Circular Electron Positron Collider
The Circular Electron Positron Collider (CEPC) is a proposed double-ring electron-positron collider designed as a Higgs factory to enable precision measurements of the Higgs boson and other fundamental particles, featuring a circular underground tunnel approximately 100 km in circumference to be hosted in China.[1] Initiated by Chinese physicists in September 2012 shortly after the discovery of the Higgs boson at the Large Hadron Collider, the CEPC aims to produce around 2.6 million Higgs events, 2.5 trillion Z-boson events, and 130 million W-boson events over a multi-year operation, facilitating high-precision studies of electroweak symmetry breaking, searches for physics beyond the Standard Model, and tests of fundamental symmetries.[2][3] The collider would operate in multiple modes, including at 240 GeV center-of-mass energy for Higgs production, 91 GeV for Z bosons, and 160 GeV for W bosons, with peak luminosities reaching up to 116 × 10³⁴ cm⁻²s⁻¹ in the Z mode, supported by two interaction points equipped with advanced detectors featuring innovations such as high-resolution calorimeters and low-power readout chips.[1][4] Technical design reports for both the accelerator and detectors were completed between 2018 and 2023, with the detector technical design report finalized in October 2025 following international review, marking significant progress in R&D including prototyping of key components like silicon-tungsten electromagnetic calorimeters with 2-3% energy resolution.[5][4] As of November 2025, however, the project remains without formal government approval or inclusion in China's 2026-2030 five-year plan, facing challenges such as high estimated costs of 36.4 billion yuan (approximately US$5.1 billion) and debates over scientific priorities, with lead scientists indicating a potential resubmission in 2030 or possible collaboration on Europe's Future Circular Collider if that project advances first.[6][5] Construction, if approved, is projected to span 2027-2035, followed by commissioning and experiments through 2049.[1]History and Development
Proposal and Initial Studies
The discovery of the Higgs boson at the Large Hadron Collider (LHC) in July 2012 highlighted the need for a dedicated facility to perform high-precision studies of this particle, as hadron colliders like the LHC produce complex event environments that limit measurement accuracy.[7] In response, the Chinese high-energy physics community, motivated by the opportunity to leverage electron-positron collisions for cleaner signatures and tunable center-of-mass energies, began conceptualizing a circular collider optimized for Higgs production.[8] This initiative aimed to achieve precisions unattainable at the LHC, focusing on thresholds for Z-pole, W-pair, and Higgs processes up to around 250 GeV.[9] In September 2012, physicists at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), led by director Wang Yifang, formally proposed the Circular Electron Positron Collider (CEPC) as a Higgs factory.[8] The proposal emphasized the advantages of a circular design over linear alternatives like the International Linear Collider (ILC), citing cost-effectiveness and synergy with existing Chinese accelerator expertise from the Beijing Electron Positron Collider (BEPC) and BEPCII.[10] Initial motivations included providing a complementary facility to global efforts, with e+e- collisions offering reduced background noise for electroweak and Higgs precision measurements.[7] From 2012 to 2014, the project entered a conceptual design phase, involving preliminary feasibility studies coordinated by a provisional management team primarily from Chinese institutions.[8] This period featured international consultations, including discussions on alignment with the ILC and European initiatives, as well as early site surveys near Qinhuangdao, approximately 300 km from Beijing, selected for its stable geology and low seismic activity.[8] Key workshops, such as the Xiangshan Forum in June 2013 and subsequent meetings in September and December 2013, gathered community input to refine the vision.[8] A pivotal milestone came in 2014 with the release of an initial white paper outlining basic parameters, including a proposed tunnel circumference of 50-70 km to accommodate the CEPC ring at beam energies up to 120 GeV per beam.[8] This document, prepared under Wang Yifang's steering committee, targeted R&D funding and set the stage for deeper engineering assessments, while advocating for government support to position China as a leader in particle physics infrastructure.[8] These early efforts established the CEPC as a strategic response to post-Higgs physics priorities, emphasizing international collaboration without delving into detailed technical specifications.[9]Design Evolution and Key Reports
The design of the Circular Electron Positron Collider (CEPC) evolved through a series of iterative studies beginning in the mid-2010s, with preliminary efforts from 2014 to 2018 culminating in key foundational documents. The CEPC-SPPC Preliminary Conceptual Design Report (Pre-CDR), often referred to as the White Paper and published in March 2015, outlined initial accelerator concepts, including the lattice design for the 100 km circumference ring and injection systems to achieve beam energies up to 120 GeV per beam.[11] This was followed by the full Conceptual Design Report (CDR) in November 2018, which established baseline parameters such as the circular layout to mitigate synchrotron radiation losses inherent in electron-positron colliders, targeting a center-of-mass energy of 240-250 GeV for Higgs production.[12] Between 2020 and 2025, the design advanced through international reviews and targeted R&D, incorporating feedback to refine components for reliability and performance. Updates addressed challenges in achieving high luminosity of approximately $10^{34} cm^{-2} s^{-1} by optimizing beam dynamics and minimizing energy losses from synchrotron radiation, a primary limitation of circular e^+e^- accelerators compared to linear designs.[13] Significant progress included prototypes for superconducting magnets using niobium-titanium windings to handle the 100 km ring's scale and advancements in crab-waist crossing schemes for RF systems to enhance luminosity without excessive beam disruption.[14] These efforts were documented in progress reports and culminated in the accelerator's Technical Design Report (TDR) released in December 2023, providing detailed engineering blueprints for the full complex.[15] Major reports in 2025 further solidified the integrated design, with the CEPC Technical Design Report for the Reference Detector (RefTDR) released in October, detailing the detector's integration with the accelerator baseline for optimal physics reach.[16] This document, available as arXiv preprint 2510.05260, emphasizes modular detector technologies compatible with the collider's high-precision requirements.[4] A key milestone was the 2025 International Workshop on the High Energy Circular Electron Positron Collider (CEPC2025), held in Guangzhou from November 6 to 10, which validated the matured accelerator-detector baseline through global expert discussions and simulations.[17] These developments addressed core challenges by balancing luminosity goals with radiation mitigation, paving the way for potential engineering design phases.Design and Specifications
Accelerator Complex
The Circular Electron Positron Collider (CEPC) features a 100 km circumference circular collider with a double-ring design, accommodating separate beam pipes for electrons and positrons, and is planned to be constructed underground at a typical depth of approximately 100 meters.[18] This layout includes eight arcs and eight straight sections, with two interaction points for particle collisions, and shares a tunnel infrastructure compatible with a future Super Proton-Proton Collider (SPPC).[18] Key parameters of the CEPC accelerator include a tunable center-of-mass energy ranging from 91 GeV at the Z pole to 240 GeV for Higgs production, with beam energies of 45.6 GeV in Z mode, 80 GeV in WW mode, and 120 GeV per beam in Higgs mode.[18] The design targets luminosities of up to 1.2 ab^{-1} per year at the Higgs threshold with a baseline RF power of 30 MW, potentially upgradable to 2 ab^{-1} per year at 50 MW, enabling integrated luminosities exceeding 20 ab^{-1} over the machine's operational lifetime.[18] These parameters support precision studies by providing high event rates while managing beam dynamics in the large ring.[18] The accelerator complex comprises several core components for beam production and acceleration. Injection begins with a superconducting linac delivering electrons and positrons up to 30 GeV over approximately 1.8 km, utilizing C-band and S-band structures for efficient acceleration.[18] A booster synchrotron, housed in the same 100 km underground tunnel as the main ring, further accelerates beams to up to 180 GeV using a triple-bend achromat lattice.[18] The main collider ring employs a double-ring configuration with eight 650 MHz RF sections providing 2.2 GV of voltage, and incorporates low-emittance optics to minimize synchrotron radiation losses, which are significant at these energies due to the large circumference and lepton beams.[18] A 147 m damping ring with dual 650 MHz cavities helps reduce beam emittance prior to injection, ensuring stable operation.[18] Operational modes are tailored to specific physics thresholds, with top-up injection enabling continuous running in higher-energy configurations. In Z mode, the collider operates at 91 GeV center-of-mass energy with a luminosity of 1.15 \times 10^{36} , \mathrm{cm}^{-2} \mathrm{s}^{-1}, targeting an integrated luminosity of approximately 60 ab^{-1} over two years to produce trillions of Z bosons.[18] WW mode runs at 160 GeV with 1.6 \times 10^{35} , \mathrm{cm}^{-2} \mathrm{s}^{-1}, accumulating about 4.2 ab^{-1} in one year for electroweak studies.[18] Higgs mode, the primary focus, achieves 240 GeV with 5 \times 10^{34} , \mathrm{cm}^{-2} \mathrm{s}^{-1} and top-up injection to maintain steady luminosity, aiming for 13 ab^{-1} over ten years to yield over a million Higgs events.[18] Luminosity in the CEPC is governed by the formula L = \frac{N_e N_p f_\mathrm{rev}}{4 \pi \sigma_x \sigma_y} \times F, where N_e and N_p are the bunch populations for electrons and positrons (typically \sim 10^{11}), f_\mathrm{rev} is the revolution frequency, \sigma_x and \sigma_y are the horizontal and vertical beam sizes (on the order of microns at the interaction point), and F is the crossing angle enhancement factor.[18] This expression highlights the importance of high bunch charges and tight beam focusing to counteract beamstrahlung and synchrotron radiation effects.[18] Ongoing research and development emphasizes enhancements for performance and future scalability. The 650 MHz crab-waist scheme is under development to boost luminosity by optimizing beam crossing angles, drawing from tests at facilities like DAΦNE and integrated into the main ring and damping ring designs.[18] Energy efficiency considerations include klystron systems achieving 55% efficiency and overall power management, positioning the CEPC as a potential precursor to a 100 TeV proton super-LEP upgrade by reusing the tunnel infrastructure for higher-energy lepton operations up to 360 GeV.[18]Detectors and Instrumentation
The Circular Electron Positron Collider (CEPC) reference detector, detailed in the Technical Design Report (RefTDR) released in October 2025, features a hermetic, multi-purpose design optimized for precision measurements in Higgs, electroweak, and top physics, with near-complete solid angle coverage exceeding 98%.[4] This detector integrates a central tracking system, electromagnetic and hadronic calorimeters, and a muon identification system within a superconducting solenoid providing magnetic fields up to 3 T, ensuring high reconstruction efficiency for event topologies like Higgs decays to photons or Z bosons.[4] The overall structure is modular and cylindrical, with a total mass of approximately 5,200 tons, including barrel and endcap regions to minimize dead zones through staggered geometries and inclined sensor placements.[4] Key subsystems include the central tracker, which comprises a silicon pixel vertex detector for precise vertexing, an inner silicon tracker using HV-CMOS sensors, a time projection chamber for momentum measurements, and an outer silicon tracker with AC-LGAD sensors for timing. The vertex detector achieves impact parameter resolutions better than 5 μm, enabling secondary vertex reconstruction essential for flavor tagging in Higgs analyses.[4] The electromagnetic calorimeter (ECAL) employs high-granularity crystal technology, such as bismuth germanate (BGO) or lead tungstate, with cell sizes down to 15×15 mm² and depths of 18–24 radiation lengths, targeting photon energy resolutions of approximately 2–3%/√E (in GeV) for clean identification of Higgs → γγ events, with prototypes achieving under 2%/√E.[4] The hadronic calorimeter (HCAL) utilizes a dual-readout or glass scintillator design with over 5 million channels and 4.7–6 interaction lengths, supporting jet energy reconstruction with resolutions around 30%/√E for hadronic decays like Higgs → ZZ → 4q.[4] Complementing these, the muon system, embedded in the iron yoke with resistive plate chambers or plastic scintillators, provides identification efficiencies exceeding 99% for leptons above 2 GeV while suppressing pion misidentification below 1%.[4] Performance targets emphasize robust particle reconstruction across pseudorapidity |η| < 3.5, covering nearly 99% of the solid angle, with lepton identification efficiencies above 99% and photon efficiencies over 95% for energies greater than 3 GeV.[4] Jet energy resolution reaches ~30%/√E for light jets, crucial for distinguishing beyond-Standard-Model signals in electroweak processes, while overall momentum resolution from the tracker is σ_{p_T}/p_T ≈ 2.1 × 10^{-5} p_T ⊕ 2.2 × 10^{-3} / √(sin θ), with p_T in GeV/c.[4] These capabilities are validated through full GEANT4 simulations within the CEPCSW framework, incorporating event generators like Whizard and Pythia, which demonstrate Higgs mass resolutions below 10 MeV and effective mitigation of beam-induced backgrounds via integrated accelerator shielding, such as tungsten masks reducing hit rates by 20–50%.[4] Instrumentation includes forward calorimeters like LumiCal for precise luminosity monitoring, achieving relative precisions of 0.1% in Higgs mode through Bhabha scattering events, and a multi-level trigger system capable of handling interaction rates up to 20 kHz at the Higgs energy while selecting events at rates below 500 Hz.[4] R&D efforts highlighted in the 2025 RefTDR feature radiation-hard silicon sensors (e.g., JadePix with 25 μm pitch), prototype ECAL crystals yielding resolutions under 2%/√E, and high-rate HCAL readouts with 60–100 photoelectrons per minimum ionizing particle, all tested for occupancy below 1% under nominal backgrounds.[4] The design's unique modularity supports future upgrades, such as scalable supercells in the ECAL, and prioritizes sensitivity to Higgs channels like H → γγ and ZZ, with simulation studies showing boson mass resolutions of ~0.4 GeV for Z decays.[4]| Subsystem | Key Components | Resolution Targets | Coverage |
|---|---|---|---|
| Vertex Tracker | Silicon pixels (6 layers, 25 μm pitch) | <5 μm (impact parameter) | |cos θ| < 0.99 |
| ECAL | Crystal (BGO/PWO, SiPM readout) | 2–3%/√E (photons) | |η| < 3.5 |
| HCAL | Dual-readout/glass scintillator (5M+ cells) | ~30%/√E (jets) | |η| < 3.5 |
| Muon System | RPC/plastic scintillator in yoke | >99% efficiency, <1% fake rate | Full azimuthal, |η| < 3.5 |
Physics Goals
Higgs Boson Precision Studies
The Circular Electron Positron Collider (CEPC) is proposed as a Higgs factory operating at a center-of-mass energy of 240 GeV, with the primary goal of achieving precision measurements of Higgs boson properties that are approximately ten times more accurate than those from the High-Luminosity Large Hadron Collider (HL-LHC). This is enabled by collecting an integrated luminosity of approximately 20 ab^{-1} over ten years, yielding about 4 million Higgs bosons predominantly produced via the Higgsstrahlung process e^+ e^- \to Z H. Such a dataset allows for model-independent determinations of Higgs couplings and decays, serving as a critical test of the Standard Model and probe for new physics.[19][20] Key measurements focus on fundamental Higgs parameters, including its mass, total decay width, branching ratios to major final states, and couplings to vector bosons and fermions. Based on updated simulations incorporating the 2023 accelerator and 2025 detector technical design reports, the Higgs mass is projected to be measured using the recoil technique against the Z boson in clean final states like Z \to \ell^+ \ell^-, achieving a precision better than 3 MeV when combining electron and muon channels. The total width is determined to better than 1.5% through lineshape analysis. Branching ratios reach sub-percent levels for dominant modes, such as H → b\bar{b} at ~0.3% and H → τ^+ τ^- at ~0.7%, while rarer decays like H → γγ are constrained to ~3.5% and H → μ^+ μ^- to ~9%. Couplings to the Z boson (g_{HZZ}) are measured to ~0.13%, to the W boson (g_{HWW}) to ~0.7%, and to third-generation fermions like the top (y_t) and bottom (y_b) quarks to around 0.5-1%, with sensitivities extending to per-mille levels in global fits. These precisions are derived from full detector simulations scaled to the current baseline luminosity, where the total uncertainty on observables like the Higgs mass is given by \sigma(m_H) = \sqrt{\sigma_{\rm stat}^2 + \sigma_{\rm syst}^2}, with the statistical component scaling as $1/\sqrt{N} from the event yield N and systematics dominated by jet reconstruction and energy scales.[20][19] The primary production mode is direct e^+ e^- → Z H, supplemented by vector boson fusion (e^+ e^- → ν\bar{ν} H) and associated top production for specific couplings. Threshold energy scans near the Z H peak refine the production lineshape and width extraction, while angular distributions in events such as e^+ e^- \to Z H \to \ell^+ \ell^- b \bar{b} confirm the Higgs spin-0 and positive parity with high fidelity. For the Higgs self-coupling, double Higgs production via e^+ e^- → H H (at higher energies around 550 GeV) provides sensitivity to the trilinear coupling λ_{HHH}, with projected constraints of δκ_λ ≈ ±1.5% at 1σ, enabling indirect probes of new physics models altering the Standard Model value.[20]| Higgs Property | Projected Precision (with 20 ab^{-1}) |
|---|---|
| Mass (m_H) | < 3 MeV |
| Total Width (Γ_H) | < 1.5% |
| BR(H → b\bar{b}) | ~0.3% |
| BR(H → γγ) | ~3.5% |
| g_{HZZ} | ~0.13% |
| g_{HWW} | ~0.7% |
| y_b (bottom Yukawa) | ~0.7% |