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Plans for world’s next major particle collider stuck in limbo

Japan delays decision on whether to host a US$7-billion linear accelerator — but competing proposals are also in development.

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Inside the ALICE detector empty skeleton

Physicists hoped that he Large Hadron Collider near Geneva, Switzerland, would discover more new particles that could be studied by a newer collider.Credit: Julien Ordan/CERN

Japan’s government has said that it is not ready to commit to hosting the world’s next major particle accelerator — the planned International Linear Collider (ILC). The decision appears to deal another blow to a project that has been more than a decade in the making, although some physicists are hopeful that the government might finally be making progress on the proposal.

“There was disappointment,” said Geoffrey Taylor, chair of the International Committee for Future Accelerators, at a press conference at the University of Tokyo on 7 March. The press conference followed a meeting with representatives of Japan’s science and technology ministry, who delivered a statement on the government’s position.

The particle-physics community conceived the ILC more than 15 years ago, as a follow-up to the Large Hadron Collider at CERN, Europe’s particle-physics lab near Geneva, Switzerland. The ILC would be a straight, 20-kilometre collider that would make detailed studies of the Higgs boson, the last puzzle piece in physicists’ standard model.

Japan has been the only country in the running to build the US$7-billion machine, after its physicists pitched to host the facility in 2012 following the discovery of the Higgs at CERN. As host, the nation would need to pay around half the construction costs, and other countries would contribute the rest. But despite years of discussions, the government hasn’t thrown its weight behind the project and has shown little formal interest in it.

European pressure

The International Committee for Future Accelerators, which oversees work on the ILC, had asked Japan’s government to decide whether to host the facility by 7 March, so that the decision could feed into major discussions over the direction of particle physics in Europe, where researchers are itching to begin planning their next collider.

“There’s no way in which one can be anything other than disappointed about the future of the project,” says Brian Foster, a particle physicist at the University of Oxford, UK, and former European regional director for the ILC. “One gets the very distinct impression that this is just another way of saying no.”

But Taylor thinks the delay doesn’t mean the project is dead, and says that he was encouraged by interest from some Japanese lawmakers.

Despite the lack of commitment, many physicists welcomed the Japanese officials’ statement because the government has now formally expressed its interest in the project: it said that it would ask senior Japanese physicists to begin official discussions about sharing costs with other countries, said Taylor.

Those discussions would need to be successful and the project would need support from the Japanese scientific community for the accelerator to move forward, he said.

Fundamental particles

The ILC is intended to smash together electrons and their antimatter counterparts, positrons, and study the products. Because electrons and positrons are fundamental particles, the collisions would be cleaner and easier to study than the proton–proton smashes at the LHC.

Most physicists want a ‘Higgs factory’ of some sort, and the ILC is the most developed proposal. But China and CERN want to build circular versions, and CERN is also working on designs for a Compact Linear Collider (CLIC), which could reach much higher energies than the ILC.

The science case for the ILC has weakened over the years, compared with those for rival accelerators, says Alain Blondel, a physicist developing one of the CERN-led competitors. Physicists once hoped that the ILC would also explore physics beyond the standard model, by studying new particles discovered by the LHC. But the LHC hasn’t found any more particles since the Higgs, meaning that any remaining ones probably lie beyond the ILC’s proposed energy range, says Blondel. However, ILC advocates say that indirect signs of new physics could still emerge in the facility’s range.

Delayed decision

Japan has repeatedly delayed making a final decision on hosting the facility, deterred in part by the project’s price tag. In 2017, physicists scaled back their ambitions for the ILC, in favour of a cheaper, lower-energy design.

And in December, a report by members of the Science Council of Japan, which advises the government, said the council couldn’t yet support the plans, citing concerns over the collider’s value for money. More work is needed to convince the council of the ILC’s benefits, said Taylor.

But the longer the delay, the greater the competition the collider faces, said Tatsuya Nakada, a particle physicist who leads the panel in charge of the project’s design, at the press conference. The rival proposals are now competing for the same pots of cash.

Japan’s lack of commitment makes it harder for European physicists to consider the ILC in a major exercise designed to guide funding for the next six years. The exercise, called the European Strategy for Particle Physics, must make its recommendations by May 2020. If Japan doesn’t commit by then, the ILC risks being seen as a low priority, making it harder to get European funding.

doi: 10.1038/d41586-019-00824-4
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