Chinese scientists have released the full design report of a major future particle collider they plan to build the next decade. Pegged at $5 billion, the Circular Electron Positron Collider (CEPC) is expected to begin construction in 2022 and operation in 2030 if the Chinese government agrees to fund it. Once running, the CEPC will function as a ‘Higgs factory’ – a collider adept at producing Higgs bosons for detailed study.
The Institute for High Energy Physics (IHEP), which prepared the report, has also said that once the CEPC completes its physics goals in 10 years, the collider complex can be upgraded to a proton-proton collider complementing the Large Hadron Collider in Europe.
Paralleling China’s announcement was one from Japan. Japanese physicists have been considering hosting another $5 billion machine called the International Linear Collider, also to produce and study Higgs bosons. They said they would announce their final plans by the end of December.
Considering the US doesn’t have plans for Higgs factories in the near future and Europe’s plans are decades away, Asia could be the new home of Higgs boson studies for most of this century.
The CEPC design report is the last of three reports, the first two published in 2015 and 2017. It details what the particle accelerator and collider will be capable of as well as the abilities and goals of the physics experiments that will be run with it. According to the report, the CEPC will consist of an underground tunnel 100 km in circumference. It will receive electrons and positrons (a.k.a. anti-electrons) from a pre-accelerator located on the surface at an energy of 10 GeV, followed by a booster. The particles will then be circulated within two concentric rings in the CEPC tunnel to a higher energy and collided head-on.
The report states that the highest centre-of-mass collision energy will be 240 GeV – i.e. the total energy carried by the electrons and positrons together at the moment of collision. At this energy, the CEPC will function as a Higgs factory, producing about 1 million Higgs bosons. At a collision energy of 160 GeV, it will produce 15 million W bosons and at 91 GeV, over one trillion Z bosons. The Higgs, W and Z bosons are all force-carrier particles within the overarching framework called the Standard Model of particle physics, used to understand the types and interactions of elementary particles.
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The Large Hadron Collider (LHC) in Europe has a much higher collision energy than the CEPC, with one-fourth the circumference, so why are the Chinese trying to build a ‘weaker’ collider? The answer lies in the purposes of the two machines. The LHC operates at the energy frontier, probing higher and higher energies looking for new particles. The CEPC operates at the intensity frontier, producing copious amounts of known particles for precision studies. For example, the LHC first elucidated that the Higgs boson weighs about 125 GeV by producing a few of them at that energy. The CEPC will now operate at this beam energy (approx. half the collision energy) to produce more Higgs bosons and study them in further detail.
According to the report, “The tunnel hosting the collider and booster will be mostly in hard rock so there is a strong and stable foundation to support the accelerators.” The tunnel will also be big enough to accommodate a future proton-proton collider.
There are other crucial differences between the LHC and the CEPC. For example, in particle physics experiments where a charged particle is accelerated through a magnetic field that bends its path, the particle emits the so-called synchrotron radiation. The lighter the particle, the more synchrotron radiation it emits. This means the electrons and positrons in the CEPC will lose more energy as they traverse the ring than the protons do at the LHC. The Chinese scientists plan to extract this radiation for use in other experiments, especially in the study of crystals. Synchrotron radiation has many desirable properties, such as high brilliance and stability, that make them easy to work with.
The IHEP expects the Chinese government will fund 75% of the project while 25% will be sourced from an international collaboration. The project has currently entered the R&D and prototyping phase, which will end in 2022. Then, construction on the CEPC will begin, followed by operationalisation in 2030. The CEPC will run in its Higgs mode for seven years, in the Z boson for two years and in the W boson mode for one. After that, in 2040, the IHEP expects the superconducting magnets needed to upgrade the CEPC to the SPPC – super proton-proton collider – will be ready to install.
The current plan is for the SPPC to operate with a collision energy of 75 TeV, over five-times higher than the LHC’s current level. The forecast factors in the availability of a new class of iron-based superconducting magnets at lower price.
In this period, the LHC will not be dormant. There have been proposals from scientists at CERN, the European lab for nuclear research that runs the LHC, to use the machine in different ways to accommodate the questions physicists need answered. For example, according to one pitch, the LHC can be modified to include a large device called the Energy Recovery Linac (ERL). The ERL will accelerate electrons and collide them with protons accelerated by the LHC.
This configuration of the machine will be called the LHeC – large hadron-electron collider. It will be used to study the strong nuclear force in greater detail. By increasing the energy of the ERL, the LHeC will also be able to operate as a high-luminosity collider by the 2030s, in conjunction with the CEPC. Luminosity is a measure of the number of particles produced in a collision.
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Another proposal suggests that by the late 2030s, when the CEPC is preparing to become the SPPC, the high-luminosity LHeC should be succeeded by the Future Circular Collider (FCC). CERN scientists working on this idea have conceived of a 100-km long tunnel accommodating a circular collider with collision energies comparable to the CEPC.
Multiple scientists have written in favour of the CEPC in the past, and more generally of a post-LHC collider. However, Chen-Ning Yang, a physicist and Nobel laureate, has been a notable detractor. Yang has argued that the CEPC will extract too great a price from China and that the money can be put to better use. Steven Weinberg, another Nobel laureate, countered this argument, saying, “The fundamental character of elementary particle physics makes it very attractive to bright young men and women, who then provide a technically sophisticated cadre available to deal with problems of society.”
The CEPC’s prospects will become clearer by 2021, when China’s next five-year plan will be introduced. Japan will know of the ILC’s fate by next year, when the European Union will finalise a deal to fund the project. Apart from moving the focus of particle physics research to Asia, it will be important for Japan to keep pace with China. If it doesn’t, then the CEPC will get a head-start that the ILC might never be able to catch up with.