China plans a Higgs factory

Beijing’s Institute of High Energy Physics has unveiled plans for a Circular Electron Positron Collider (CEPC). A two-volume conceptual design report released last month details the accelerator and detector, physics performance, and site requirements. Longer term, the beam lines in the 100 km tunnel could be retooled to make a 100 TeV proton–proton collider.

The estimated $6 billion project still awaits approval from China’s Ministry of Science and Technology (MOST), which in two years will weigh the CEPC against several other contenders. If it goes ahead, it will be “a truly major high-energy physics facility that Asia deserves,” says George Wei-Shu Hou of National Taiwan University and chair of the Asia-Pacific High Energy Physics Panel.

The CEPC would accelerate electrons and positrons in opposite directions in separate beam pipes and collide them at two points. It would run as a Higgs factory with collisions at 240 GeV, producing at least a million Higgs bosons over a planned seven years. With so much statistical power, researchers would obtain precise measurements of mass, spin, coupling, and other properties (see the article by Joe Lykken and Maria Spiropulu, Physics Today, December 2013, page 28 ). Electron–positron collisions are much cleaner than the messy proton–proton events at the Large Hadron Collider (LHC), where the Higgs was first observed in 2012.

The Higgs phase would be followed by three years running at lower energies—160 GeV to investigate W bosons and 91 GeV to probe Z bosons. Decay of the Z bosons would produce billions of bottom quarks, charm quarks, and tau leptons. The large samples would provide unprecedented precision measurements and could lead to discoveries of deviations from the standard model, says project director Xinchou Lou.

The facility would rely largely on mature technologies. Chinese industry and academia have the know-how to build much of the accelerator and detectors, says Caltech’s Barry Barish, a Nobel laureate and member of the CEPC international advisory committee. But to realize some technical aspects of the design, he adds, such as the high-temperature superconducting magnets and some specialized accelerator technologies, international partnerships will be essential. Lou agrees, pointing to the plasma-based wakefield acceleration technology being considered for the injector.

The CEPC fits neatly into a call for large international projects by MOST. If it gets the green light, construction could begin in 2022, with operations starting as soon as 2030. And the project would be open to partners. “The Chinese system is not used to functioning as an international team,” says Hou. “It’s cultural. This will be a challenge for them, which is part of the point.”

In direct competition as a Higgs factory is the proposed International Linear Collider (ILC), on which a decision in Japan is expected imminently (see Physics Today, March 2018, page 25 ). The ILC could be upgraded to higher electron–positron collision energies, whereas the CEPC could be converted to a hadron machine with energy nearly an order of magnitude higher than that of the LHC. Each project gives the other credibility, says Hou, and it’s likely that neither China nor Japan wants to cede dominance in the field to the other.