studied them in detail at electron-positron colliders.
That’s the game particle physicists around the world are trying to play today. In 2012, the proton-smashing LHC blasted out the Higgs boson, the last particle predicted by physicists’ standard model and the linchpin to their explanation how all other fundamental particles get their mass. Many would now like to build an electron-positron collider and run it as a Higgs factory, to make the particle in large numbers and see whether it has exactly the predicted properties. Any deviation from the predictions would be signs of new physics beyond the 40-year-old standard model, something particle physicists are desperate to find. Physicists in Japan would like to host such a linear collider.
A few years ago, however, some physicists proposed another approach, building an 80- to 100-kilometer-long circular electron-positron collider to study the Higgs. That machine would have a major drawback: As light-weight electrons go around in circles, they radiate copious x-rays and lose energy, so such a machine is inefficient and limited in its energy reach. But it has a big practical upside: The tunnel it needs could also later be used to house a higher energy proton collider. This is exactly what CERN did with the LHC, which was built in an existing tunnel dug for the Large Electron-Positron Collider, which ran from 1989 to 2000. (It studied in detail particles called the W and Z bosons that had been discovered previously with a proton-antiproton collider at CERN.)
Now, CERN physicists envision a future in which, around 2040, they build a huge circular electron-positron collider to study the Higgs. Then, they would follow up with a more powerful proton collider to reach a new high-energy frontier. Today, the CERN council took a step in that direction, announcing an update to its long-range strategy, the first since 2013.
Just how much CERN’s plans have changed remains murky, however. Some physicists there have long been working on CERN’s own design for a linear collider. And it appears the new long-range strategy does not completely sideline that idea. “We also recommend continued accelerator R&D to ensure that we do not miss an opportunity to improve our accelerator technology,” said Halina Abramowicz, a physicist at Tel Aviv University who led the planning exercise, during an online question-and-answer session. “I think it’s important to convey this message very clearly.”
The feasibility study for the big new machine should be done by 2026 or 2027, when CERN will next update its long-term strategy. CERN may also have competition in the presumed collider arms race, as physicists in China have similar plans to build big circular colliders. Of course, all may depend on whether the LHC, which is now undergoing an upgrade and should run until the mid 2030s, finds anything beyond the Higgs boson to study. If it doesn’t, convincing the governments of Europe to spend $20 billion to study just the Higgs may prove a daunting political challenge.