China’s bid for a circular electron–positron collider

Physicists in China have completed a conceptual design report for a 100 km-circumference collider that, in conjunction with a possible linear collider in Japan, would open a new era for high-energy physics in Asia.


La Chine candidate pour un collisionneur circulaire électron-positon

La découverte du boson de Higgs et de sa masse relativement faible fait naître de nouvelles perspectives pour les grands accélérateurs. En septembre 2012, des scientifiques chinois ont proposé un collisionneur électron-positon circulaire (CEPC) de 240 GeV, d’une circonférence de 50 à 70 km, qui serait situé en Chine et permettrait de réaliser des mesures de précision du boson de Higgs. Le tunnel qui serait construit pour cette machine pourrait également accueillir un supercollisionneur proton-proton (SppC), qui viserait à atteindre de nouvelles frontières d’énergie. Le projet du CEPC progresse rapidement, une étude préliminaire de conception devant être publiée en juillet. Ce projet et un éventuel collisionneur linéaire au Japon pourraient marquer le début d’une nouvelle ère pour la physique des hautes énergies en Asie.

Chinese accelerator-based research in high-energy physics is a relatively recent affair. It began in earnest in October 1984 with the construction of the 240 m-circumference Beijing Electron Positron Collider (BEPC) at the Institute of High Energy Physics. BEPC’s first collisions took place in 1988 at a centre-of-mass energy of 1.89 GeV. At the time, SLAC in the US and CERN in Europe were operating their more energetic PEP and LEP electron–positron colliders, respectively, while the lower-energy electron–positron machines ADONE (Frascati), DORIS (DESY) and VEPP-4 (BINP Novosibirsk) were also in operation.

Beginning in 2006, the BEPCII upgrade project saw the previous machine replaced with a double-ring scheme capable of colliding electrons and positrons at the same beam energy as that of BEPC but with a luminosity 100 times higher (1033 cm−2 s−1). BEPCII, whose collisions are recorded by the Beijing Spectrometer III (BES III) detector, switched on two years later and continues to produce results today, with a particular focus on the study of charm and light-hadron decays. China also undertakes non-accelerator-based research in high-energy physics via the Daya Bay neutrino experiment, which was approved in 2006 and announced the first observation of the neutrino mixing angle θ13 in March 2012.

The discovery of the Higgs boson at CERN’s Large Hadron Collider in July 2012 raises new opportunities for a large-scale accelerator. Thanks to the low mass of the Higgs, it is possible to produce it in the relatively clean environment of a circular electron–positron collider – in addition to linear electron–positron colliders such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) – with reasonable luminosity, technology, cost and power consumption. The Higgs boson is the cornerstone of the Standard Model (SM), yet is also responsible for most of its mysteries: the naturalness problem, the mass-hierarchy problem and the vacuum-stability problem, among others. Therefore, precise measurements of the Higgs boson serve as excellent probes of the fundamental physics principles underlying the SM and of exploration beyond the SM.

In September 2012, Chinese scientists proposed a 50–70 km circumference 240 GeV Circular Electron Positron Collider (CEPC) in China, serving two large detectors for Higgs studies. The tunnel for such a machine could also host a Super Proton Proton Collider (SppC) to reach energies beyond the LHC (figure 1). CERN is also developing, via the Future Circular Collider (FCC) study, a proposal for a large (100 km circumference) tunnel, which could host high-energy electron–positron (FCC-ee), proton–proton (FCC-hh) or electron–proton (FCC-he) colliders (see CERN thinks bigger). Progress in both projects is proceeding fast, although many open questions remain – not least how to organise and fund these next great steps in our exploration of fundamental particles.

Precision leap

CEPC is a Higgs factory capable of producing one million clean Higgs bosons over a 10 year period. As a result, the couplings between the Higgs boson and other particles could be determined to an accuracy of 0.1–1% – roughly one order of magnitude better than that expected of the high-luminosity LHC upgrade and challenging the most advanced next-to-next-to-leading-order SM calculations (figure 2). By lowering the centre-of-mass energy to that of the Z pole at around 90 GeV, without the need to change hardware, CEPC could produce at least 10 billion Z bosons per year. As a super Z – and W – factory, CEPC would shed light on rare decays and heavy-flavour physics and mark a factor-10 leap in the precision of electroweak measurements.

The latest CEPC baseline design is a 100 km double ring (figure 3, left) with a single-beam synchrotron-radiation power of 30 MW at the Higgs pole, and with the same superconducting radio-frequency accelerator system for both electron and positron beams. CEPC could work both at Higgs- and Z-pole energies with a luminosity of 2 × 1034 cm–2 s–1 and 16 × 1034 cm–2 s–1, respectively. The alternative design of CEPC is based on a so-called advanced partial double-ring scheme (figure 3, right) with the aim of reducing the construction cost. Preliminary designs for the two CEPC detectors are shown in figure 4.

Concerning the SppC baseline, it has been decided to start with 12 T dipole magnets made from iron-based high-temperature superconductors to allow proton–proton collisions at a centre-of-mass energy of 75 TeV and a luminosity of 1035 cm–2 s–1. The SppC SC magnet design is different to the Nb3Sn-based magnets planned by the FCC-hh study, which are targeting a field of 16 T to allow protons to collide at a centre-of-mass energy of 100 TeV. The Chinese design also envisages an upgrade to 20 T magnets, which will take the SppC collision energy to beyond 100 TeV. Discovered just over a decade ago, iron-based superconductors have a much higher superconducting transition temperature than conventional superconductors, and therefore promise to reduce the cost of the magnets to an affordable level. To conduct the relevant R&D, a national network in China has been established and already more than 100 m of iron-based conductor cable has been fabricated.

The CEPC is designed as a facility where both machines can coexist in the same tunnel (figure 5). It will have a total of four detector experimental halls, each with a floor area of 2000 m2 – two for CEPC and another two for SppC experiments. The tunnel is around 6 m wide and 4.8 m high, hosting the CEPC main ring (comprising two beam pipes), the CEPC booster and SppC. The SppC will be positioned outside of CEPC to accommodate other collision modes, such as an electron–proton, in the far future. The FCC study, which is aiming to complete a Conceptual Design Report (CDR) by the end of the year, adopts a similar staged approach (see CERN thinks bigger).

China on track

Since the first CEPC proposal, momentum has grown. In June 2013, the 464th Fragrant Hill Meeting (a national meeting series started in 1994 for the long-term strategic development of China’s science and technology) was held in Beijing and devoted to developing China’s high-energy physics following the discovery of the Higgs boson. Two consensuses were reached: the first was to support the ILC and participate in its construction with in-kind contributions, with R&D funds to be requested from the Chinese government; the second was a recognition that a circular electron–positron Higgs factory – the next collider after BEPCII in China – and a Super proton–proton collider built afterwards in the same tunnel is an important historical opportunity for fundamental science.

In 2014, the International Committee for Future Accelerators (ICFA) released statements supporting studies of energy-frontier circular colliders and encouraged global coordination. ICFA continues to support international studies of circular colliders, in addition to support for linear machines, reflecting the strategic vision of the international high-energy community. In April 2016, during the AsiaHEP and Asian Committee for Future Accelerators (ACFA) meeting in Kyoto, positive statements were made regarding the ILC and a China-led effort on CEPC-SppC. In September that year, at a meeting of the Chinese Physics Society, it was concluded that CEPC is the first option for a future high-energy accelerator project in China, with the strategic aim of making it a large international scientific project. Pre-conceptual design reports (pre-CDRs) for CEPC-SppC were completed at the beginning of 2015 with an international review, based on a single ring-based “pretzel” orbit scheme. A CEPC International Advisory Committee (IAC) was established and, in 2016, the Chinese Ministry of Science and Technology (MOST) allocated 36 million RMB (€4.6 million) for the CEPC study, and in 2018 another 32 million RMB (€4.1 million) has been approved by MOST.

Ensuring that a large future circular collider maximises its luminosity is a major challenge. The CEPC project has studied the use of a crab-waist collision scheme, which is also being studied for FCC-ee. Each of the double-ring schemes for CEPC have been studied systematically with the aim of comparing the luminosity potentials. On 15 January last year, CEPC-SppC baseline and alternative designs for the CDR were decided, laying the ground for the completion of the CEPC CDR at the end of 2017. Following an international review in June, the CEPC CDR will be published in July 2018.

While technical R&D continues – both for the CEPC machine and its two large detectors – a crucial issue is how to pay for such a major international project. In addition to the initial funding from MOST, other potential channels include the National Science Foundation of China (NSFC), the Chinese Academy of Sciences (CAS) and local governments. For example, two years ago Beijing Municipal allocated more than 500 million RMB (€65 million) to the Institute of High Energy Physics for superconducting RF development, and in 2018 CAS plans to allocate 200 million RMB (€26 million) to study high-temperature superconductors for magnets, including studies in materials science, industry and projects such as SppC. While not specifically intended for CEPC-SppC, such investments will have strong synergies with high-energy physics and, in November 2017, the CEPC-SppC Industrial Promotion Consortium was established with the aim of supporting mutual efforts between CEPC-SppC and industry.

A five-year-long Technical Design Report (TDR) effort to optimise the CEPC-SppC design and technologies, and prepare for industrial production, started this year. Construction of CEPC could begin as early as 2022 and be completed by the end of the decade. CEPC would operate for about 10 years, while SppC is planned to start construction in around 2040 and be completed by the mid-2040s. The CEPC-SppC TDR phase after the CDR is critical, both for key-component R&D and industrialisation. R&D has already started towards high-Q, high-field 1.3 GHz and 650 MHz superconducting cavities; 650 MHz high-power high-efficiency klystrons; 12 kW cryogenic systems, 12 T iron-based high temperature superconducting dipoles, and other enabling technologies. Construction of a new 4500 m2 superconducting RF facility in Beijing called the Platform of Advanced Photon Source began in May 2017 to be completed in 2020, and could serve as a supporting facility for different projects.

International ambition

CEPC-SppC is a Chinese-proposed project to be built in China, but its nature is an international collaboration for the high-energy physics community worldwide. Following the creation of the CEPC-SppC IAC in 2015, more than 20 MoUs have been signed with many institutes and universities around the world, such as the Budker Institute of Nuclear Physics (BINP; Russia); National Research Nuclear University MEPhI (Moscow, Russia) and the University of Rostock (Germany).

In August 2017, ICFA endorsed an ILC operating at a centre-of-mass energy of 250 GeV (ILC250), with energy-upgrade possibilities in the future (CERN Courier January/February 2018 p7). Although CEPC and ILC250 start with the same energy to study the Higgs boson, the ultimate goals are totally different from each other: SppC is for a 100 TeV proton–proton collider and ILC is a 1 TeV (maximum) electron–positron collider. The existence of both, however, would offer a highly complementary physics programme operating for a period of decades. The specific feature of CEPC is its small-scale superconducting RF system, (and its relatively large AC power consumption (300 MW for CEPC compared to 110 MW for ILC250). As for the cost, CEPC in its first phase includes part of the cost of SppC for its long tunnel, whereas ILC would upgrade its energy by increasing tunnel length accordingly later.

Deciding where to site the CEPC-SppC involves numerous considerations. Technical criteria are roughly quantified as follows: earthquake intensity less than seven on the Richter scale; earthquake acceleration less than 0.1 g; ground surface-vibration amplitude less than 20 nm at 1–100 Hz; granite bedrock around 50–100 m deep, and others. The site-selection process started in February 2015, and so far six sites have been considered: Qinhuangdao in Hebei Province; Huangling county in Shanxi Province; Shenshan Special District in Guangdong Province; Baoding (Xiongan) in Heibei Province; Huzhou in Zhejiang Province and Changchun in Jilin Province, where the first three sites have been prospected underground (figure 6). More sites, such as Huzhou in Zhejiang Province, will be considered in the future before a final selection decision. According to Chinese civil construction companies involved in the siting process, a 100 km tunnel will take less than five years to dig using drill-and-blast methods, and around three years if a tunnel boring machine is employed.

2018 is a milestone year for Higgs factories in Asia. As CEPC completes its CDR, the global high-energy physics community is waiting for a potential positive declaration from the Japanese government, by the end of the year, on their intention to host ILC250 in Japan, upgradable to higher energies. It is also a key moment for high-energy physics in Europe. FCC will complete its CDR by the end of the year, while CLIC released an updated 380 GeV baseline-staging scenario (CERN Courier November 2016 p20), and the European Strategy for Particle Physics update process will get under way (CERN Courier April 2018 p7). Hopefully, both ILC250 and CEPC-SppC will be included in the update together with FCC, while with respect to the US strategy we are looking forward to the next “P5” meeting following the European update.

During the past five years, CEPC-SppC has kept to schedule both in design and R&D, together with strong team development and international collaboration. On 28 March this year, the Chinese government announced the “Implementation method to support China-initiated large international science projects and plans”, with the goal of identifying between three and five preparatory projects, one or two of which will be put to construction, by 2020. Hopefully, CEPC will be among those selected.

Further reading

T J Bian 2017 Int. J. Mod. Phys. A 32 1746009.

J Gao 2016 IHEP-AC-LC 2016-002.

J Gao 2017 Int. J. Mod. Phys. A 32 1746003.

F Su 2017 Int. J. Mod. Phys. A 32 1746005.

D Wang 2017 Int. J. Mod. Phys. A 32 1746006.

Y F Wang 2018 Talk given at FCC Week 2018, Amsterdam.

Y W Wang 2017 Int. J. Mod. Phys. A 32 1746004.

About the author

Jie Gao, Institute of High Energy Physics and Chinese Academy of Sciences, University of Chinese Academy of Sciences.