XinChou Lou describes the plans for the Circular Electron Positron Collider, a large accelerator complex that would be built in China.
The discovery of the Higgs boson in 2012 by the ATLAS and CMS experiments at CERN was great news for the Chinese particle physics community, which had been contemplating the next major accelerator project in China beyond the Beijing Electron–Positron Collider II (BEPCII). Chinese physicists proposed the Circular Electron Positron Collider (CEPC) in September 2012. CEPC would be a very large accelerator, a Higgs factory producing at least 1 million clean Higgs events, enabling the precision study of the Higgs boson and facilitating the search for new physics beyond the standard model.
The CEPC study group, hosted by the Institute of High Energy Physics (IHEP) in Beijing, has since studied various accelerator design options. They carefully evaluated and balanced the factors of luminosity, component performance, cost and power consumption in the design. The group released the CEPC Conceptual Design Report1,2 in November 2018.
The CEPC is envisioned to be a large international scientific project initiated and hosted by China. The CEPC would be housed in a 100-km-circumference underground tunnel. The proposed accelerator complex consists of a linear accelerator (linac), a damping ring, the booster, the collider and several transport lines (see Fig. 1). There would be two interaction points at which detectors with high precision tracking and high granularity calorimeters will be installed. The tunnel would be ~6 m wide, sufficiently large to allow for a future super proton–proton collider (SppC) without dismantling the electron accelerator, thus creating opportunities for future electron–positron and electron–ion physics programmes.
CEPC compared with other accelerators
Historically, the discovery of new particles, such as the beauty quark and the W and Z bosons at hadron accelerators3, have been followed by precision measurements of these heavy particles at electron–positron colliders. The clean environment of such colliders made possible the observation of the CP (charge conjugation parity) violation in B meson decays at B factories4,5, the constraint on the standard model at the Large Electron–Positron Collider (LEP)6 and other insights into the elementary particle physics world. Now particle physics needs a high-luminosity electron–positron collider — a Higgs factory — that can deliver high statistical samples of clean Higgs collisions to fully probe the Higgs boson with a precision of 1% or better.
The precision with which current experiments such as ATLAS and CMS can study the Higgs boson is limited to approximately 5–10% by the large quantum chromodynamics background and overlaps of multiple proton–proton collisions. This level of precision will remain even after the High-Luminosity Large Hadron collider (HL-LHC) programme comes to completion by 2035.
The CEPC, the Future Circular Collider (FCC), the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) are all proposed Higgs factories. For linear colliders, the longer the accelerator, the higher the energy and the smaller the beam size. The luminosity, which is the performance indicator for colliders, will also increase with the energy. Therefore, the performance of linear colliders is expected to improve as the energy increases, making them particularly advantageous at energies beyond 350 GeV, which is the minimum energy required in an electron–positron collision to produce a pair of top quarks (top quark threshold). However, their performance falls off towards lower energies. In addition, because the electron and positron beam collide only once, in general, linear colliders only allow one detector at a time to take data. The circular colliders CEPC and FCC would offer higher luminosities at the Higgs energy (240 GeV) and superior luminosities at the energies needed to produce pairs of W bosons (~160 GeV) and the Z bosons (91 GeV), and could accommodate multiple detectors simultaneously. Thus, circular colliders offer better Higgs factory options and ideal factories for producing Z bosons (Z mode) and W boson pairs (WW mode), whereas linear colliders are the only hope of an electron–positron accelerator extending energies to multi-tera-electron-volts. Circular and linear electron–positron colliders are complimentary to each other, and together they could cover a wide range of energies and have great potential for physics discoveries.
The CEPC and FCC have many similarities and both have recently published the Conceptual Design Reports. The CEPC expects to benefit from a relatively low cost of tunnel construction in China, and an earlier start-up time in 2030 than the FCC. The CEPC will provide electron–positron collisions at 240 GeV, at which ZH events (in which the Z and Higgs bosons are simultaneously produced) are to occur3. By design, the CEPC can switch to the Z pole and the WW region without major hardware alternations.
In addition to enabling particle physics experiments, the CEPC can simultaneously operate as a powerful synchrotron radiation light source. It will extend the usable synchrotron radiation spectrum into an unprecedented energy and brightness range (fluxes of 1012–1016 photons per second). Two gamma-ray beamlines are included in the Conceptual Design Report1,2.
The plan for CEPC
Before the construction there will be a 5-year research and development period (2018–2022), during which prototypes of key technical components will be built and the industrial infrastructure will be established for manufacturing the large number of components needed in the construction stage.
Construction could start in 2022 and be completed in 2030. After commissioning, a tentative operation plan will be to perform Higgs physics for 7 years, followed by Z mode operations for 2 years and WW mode operations for 1 year. This 10-year operations plan brings us to ~2040, at which point three upgrade paths are possible: increase the collision energy to the top quark threshold (~350 GeV); increase the luminosity at 240 GeV to improve the precision of the Higgs measurements; or upgrade to a 100 TeV SppC, if cost-effective high-field superconducting magnets have been developed by then. For the latter, a strong collaboration of scientists across various disciplines has been established in China devoted to the development of advanced superconducting materials with a special focus on iron-based superconductors (IBS), which may provide a magnetic field of 12–24 Tesla with operating temperatures much higher than the liquid helium temperature. The IBS materials are expected to have very good mechanical properties and be easy to fabricate, thus making the IBS magnets inexpensive to produce and, consequently, the SppC affordable.
There are numerous considerations in choosing the site. At this time, six sites that all satisfy the technical requirements have been considered. The civil engineering and infrastructure construction plan has been developed. The CEPC will build on the expertise and the experience with electron–positron colliders that the IHEP has developed through the BEPC accelerators, and will further benefit from the construction of advanced light sources in China. Lessons learned from LEP, LEPII and SuperKEKB will also help advance the CEPC.
By using superconducting radiofrequency (RF) cavities, high efficiency klystrons, 2-in-1 magnets, combined function magnets, large coil cross section in the quadruples and permanent magnets in the transport lines, the total facility power consumption would be kept below 300 MW. The power conversion efficiency from the grid to the beam will be more than 20%, higher than at other accelerator facilities.
Challenges for CEPC
The optimized design of the CEPC accelerator system places stringent requirements on the quality of the components. The limit on the power budget demands highly efficient klystrons and superconducting RF cavities. The very long accelerator rings need precision low-field magnets to accurately steer the beams, ensuring beam stability. The CEPC research and development programme, and the ever-expanding CEPC Industrial Consortium established in 2017, seek to find the solutions and to get ready for the mass production of components during the construction phase.
Whether China is willing to invest in large science facilities such as the CEPC is not guaranteed. However, the growing economy, the national government’s strong support of basic research and the level of interest of the general public all seem to foster an environment that favours large science projects.
In March 2018, the Chinese government announced a plan to create several Chinese-led, large-scale, global science or engineering projects. The plan calls for the development of six to ten projects, from which several mature projects will be approved for construction. This programme presents an ideal path for CEPC to go forward.
The CEPC could be a place where physicists from many countries work together to explore the science frontier and push our understanding of the fundamental nature of matter, energy and the universe.
The author declares no competing interests.
ATLAS collaboration, projections for measurements of Higgs boson signal strengths and coupling parameters with the ATLAS detector at a HL-LHC: http://cds.cern.ch/record/1956710
CMS collaboration, projected performance of an upgraded CMS detector at the LHC and HL-LHC: contribution to the snowmass process: http://www.slac.stanford.edu/econf/C1307292/docs/submittedArxivFiles/1307.7135.pdf
The CEPC Study Group. CEPC conceptual design report: volume 1 – accelerator. Preprint at arXiv https://arxiv.org/abs/1809.00285 (2018).
The CEPC Study Group. CEPC conceptual design report: volume 2 – physics & detector. Preprint at arXiv https://arxiv.org/abs/1811.10545 (2018).
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The author thanks Y. Wang, J. Gao, and Q. Xu for insightful discussions, the CEPC study group for the knowledge this Comment is based on and M. Lou for editorial assistance. The author acknowledges the support of Chinese Academy of Science Special Grant for Large Scientific Project (113111KYSB20170005), CAS Center for Excellence in Particle Physics, Beijing Municipal Science & Technology Commission (project no. Z181100004218003) and University of Texas at Dallas.