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China unveils design for $5 billion particle smasher [sciencemag.org]

By Dennis Normile [sciencemag.org]Nov. 16, 2018 , 3:30 PM

BEIJING—The center of gravity in high energy physics could move to Asia if either of two grand plans is realized. At a workshop here last week, Chinese scientists unveiled the full conceptual design for the proposed Circular Electron Positron Collider (CEPC), a $5 billion machine to tackle the next big challenge in particle physics: studying the Higgs boson. (Part of the design was published in the summer.) Now, they’re ready to develop detailed plans, start construction in 2022, and launch operations around 2030—if the Chinese government agrees to fund it.

Meanwhile, Japan’s government is due to decide by the end of December whether to host an equally costly machine to study the Higgs, the International Linear Collider (ILC). How Japan’s decision might affect China’s, which is a few years away, is unclear. But it seems increasingly likely that most of the future action around the Higgs will be in Asia. Proposed “Higgs factories” in Europe are decades away and the United States has no serious plans.

The Higgs boson, key to explaining how other particles gain mass, was discovered at CERN, the European particle physics laboratory near Geneva, Switzerland, in 2012—more than 40 years after being theoretically predicted. Now, scientists want to confirm the particle’s properties, how it interacts with other particles, and whether it contributes to dark matter. Having only mass but no spin and no charge, the Higgs is really a “new kind of elementary particle” that is both “a special part of the standard model” and a “harbinger of some profound new principles,” says Nima Arkani-Hamed, a theorist at the Institute for Advanced Study in Princeton, New Jersey. Answering the most important questions in particle physics today “involves studying the Higgs to death,” he says.

“Physicists want at least one machine,” says Joao Guimaraes da Costa, a physicist at the Chinese Academy of Sciences’s Institute of High Energy Physics (IHEP) here, which put together the Chinese proposal. “Ideally, both should be built,” because each has its scientific merits, adds Hitoshi Murayama, a theoretical physicist at the University of California, Berkeley, and the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe in Kashiwa, Japan.

The CERN discovery relied on the Large Hadron Collider, a 27-kilometer ring in which high-energy protons traveling in opposite directions are steered into head-on collisions. This produces showers of many types of particles, forcing physicists to sift through billions of events to spot the telltale signal of a Higgs. It’s a bit like smashing together cherry pies, Murayama says: “A lot of goo flies out when what you are really looking for is the little clinks between pits.”

Smashing electrons into their antimatter counterparts, positrons, results in cleaner collisions that typically produce one Z particle and one Higgs boson at a time, says Bill Murray of The University of Warwick in Coventry, U.K. How Z particles decay is well understood, so other signals can be attributed to the Higgs “and we can watch what it does,” Murray says.

Japan’s plan to build an electron-positron collider grew from international investigations in the 1990s. Physicists favored a linear arrangement, in which the particles are sent down two straight opposing raceways, colliding like bullets in rifles put muzzle to muzzle. That design promises higher energies, because it avoids the losses that result when charged particles are sent in a circle, causing them to shed energy in the form of x-rays. Its disadvantage is that particles that don’t collide are lost; in a circular design they continue around the ring for another chance at colliding.

Along the way, Japan signaled it might host the machine and shoulder the lion’s share of the cost, with other countries contributing detectors, other components, and expertise. A 2013 basic design envisioned a 500-giga-electronvolt (GeV) linear collider in a 31-kilometer tunnel costing almost $8 billion, not counting labor. But by then, the CERN team had already pegged the Higgs mass at 125 GeV, making the ILC design “overkill,” Murayama says. The group has since revised the plan, aiming for a 250-GeV accelerator housed in a 20-kilometer-long tunnel and costing $5 billion, says Murayama, who is also deputy director of the Linear Collider Collaboration, which coordinates global R&D work on several future colliders.

IHEP scientists made their own proposal just 2 months after the Higgs was announced. They recognized the energy required for a Higgs factory “is still in a range where circular is better,” Murray says. With its beamlines buried in a 100-kilometer-circumference tunnel at a site yet to be chosen, the CEPC would collide electrons and positrons at up to 240 GeV.

Both approaches have their advantages. The CEPC will produce Higgs at roughly five times the rate of ILC, allowing research to move faster. But Murayama notes that the ILC could easily be upgraded to higher energies by extending the tunnel by another couple of kilometers. Most physicists don’t want to choose. The two colliders “are quite complementary,” Murray says.

Whether politicians and funding agencies agree remains to be seen. Construction of the CEPC hinges on funding under China’s next 5-year plan, which starts in 2021, says IHEP Director Wang Yifang. IHEP would then also seek international contributors. Murayama says Japan needs to say yes to the ILC in time to negotiate support from the European Union under a particle physics strategy to be hammered out in 2019. Missing that opportunity could mean delaying the collider by 20 years, he says—and perhaps ceding the field to China.

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