A chilled linear collider would accelerate particles nearly to the speed of light.

© REY.HORI/KEK

Lack of discoveries is damaging prospects for next big particle collider

Particle physicists are fretting that they don't know what their next collider will be or where to build it. Europe, China, and Japan have each released plans for a successor to the current biggest atom smasher, the Large Hadron Collider (LHC) at CERN, Europe's particle physics laboratory near Geneva, Switzerland. Yet the LHC's worrying lack of discoveries is making it hard for physicists to rally support for the megaprojects from governments or their colleagues.

The successor machine thought to be closest to hand, the International Linear Collider (ILC), is bringing these concerns to a head. By the time members of the Linear Collider Board meet in Tokyo on 7 March, Japan's government is expected to have issued a statement about whether it will host the $7.5 billion machine. The physicists are holding their collective breath. After a critical report published in December 2018 by the Science Council of Japan (SCJ), even a hint of official support would please some ILC supporters. "If the statement is like, ‘We would like to host the ILC, provided that the international negotiations are successful,’ it would be very positive," says Hitoshi Yamamoto, a physicist at Tohoku University in Sendai, Japan.

Just 7 years ago, particle physicists rejoiced in the discovery of the Higgs boson amid the wreckage of proton collisions at the LHC. The Higgs was the last missing piece in physicists' standard model of elementary particles and forces and the linchpin in their explanation of how all fundamental particles acquire their mass. But many were hoping to find new particles and forces not predicted by the standard model. On that account, the LHC has so far come up empty.

Many physicists have long argued that their next big machine should be a linear collider that slams electrons and positrons together. Those pointlike particles produce collisions that are much easier to interpret than collisions of protons, which are themselves bags of subatomic particles. So a linear collider would be ideal for a precision analysis of any new particles discovered by the LHC. In 2003, groups in the United States, Europe, and Japan joined forces to propose the ILC: two straight accelerators housed in a 31-kilometer-long tunnel that would smash electrons into positrons at energies up to 500 giga-electron volts (GeV).

But when physicists completed a basic design in 2007, the United States balked at the $14 billion price tag. Japan emerged as the likely host after the 2011 Tohoku earthquake and tsunami, when backers hoped the government would fund the project as part of an economic stimulus. The design was later scaled back when the Higgs discovery revealed that a cheaper 20-kilometer-long machine, capable of reaching 250 GeV, would suffice to generate lots of those particles.

As a "Higgs factory," the ILC would help physicists measure the rates or "branching ratios" with which the Higgs decays to familiar particles. Discrepancies from standard model predictions would point to new physics. Although that science is solid, it is also less exciting than searching for new particles, some physicists say. "Spending 30 years measuring Higgs branching ratios probably isn't going to grab people," says George Gollin, a particle physicist at the University of Illinois in Urbana.

In its December report, the SCJ expressed similar reservations, noting the project's unresolved technological challenges and questioning whether "the prospective scientific outcome … is sufficient to justify Japan's large share of the overall cost."

Colliding dreams

Japan, China, and Europe have all proposed machines to succeed the Large Hadron Collider (LHC).

CEPC (China) and Future Circular Collider (Europe)100-kilometer circumference LHC (Europe)27-kilometer circumference ILC (Japan)20-kilometer length
V. ALTOUNIAN/SCIENCE

The government might still offer to host the ILC for reasons beyond the scientific payoff. The ILC would be the first truly international scientific facility in Japan, and it would make the country a center for accelerator physics for decades. The candidate site, in the Kitakami Mountains that straddle Iwate and Miyagi prefectures, is in a region that bore the brunt of the 2011 earthquake. The potential boost to the regional economy has not gone unnoticed: Regional representatives to the Diet, the national parliament, formed an ILC promotion group, as did industrial interests.

This is why "people outside of the science community are so strongly supporting" the ILC, says Yasuhiro Okada, a theoretical physicist and deputy head of the ILC planning office at the High Energy Accelerator Research Organization in Tsukuba, Japan. Over the past 6 weeks, a steady stream of city councils, civic groups, and chambers of commerce has presented letters of support for the ILC to officials of the ruling Liberal Democratic Party, according to local news reports.

But the limited scope of a Higgs factory has researchers in both China and Europe considering plans for more ambitious machines. In November 2018, China unveiled plans for a proposed Circular Electron Positron Collider (CEPC), a $5 billion machine measuring 100 kilometers in circumference. Not to be outdone, in January CERN released a conceptual design for its own 100-kilometer-long circular electron-positron collider.

Because electrons and positrons radiate copious x-rays as they bend around a ring, the circular colliders would run much less efficiently than a linear collider. However, they would have an upside: The tunnels could later be used for a new proton collider that would rev the particles up to energies seven times as high as the LHC's, increasing the potential for discovering new particles. A linear collider could not be converted into such a proton smasher because in their single pass through the machine, protons could not reach such energies.

Construction of the CEPC hinges on funding under China's next 5-year plan, which starts in 2021. "It is still too early to get any response from our funding agencies," says Wang Yifang, director of the Chinese Academy of Sciences's Institute of High Energy Physics in Beijing, which is leading the project. And like the ILC, the CEPC faces questions about cost and scientific payoff. But China may be eager to show the rest of the world it can host such an international collaboration, says Young-Kee Kim, a particle physicist at the University of Chicago in Illinois and former deputy director of Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. "They want to show the world their power."

Of course, the doubts about the science case for the next collider could evaporate if the LHC does conjure up new particles. The machine, which will run into the 2030s, has collected just a 10th of its total planned data. Given that fact, some physicists say there's no rush to decide on the next machine.

If a Higgs factory does not materialize, there may still be a way forward. Some CERN researchers want to replace the LHC with a collider that has particle-guiding magnets twice as strong. That would double the LHC's energy without the expense of a new tunnel. Although that plan is less ambitious, says Robert Roser, a Fermilab physicist, "That's what I would do."