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CEPC: A Proposed Circular Electron-Positron Collider as a Higgs Factory
Gao Jie
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DOI: 10.22661/AAPPSBL.2020.30.4.56

CEPC: A Proposed Circular Electron-Positron Collider
as a Higgs Factory

Gao Jie 1,2
1 Institute of High Energy Physics, CAS, 100049, Beijing, China
2 University of Chinese Academy of Sciences, CAS, 100049, Beijing, China


On July 4, 2012, a Higgs boson particle was found at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), and the Higgs era began. While there are proposed electron positron linear colliders, such as the International Linear Collider (ILC) and Compact Linear Collider (CLIC), due to the low mass of Higgs particles, circular electron positron colliders are becoming possible. In this paper we will introduce the Circular Electron Positron Collider (CEPC), a Higgs factory in China. The Future Circular Collider (FCC) at CERN will be briefly mentioned as well.

PACS numbers: 29.20.-c; 29.20.db.


The discovery of the Higgs boson at the European Organization for Nuclear Research (CERN) Large Hadron Collider (LHC) on July 4, 2012, created new opportunities for large-scale colliders. Due to the low mass of the Higgs boson, it is possible to produce Higgs bosons in the relatively clean environment of a circular electron-positron collider, which was invented in the early 60s, with reasonable luminosity, technology, cost and power consumption. In addition, linear electron-positron colliders such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) have been proposed.

The Higgs boson is the cornerstone of the Standard Model (SM), yet it is also related to many unknown mysteries, such as the mass hierarchy between the weak scale and the Planck scale, the nature of the electroweak phase transition, the naturalness problem, the vacuum-stability problem and dark matter issues [1-8]. Precise measurements of the Higgs boson serve as excellent probes of the fundamental physics principles underlying the SM and of exploration beyond the SM; consequently, construction of a Higgs factory as soon as possible is the most urgent objective of the international high energy physics community.

In September 2012, Chinese scientists proposed the construction of the 240 GeV Circular Electron Positron Collider (CEPC), which would provide two large detectors for Higgs studies. The tunnel could also host a Super Proton-Proton Collider (SppC) that could co-exist with CEPC to reach energies beyond the LHC.

CEPC is anticipated to be a Higgs factory capable of producing one million clean Higgs bosons over a 10-year period. If this forecast is correct, then the couplings between the Higgs boson and other particles could be determined to an accuracy of 0.1-1%, which would be roughly one order of magnitude better than that expected of the high-luminosity LHC upgrade. By lowering the center-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 flavor physics and would create a factor of 10 increase in the precision of electroweak measurements.

CEPC is a large international project initiated and hosted by China. It was presented for the first time to the international community at the International Committee for Future Accelerators (ICFA) Workshop "Accelerators for a Higgs Factory: Linear vs. Circular" (HF2012) in November 2012 at Fermilab. A Preliminary Conceptual Design Report (Pre-CDR, the White Report) [9] was published in March 2015, and was followed by a Progress Report (the Yellow Report) [10] in April 2017, where the choice for the accelerator baseline for CEPC was made [11, 12]. The Conceptual Design Report (CEPC Accelerator CDR, the Blue Report) was completed by hundreds of scientists and engineers after international review from June 28-30, 2018 and was formally released in November 2018 [13]. In May 2019, the accelerator and physics/detector documents for CEPC were submitted to the European High Energy Physics Strategy Workshop for worldwide discussions [14].

The Future Circular Collider (FCC) at CERN builds on the success and the experience gained from operating the Large Electron-Positron (LEP) and Large Hadron Collider (LHC). FCC intends to integrate the complementary qualities of circular electron-positron and proton-proton colliders within a largely common, and partly existing, infrastructure [15]. FCC's kick-off meeting took place in February 2014, in Geneva, Switzerland, and the FCC Conceptual Design Reports were completed in Jan. 2019 [16].

FCC-ee will collide e+e- pairs at several center-of-mass energies, producing large numbers of Z0 bosons, Wpairs and top quarks (t) in a clean environment, thereby enabling their high precision measurement. FCC-ee also forecasts the production of Higgs bosons at = 240 GeV, and through W-boson fusion at 365 GeV. In addition, the challenging direct Higgs production at the Higgs mass of 125 GeV, is being investigated with the help of a 'monochromatization' scheme. Subsequently, FCC-hh will collide protons at = 100 TeV. For studies of quark-gluon plasma and of strong interactions at a high density and temperature, the FCC-hh could support heavy ion collisions. For further searches of new physics, and to refine our knowledge of proton structure, FCC could allow electron-proton collisions at = 3.5 TeV [15].


The design of CEPC's lattice began with a single ring pretzel scheme of head-on collision, which is similar to how LEP and the CEPC Pre-CDR was completed based on a single ring scheme without reaching the design luminosity goal due to small dynamic apertures [9]. Making use of the crab-waist collision scheme, with the aim of reducing the collider's power consumption, increasing luminosity and dynamic apertures, different and new schemes were proposed and studied, such as a partial double ring (PDR), an advanced partial double ring (APDR), and a fully partial double ring (FPDR), as shown in Fig. 1, and the luminosity potentials for each scheme were systematically studied as shown in Fig. 2. The dependence of the luminosities of different energies with respect to different circumferences was studied also in detail, as shown on Fig. 3, and finally, the circumference of CEPC was chosen to be 100 km. The detailed design process and comparisons were documented in the CEPC-SppC Progress Report [10].


Fig. 1: Different scheme layouts for CEPC.


Fig. 2: The luminosity potentials for different schemes.


Fig. 3: The luminosity potentials for different energies compared to the collider circumferences.


Fig. 4: CEPC CDR base-line layout.

The CEPC Conceptual Design Report (CDR) baseline design is a 100 km fully partial double ring, as shown in Fig. 4, with 30 MW of single-beam synchrotron-radiation power at the Higgs energy, 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 3횞1034 cm-2 s-1 and 16 (32)횞1034 cm-2 s-1, for a 2 T or 3 T detector dipole magnetic field, respectively, with the CDR parameters shown in Table 1.


Table 1: CEPC CDR parameters.

Concerning the SppC baseline, as shown in Fig. 2, we decided to start with 12 T dipole magnets made from iron-based high-temperature superconductors to allow proton-proton collisions at a center of-mass energy of 75 TeV and a luminosity of 1035 cm-2 s-1. The magnet design of SppC SC 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 center-of-mass energy of 100 TeV.

The Chinese design also envisages an upgrade to 20 T magnets, which would 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 they would reduce the cost of the magnets to an affordable level. To conduct the relevant research and development (R&D), a national network in China has been established, and more than 100 m of iron-based conductor cable has been already fabricated.


Fig. 5: The layout of SppC.

The CEPC-SppC is designed as a facility where both machines can co-exist in the same tunnel, as shown in Fig. 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 possible collision modes, such as electron-proton collision, in the far future.


According to the recommendations from the CEPC International Advisory Committee (IAC) in 2019, the CEPC accelerator formally entered into the Technical Design Report (TDR) phase, which is scheduled to be completed at the end of 2022 [17].

In the TDR phase, we will need to continue to optimize the design of the parameters, both for Higgs and Z luminosities, taking into account the possibility of a ttbar option as well. From an accelerator physics point of view, the way toward higher luminosity at Higgs energy is to reduce the colliding beam transverse beam size at the interaction point (IP) by reducing 棺y to one millimeter, and even to a sub-millimeter in the future, with the challenges of higher nonlinear effects and dynamic aperture reductions. The new Higgs energy parameter of 棺y = 1 mm is shown in Table 2, and the dynamic aperture study is shown in Fig. 6.


Table 2: CEPC's new Higgs parameters.


Fig. 6: CEPC's dynamic aperture of 棺y = 1 mm.

As for Z-pole energy, luminosity is increased by increasing the colliding beam current, and this is done by optimizing the beam-environment interaction effects, i.e., by superconducting radio frequency (SCRF) cavities and vacuum chambers. The new parameters for high luminosity for Z-pole energy by using single cell 650 MHz SCRF cavities instead of 2 cell ones, such as CDR, is shown in Table 3.


Table 3: CEPC's new parameters for Z-pole energy.

The SCRF system's corresponding new parameters are shown in Table 4. CEPC applies the aforementioned principle to use the same hardware for Higgs, W, and Z-pole energies, and therefore, a single cell 650 MHz SC cavity for Higgs running mode with an accelerating gradient of 40 MV/m and Q0 of 3횞1010 is needed. To reach this high field and high Q requirements, large grain Nb materials and special surface treatments are under study.


Table 4: CEPC SCRF's new parameters for high luminosity of Z.

CEPC's booster's performance and cost are also subjects of optimization design in the TDR phase. New booster parameters are shown in Table 5. The 1.3 GHz SCRF system parameters for the booster are shown in Table 6. The 2 K helium distribution cryogenic system for a booster and collider SCRF system is shown in Fig. 7, where four 18 kW@4.5 K cryogenic plants are needed.


Table 5: (left) Main parameters for the booster at injection energy. (right) Main parameters for the booster at extraction energy.


Table 6: CEPC booster SCRF system parameters.


Fig. 7: CEPC SCRF cryogenic system layout.

The CEPC CDR 10 GeV linac injector and parameters are shown in Fig. 8, where there is a positron damping ring and its parameters are shown in Fig. 9.


Fig. 8: CEPC's linac injector layout and parameters.


Fig. 9: CEPC's positron damping ring layout and parameters.

To balance of the cost of CEPC and the booster dipole magnet technology feasibilities, in CEPC CDR, injector linac energy was chosen to be 10 GeV, and the booster dipole starting magnetic field strength was 28 G. To conduct R&D on high precision booster low field dipoles is one of the key R&D issues for magnets, which will be discussed in the following TDR R&D section. As backups for higher booster injection energy, two options have been studied; one is to increase the linac exit energy to 20 GeV by adding a C-band linac after a S-band linac as shown in Fig. 10, with parameters shown in Table 7. The accelerating field gradients are 21 MV/m for the S-band and 45 MV/m for the C-band. The advantage of the C-band is its higher accelerating gradient, but the disadvantage is its higher wake-field which has stronger instability effects than those in the S-band linac. Another backup option to increase the booster injection energy is to add a plasma accelerator system after the 10 GeV S-band linac to provide electron and positron beams with energies up to 45 GeV, as shown in Fig. 11.


Fig. 10: CEPC's 20 GeV linac backup scheme layout.


Table 7: CEPC 20 GeV linac backup parameters.


Fig. 11: CEPC's plasma injector scheme.

The CEPC injection/extraction system includes linac to damping ring, damping ring to linac, linac to booster, booster to collider ring, and collider ring to beam dump, etc. The kicker and Septa magnet types corresponding to different injection/extraction regions are shown in Table 8.


Table 8: CEPC injection and extraction system.

The CEPC Machine Detector Interface (MDI) is composed of the interaction region beam pipes, anti-solenoids, SC quadrupoles, bending magnets, and a luminosity calorimeter (LumiCal), etc. The MDI design is very critical of the CEPC detector experiments' at background level due to various effects coming from colliding beam interactions and synchrotron radiation effects in the MDI region.


If CEPC CDR is defined as design that works on paper, then CEPC TDR is defined as that which is technically feasible by the completion of TDR R&D phase at the end of 2022 according to the CEPC timeline, as shown in Fig.12.


Fig. 12: CEPC's time line.

As for the CEPC's Accelerator TDR key R&D priority issues, they could be summarized as follows:

1) CEPC 650 MHz 800 kW high efficiency klystron (80%) (no commercial products)
2) High precision booster dipole magnet (critical for booster operation)
3) CEPC 650 MHz SC accelerator system, including SC cavities and cryomodules
4) Collider dual aperture dipole magnets, dual aperture quadrupoles, small dimension sextuples
5) Vacuum chamber system
6) SC magnets including cryostat
7) MDI mechanical system
8) Collimators
9) Linac components
10) Civil engineering design
11) Siting
11) Plasma injector
12) 18 KW@4.5 K cryoplant

In the following section we will give detailed progress status regarding R&D at CEPC.


CEPC's TDR is mainly focused on key hardware technology R&D to make the technology is available and ready for industrialization before construction for the project starts.

CEPC's linac injector

We start with the S-band injector. The new S-band accelerating structure has been fabricated and tested in high-power with an accelerating gradient of 20 MV/m, which satisfies the CEPC CDR design goal, and the conventional positron source flux concentrator from 6 T to 0.5 T and its solid-state pulsed power generator of 15 kA/15 kV/5 關s have been fabricated, as shown in Fig. 13.


Fig. 13: Key components for CEPC's linac positron source and high field accelerating structure.

As for the plasma injector, we started electron plasma acceleration experiments that will be carried out in the linear accelerator of Shanghai's soft X-ray FEL facility, as shown in Fig. 14, where there is a low emittance rf gun. The experiments will be carried out in late 2020. The demonstration of the plasma acceleration of positron beams might be conducted in facilities such as FACET-II at SLAC.


Fig. 14: The experimental layout for CEPC's plasma electron accelerator.

CEPC's booster magnets

The CEPC CDR injection-energy to booster of 100 km circumference is 10 GeV for both electron and positron beams. The starting dipole magnetic field in the booster is 28 G, which is very low and the precision required by the booster is therefore very high. The R&D on high precision booster dipoles was carried out by two types of designs, one with an iron core and another without an iron core, as shown in Fig. 15. Two types of 1 m long dipole magnets were fabricated and tested as shown in Fig. 16. It turned out that the dipole without the iron core reached the design precision objectives and the dipole with the iron core has not yet been achieved the goal and improvement continues. The good news is that the material used as the iron core satisfies the requirement, and only the detailed iron core dimensions should be adjusted in the next tries.


Fig. 15: CEPC booster dipole magnets, (left) with an iron core, (right) without an iron core.


Fig. 16: CEPC booster dipole magnets' test models with test results, on the left with iron core and on the right without iron core.

CEPC collider ring magnets

The CEPC main collider rings employ dual aperture dipoles and quadrupoles, and the difficulties are their precision, material quality and mechanical rigidity for a magnet of about 6 meters long. For dual aperture dipole and quadrupoles, 1 meter long models for both types of magnets have been fabricated and further improvements are needed to satisfy the design requirements as shown in Fig. 17. The next step is the real size magnet model design and fabrication.


Fig. 17: CEPC collider ring dual aperture dipole and quadrupole magnets.

CEPC's collider ring electro-magnet separator

CEPC's CDR design for different energy operation modes, such as Higgs, W and Z-pole, adopts different lattice configurations; for example, for a Higgs run, a colliding ring is fully partial double ring (electron and positron beams share the same SCRF accelerator system), and for W and Z modes the colliding ring uses a double ring scheme (electron and positron beams have their own independent SCRF accelerator system). For the Higgs operation mode, the key elements, which combine the two independent electron and positron beam pipes into a single beam pipe in the SCRF region, are electro-magnet separators, as shown in Fig. 18, and the test model will be fabricated in 2020. The key difficulty is the high order modes' heating problem in the separator generated by the passing beams in the Z-mode operation. The alternative solution is to use kickers to replace the electro-magnet separator, which is studied in parallel.


Fig. 18: CEPC's collider ring electro-magnet separator.

CEPC's collider ring vacuum chamber system

CEPC's vacuum chamber system is critical for pumping the vacuum and the electron cloud effects due to synchrotron light induced electrons trapped by positron beams. The vacuum chamber will be coated with a non-evaporable getter (NEG) coating. The 6 m long copper and aluminum vacuum chambers have been fabricated and a NEG coating of 1 關m on copper vacuum chamber test is underway as shown in Fig. 19.


Fig. 19: CEPC vacuum chamber system.

CEPC's booster and collider ring SCRF accelerator system

CEPC's booster uses a 1.3 GHz SC accelerator system with cryomodules similar to those used in an ILC composed of 9-cell TESLA type SC cavities. The CEPC collider ring uses a 650 MHz SC accelerator system with cryomodules composed of 2-cell SC cavities in CDR or single cavities in TDR studies. For CDR 650 MHz accelerator R&D, a test cryomodule of two 2-cell cavities was constructed as shown in Fig. 20. High-power couplers for 1.3 GHz and 650 MHz were also fabricated and have passed a high-power test as shown in Fig. 21.


Fig. 20: CEPC 650 MHz SC test cryomodule.

As for CEPC's SC accelerator system test experiment with beam and cryomodule assembly, IHEP has in development a new 4500 m2 superconducting technology laboratory, where cavity vertical and horizontal tests, cryomodule assembly, and a SC accelerator test facility where the beam will be incorporated, as shown in Fig. 22.


Fig. 21: CEPC's 650 MHz SC test cryomodule components.


Fig. 22: CEPC's new SC technology laboratory.

CEPC collider ring 650 MHz high efficiency klystron

CEPC, as a circular collider Higgs factory, obtains its luminosity mainly through colliding beam powers that are provided by SC cavities powered by klystrons. The wall plug AC power largely depends on the klystron efficiency. CEPC's 650 MHz CW 800 kW klystron efficiency goal is larger than 80%. As a first 650 MHz high power CW klystron, a single beam test klystron of 800 kW with efficiency of 65% was designed, fabricated and tested as shown in Figs. 23 and 24, and the output power reached a pulsed power of 800 kW (400 kW CW due to test load limitation), an efficiency of 62% and a band width > 짹0.5 MHz. As for CEPC's 800 kW CW klystron of 80% efficiency, a multi beam klystron (MBK) was designed and an efficiency of 80.7% was achieved by 3D simulation as shown in Fig. 25.


Fig. 23: CEPC's first single beam 650 MHz test klystron design.


Fig. 24: CEPC's new SC technology laboratory and fabricated klystron.


Fig. 25: CEPC 650 MHz multi beam klystron design.

CEPC MDI system

One of the key systems of CEPC is the machine (accelerator) and detector interface (MDI) as shown in Fig. 26, where many critical issues are detector backgrounds, luminosity and beam position monitoring, hardware installations, etc. as shown in Fig. 27. SC magnets in the MDI region, such as QD0 and QD1, are also subjects for R&D.


Fig. 26: CEPC Machine and Detector Interface (MDI) layout.


Fig. 27: CEPC MDI mechanical installation.

SppC status and CEPC-SppC compatibility

Concerning the SppC baseline, we have decided to start with 12 T dipole magnets made from iron-based high-temperature superconductors to allow proton-proton collisions at a center-of-mass energy of 75 TeV and a luminosity of 1035 cm-2 s-1 as shown in Table 9. 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 center-of-mass energy of 100 TeV.

The SppC design also envisages an upgrade to 20 T magnets in the future. 12 T dipole magnets made from iron-based high-temperature superconductors have been designed as shown in Fig. 28. The goal of SppC iron-based high-temperature dipoles is to reach the design goal in about 10~15 years from now on.

As for the compatibility of CEPC and SppC installed in the same tunnel, to maintain the same circumferences for both CEPC and SppC, which permits the electron proton collision in the far future, the relation of the lengths between SppC bypass and collimation section has been studied and our solutions are shown in Fig. 29.


Table 9: SppC parameters (CDR parameter is phase I).


Fig. 28: SppC's 12 T Fe-based dipole magnet design.


Fig. 29: CEPC-SppC's compatibility relationship.

CEPC's civil engineering design

CEPC's civil engineering design is composed of the tunnel system design and components' numerical installation design. Numerical modelling design techniques (BIM) have been used as shown in Figs. 30 and 31. To verify the installation techniques and precision of CEPC components into CEPC tunnel, a 40 m long 1:1 scale mockup tunnel has been designed as shown in Fig. 32. Due to the long tunnel and the huge number of components that require high installation precision, we found that installation of the components is the critical path of the CEPC project time line as shown in Fig. 33, and the high precision alignment and high efficiency installation technologies are under R&D.


Fig. 30: CEPC tunnel civil engineering design.


Fig. 31: CEPC tunnel components' installation BIM design.


Fig. 32: CEPC tunnel mockup layout.


Fig. 33: CEPC timeline with installation as a critical path.

CEPC site selection

Deciding where to site CEPC-SppC involves numerous considerations. The technical criteria were roughly quantified as follows: the earthquake intensity should be less than seven on the Richter scale; the earthquake acceleration should be less than 0.1 g; ground surface-vibration amplitude should be less than 20 nm at 1-100 Hz; and that granite bedrock should be around 50-100 m depth. Other criteria were also addressed, such as social environment factors. The site-selection process started in February 2015, and so far six sites have been considered as shown in Fig. 34: Qin Huangdao in Hebei Province; Huangling county in Shanxi Province; Shenshan Special District in Guangdong Province; Huzhou in Zhejiang Province and Changchun in Jilin Province, where the first three sites have been prospected underground. The distribution of the selected sites' yearly average temperatures ranges from 7oC (Jilin) to 23oC (Shenshan).

Corresponding to three sites, Qinhuangdao, Huzhou, and Changsha, three BIM design of CEPC videos have been done [18-20].


Fig. 34: CEPC's site selection status.

International collaboration and perspectives

CEPC is a Chinese proposed project but also an international collaboration in the sense that international participation has been encouraged from the very beginning. We see the project as being an open door on a continually moving train; we welcome every partner and collaborator on board at any time but we will continue without stopping or waiting.



Fig. 35: Electron-positron circular colliders: history and perspectives.

As for CEPC's place in history, it is worthwhile to make a complete review of different colliders, especially electron positron colliders that became operational in the beginning of the 60s as shown in Fig. 35. By entering the Higgs era in 2012, electron positron colliders, such as CEPC and FCC-ee, together with ILC and CLIC, will help us search for the fundamental laws for the unknown Universe.

CEPC-SppC vs FCC-ee/hh

CEPC-SppC and FCC-ee/hh are both 100 km in circumference, and there are many similarities between the two proposals. The design philosophy, the optimization criteria and operation scenario, however, are not exactly the same, and it is interesting to provide a brief comparison between them as shown in Tables 10 and 11 [21]. On June 19, 2020, the Update of the European Strategy for Particle Physics (CERN-ESU-013, June 2020) was formally realized, and it concluded that an electron-positron Higgs factory is the highest-priority next collider. As part of their long-term perspective, the European particle physics community has aspirations to operate a proton-proton collider at the highest achievable energy. The updated European Strategy for Particle Physics is highly similar to the strategy of Chinese particle physics, which was proposed in Sept. 2012, where CEPC is to be operated as a Higgs factory first, and as a SppC afterwards.


Table 10: Key parameters of proposed circular electron positron colliders.


Table 11: Collider parameters for FCC-hh and SppC.


In this paper, we give a full spectrum review of the status of CEPC-SppC after the CDR was released, with a detailed section regarding the progress made in research and development for CEPC-SppC. CEPC TDR will be completed at the end of 2022 and construction is planned to be completed around 2030. FCC is also briefly reviewed, with comparisons with CEPC-SppC.

Acknowledgements: The author thanks the Circular Electron Positron Collider (CEPC) - Super Proton-Proton Collider (SppC) team, international collaborators, the CEPC Institution Board, the CEPC Steering Committee, the CEPC International Advisory Committee, and the CEPC International Review Committee.

This work is supported by (1) the National Natural Science Foundation of China (NSFC):11975252; (2) the National Key Program for S&T Research and Development (Grant No. 2016YFA0400400); and (3) the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-SLH004).


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