Particle physicists love speed. They hope that, some years from now, they’ll have a brand-new machine capable of colliding particles stupefyingly fast—99.999999999 percent of the speed of light. Planning such a collider, though, has been a much slower process.*
One contender is the International Linear Collider (ILC), a $7-billion, 20-kilometer-long machine that would be built in Japan. The ILC’s prospects have always been uncertain—a trend that continued last March, when Japanese officials announced that they would not commit to fund it. Without a firm decision, headlines described the ILC as “stuck in limbo”—and some physicists despaired that it was the Japanese government’s way of saying no.
A year later, at a meeting of the International Committee for Future Accelerators (ICFA) on February 20, Japanese officials again did not commit to funding the ILC. Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) reiterated that it would “discuss the ILC project with the U.S. and European counterparts while having an interest in the project.” What the tepid bureaucratic language obscures is a slow march of progress.
Domestically, the ILC finally cleared the Science Council of Japan (SCJ), a scientific advisory group to the Japanese government which had been holding it up for years. “The SCJ process is over and MEXT can move forward,” says Hitoshi Murayama, deputy director of the Linear Collider Collaboration and a theoretical physicist at the University of California, Berkeley.
On the international side, Japan has begun formal discussions with France, Germany, the U.K. and the U.S. about cost-sharing for the ILC. While the U.S. has signalled that it is ready to support the ILC, none of the European countries Japan has spoken with are currently prepared to promise financial support.
This summer MEXT will release a roadmap that points out a path for Japanese science over the next few years. If the ILC is included in the roadmap’s projects, it will have cleared another hurdle. In its postmeeting recommendations, ICFA advocated Japan transitioning from preplanning to a preparatory stage within a year.
Timing is critical: the next few years will determine the fate of the ILC and the field of particle physics. Even as MEXT formulates a roadmap for Japan, other nations are drawing up their plans—which may or may not include the ILC. The European Strategy Group (a special task force convened every five years by CERN) will release its report about which projects to prioritize in May; physicists in the U.S. will begin to sketch out their own particle physics roadmap next April. Whether the ILC is built depends in no small part on its plans being ready in time.
A Nonlinear Path
When electrons turn as they accelerate, they emit photons, bleeding off energy in a process called synchrotron radiation. In traditional circular colliders, this loss limits the particles’ maximum energy. Enter linear colliders: as their name suggests, they accelerate and collide particles in a straight path, avoiding synchrotron radiation so that electrons can reach higher energies.
The goal of the ILC would be to pick up where the LHC left off: with the Higgs boson. The Higgs, which was predicted in the 1960s and discovered at the LHC in 2012, is central to the mechanism that gives other elementary particles mass. Until its discovery, the Higgs was also the last missing piece of the vaunted Standard Model of particle physics, a unified description of all known fundamental particles and forces (except for gravity). Questions about the Higgs still remain—physicists want to know if it’s a “vanilla” Higgs, or if it exhibits unusual phenomena that could point the way to new physics.
Whereas the LHC collided protons to investigate the Higgs, the ILC would collide electrons with their antimatter counterparts, positrons. Unlike protons, which are made of a jumble of quarks and gluons, electrons and positrons are “fundamental”—just themselves. This produces “cleaner” collisions, minimizing the “noise” from unwanted secondary particles that could obscure the signals physicists seek to more deeply probe the Higgs.
From there the hope is that in such clean collisions, physicists could spot ultrarare events—following them like bread crumbs through a maze to reach the promised land of new physics beyond the Standard Model.
The SCJ recognized this importance, writing that “there is no doubt that the search for ‘new physics beyond the Standard Model’ is the most important task” for particle physics. But that alone was not enough to give such an expensive project priority. Other experiments such as Hyper-Kamiokande, a recently approved massive neutrino detector with a lesser price tag of $600 million, have proved more appealing.
If the ILC is eventually built, it will have supporters in Japanese industry and politics to thank. Both communities see the particle collider as a political and economic coup for the country. Additionally, because of its planned location, the ILC also represents a revitalization effort in the Tohoku region, which is still recovering from the devastating 2011 earthquake-induced tsunami and associated nuclear disaster at the Fukushima Daiichi power plant.
But even ardent supporters of the ILC must contend with the fact that it is not the only path forward in particle physics. Several emerging technologies—such as muon accelerators, energy-recovery linear accelerators and plasma wakefield acceleration—could offer equivalent or superior performance at lower costs, potentially undercutting the rationale for building future colliders like the ILC with conventional technology.
Internationally, the ILC is also in competition with other proposed colliders. Ongoing incremental upgrades to the LHC are underway at CERN and are planned to continue into the 2030s. Beyond the LHC, CERN has plans to build either its own linear collider, the CLIC, or the Future Circular Collider, a behemoth with a 100-kilometer-wide ring, or both devices. Led by Yifang Wang, physicists in China are also at work planning what would be that nation’s first major collider: the Circular Electron Positron Collider.
But dueling collider plans aren’t necessarily bad, according to Wang. “I think healthy competition is actually good,” he says. “If the ILC is approved, certainly it proves that the scientific interest is there and there is international support with this kind of science.”
There is no guarantee that any of these machines will uncover any new physics. Investigating the Higgs could well be a subatomic wild goose chase, only useful insofar as it tells researchers where not to look. Or it could reveal new physics beyond the scope of our current understanding of the universe.
In either case, particle physicists are prepared to do whatever it takes. “This is a very special particle, unlike any we've studied in the past,” Murayama says about the Higgs. “This one, in particular, should be studied to death.”
*Editor’s Note (2/27/20): This sentence was revised after posting. It originally gave a figure of 0.99999999999 percent.