Circular Electron Positron Collider (CEPC)

• Circular Electron Positron Collider (CEPC) is a high-luminosity, 100 km e⁺e⁻ collider designed to study Higgs, W, and Z boson properties with unprecedented precision.
• It employs dual-ring configurations, advanced particle flow detectors, and robust accelerator technologies to optimize luminosity and minimize background interference.
• Its multi-mode physics program enables precision measurements in electroweak, flavor physics and prepares for future upgrades including top-quark studies and hadron collider applications.

The Circular Electron Positron Collider (CEPC) is a proposed high-luminosity, high-precision $e^+e^-$ collider designed as a Higgs, W, and Z boson factory. Sited in China, the CEPC features a baseline 100 km tunnel with a double-ring configuration and aims to explore the scalar sector, electroweak symmetry breaking, and flavor physics at unprecedented precision. Its multi-mode physics program encompasses the Z-pole, WW threshold, Higgs factory operation, and upgrades toward top-quark physics and hadron collider applications. The CEPC leverages advanced particle flow detectors and robust accelerator technology, providing a clean environment free from strong-interaction backgrounds typical of hadron colliders.

1. Machine Architecture and Operation Modes

The CEPC's principal layout includes a 100 km underground tunnel hosting two collider rings (one for electrons, one for positrons) with 2 interaction points. There are also single-ring and double-ring designs considered for optimized luminosity across different energy regimes (Group, 2022, Xiao et al., 2015). The baseline operation parameters are:

• Tunnel Circumference: 100 km
• Center-of-mass energy ($\sqrt{s}$): 91.2 GeV (Z-pole), 158–172 GeV (WW threshold), 240–250 GeV (Higgs factory), up to 360 GeV (top threshold).
• Peak Luminosity ($\mathcal{L}$): $3\times10^{34}\,\mathrm{cm}^{-2}\mathrm{s}^{-1}$ at 240 GeV per IP (Higgs mode); up to $1.5\times10^{36}\,\mathrm{cm}^{-2}\mathrm{s}^{-1}$ (Z mode) with double-ring and 50 MW synchrotron radiation (SR) power.
• Integrated Luminosity: Nominal scenario yields $5-5.6\,\mathrm{ab}^{-1}$ for Higgs mode in 7–10 years, $\sim16\,\mathrm{ab}^{-1}$ for Z-pole in 2 years, $2.6\,\mathrm{ab}^{-1}$ for WW in 1 year (An et al., 2018, Group, 2019, Group, 2022).

These parameters allow the CEPC to produce $\sim$1--4 million Higgs bosons, $10^{8}$ W pairs, and up to $4\times10^{12}$ Z bosons (Ai et al., 27 Dec 2024).

2. Higgs Factory Physics: Production and Measurement

Production Processes

At $\sqrt{s}=240$–250 GeV, Higgs bosons are synthesized predominantly via "Higgsstrahlung" ($e^+e^- \rightarrow ZH$) and to a lesser extent by vector boson fusion ($WW$ and $ZZ$ channels):

• $\sigma_{ZH}\simeq 206$–$240$ fb ($m_H=125$ GeV) (Chen et al., 2016, An et al., 2018, Ruan, 2014).
• $\sigma_{WW\to H} \sim 7$–$16$ fb; $\sigma_{ZZ\to H} \sim 0.6$–$5$ fb.

Recoil Mass Technique

The recoil-mass measurement provides model-independent determination of $\sigma_{ZH}$ and $m_H$:

$$M_\text{recoil}^2 = s + M_{f\bar{f}}^2 - 2\sqrt{s}(E_{f}+E_{\bar{f}})$$

This method yields:

• Statistical precision on $\sigma_{ZH}$: 0.97% with $Z\rightarrow\mu^+\mu^-$ channel (5 ab$^{-1}$)
• Higgs mass resolution: $6.9$ MeV (model-independent, Z-only), improved to $5.4$ MeV including tagged Higgs decay (Chen et al., 2016).

Precision on absolute couplings from recoil analyses achieves sub-percent levels: $\Delta g_{HZZ}/g_{HZZ} = 0.35\%$ (Ruan, 2014, An et al., 2018).

Invisible and Exotic Decays

The clean recoil environment enables limits on exotic decays:

• $H\rightarrow$ invisible: $<0.3$% at 95% CL (global fits), exclusive Z$\rightarrow\mu^+\mu^-$ channel at $<1.2$% (Chen et al., 2016, Group, 2019).

Multiparameter Coupling Fits

Global $\kappa$-framework fits, incorporating inclusive and exclusive Higgs channels, produce:

Coupling	CEPC Precision	HL-LHC Precision
$\kappa_Z$	$0.13$–$0.25\%$	$1.9\%$
$\kappa_W$	$0.35$–$1.3\%$	$2.7\%$
$\kappa_b$	$0.27$–$1.2\%$	$4.2\%$
$\kappa_c$	$2.1$–$3.3\%$	$5.0\%$
$\kappa_\gamma$	$3.7$–$6.8\%$	$3.9\%$
$\kappa_\mu$	$16$–$17\%$	$7.6\%$
$\Gamma_H$	$2.4$–$3.5\%$	$4.1\%$

(An et al., 2018, Group, 2019)

Sensitivity to New Physics

CEPC’s per-mille-level coupling measurements probe new physics scales up to multi-TeV via effective field theory fits and are sensitive to extended scalar sectors, top-partner scenarios, strong first-order electroweak phase transitions, and Higgs portal models (An et al., 2018).

3. Detector Design and Reconstruction Performance

Particle Flow and Tracking

The CEPC v_1 detector concept is particle flow optimized, featuring:

• Vertex detector: silicon pixel (single-point resolution $\sim$3-$5\,\mu$m, impact-parameter $\sim$5\,$\mu$m), material budget $<0.15\%\,X_0$/layer (Li et al., 2 Apr 2024, Ruan et al., 2018).
• Tracking: large TPC (standalone momentum resolution $\delta(1/p_T)=1\times10^{-4}$ GeV$^{-1}$, combined to $2\times10^{-5}$ GeV$^{-1}$).
• Calorimetry: ECAL Si/W, $24$–$30 X_0$, $5\times5$ mm$^2$ cell ($\sigma_E/E = 16\%/\sqrt{E}\oplus 1\%$); HCAL Fe/RPC, $6\,\lambda$, $10\times10$ mm$^2$ cell ($\sigma_E/E \simeq 60\%/\sqrt{E}\oplus 5-10\%$).
• Muon system: RPC in return yoke (Ruan et al., 2018, An et al., 2018).

The Arbor particle flow algorithm reconstructs tree-like shower topologies, achieving:

• Track finding efficiency $>99\%$, lepton ID $>99.5\%$, jet energy resolution $3$–$6\%$ (20–200 GeV), flavor tagging at 80% ($b$) and 60% ($c$) (Ruan et al., 2018).
• Vertex detector prototype performance: $\sigma_\text{spatial}\simeq 5 \,\mu$m, efficiency $>99\%$ (Li et al., 2 Apr 2024).

Object Resolution Benchmarks

Object	Resolution
$\frac{1}{p_T}$	$2\times10^{-5}$ GeV$^{-1}$
$e, \gamma$	$16\%/\sqrt{E} \oplus 1\%$
jet mass (dijet)	$3.8\%$ (post-cleaning)
$b$-tag	$80\%$ efficiency (90% purity)

(Ruan et al., 2018, An et al., 2018)

Impact of Beamstrahlung and Backgrounds

Beamstrahlung-induced backgrounds are characterized by $\Upsilon_\text{av}\simeq 4.7\times10^{-4}$, generating $\sim 10^{10}$ photons, $\sim 10^3$ $e^+e^-$ pairs per bunch crossing, with negligible detector occupancy ($<0.5\%$) and annual radiation doses of $10^{11}$ n$_\text{eq}$/cm$^2$ and 300 kRad/year at the inner vertex layer (Xiu et al., 2015). These conditions are compatible with robust pixel detector operation.

4. Electroweak and Flavor Physics Capabilities

Z/W Running and Precision Measurements

The CEPC is capable of delivering $10^{12}$ Z bosons ("Tera-Z") and $10^8$ W pairs, enabling:

• $M_Z$ resolution: 0.5 MeV, $A_\text{FB}^l$ to 0.1%, $M_W$ to 1 MeV, $\sin^2\theta_\text{eff}^l$ to $10^{-5}$ (Group, 2019).
• Oblique parameters ($S,T$) at $\Delta S,\Delta T\sim 0.005$.
• QCD studies, event-shape analyses, and $\alpha_s$ at percent or better accuracy.

Flavor Physics Reach

At the Z-pole, CEPC’s heavy-flavor yields eclipse Belle II and rival LHCb with O($10^{12}$) $B$, $D$, and $\tau$ decays (Ai et al., 27 Dec 2024). Sensitivity benchmarks include:

• $B_c^+\rightarrow\tau^+\nu$: $\delta\text{BR}/\text{BR}\sim0.5\%$
• $R_{J/\psi},R_{D_s},R_{\Lambda_c}$: $2.1\times10^{-2}$, $1.6\times10^{-3}$, $4.9\times10^{-4}$, respectively
• FCNC $b\rightarrow s\tau\tau$: $\text{BR}\lesssim10^{-6}$, $b\rightarrow s\nu\nu$: $\lesssim1\%$ uncertainty
• $\tau$ LFV limits: $\text{BR}<10^{-10}$, LFU tests at $10^{-4}$ sensitivity
• Higgs FCNC for $H\rightarrow bs, sd, uc$ at $\text{BR}\lesssim10^{-4}$, top FCNC at $t\rightarrow cH$ at $\text{BR}\lesssim10^{-5}$

These results utilize detector capabilities including vertex impact parameter resolution ($\lesssim5\,\mu$m), powerful particle identification, and efficient flavor tagging.

5. Accelerator Technologies, R&D, and Upgrades

Magnet and RF Systems

The CEPC accelerator employs high-efficiency superconducting RF cavities at 650 MHz (Q$_0>2\times10^{10}$), arc dipoles (twin-aperture, aluminum coils), final focus with "crab-waist" optics (16.5 mrad crossing angle, $\beta^*_y\sim 1$ mm), and advanced feedback systems (BPM accuracy $10$ nm) (Group, 2022).

Key R&D addresses:

• Iron-based superconductors (IBS) and Nb$_3$Sn prototypes for possible future SppC (20–24 T magnets)
• Vacuum chamber NEG coatings achieving $<3\times10^{-10}$ Torr
• High-power klystron development ($\eta>75\%$, output $>750$ kW)

Upgrade Pathways

• 50 MW SR power per beam: Increases luminosity by 60% in Higgs mode
• Top threshold running: RF systems can be expanded for $E_{\rm beam}=180$ GeV
• SppC in same tunnel: pp collisions at $\sqrt{s}\sim 125$ TeV
• $e$-$p$ collisions: $\sqrt{s}\approx 6.7$ TeV possible

Timeline and Cost

The design schedule projects construction and commissioning through the 2030s, physics running in the 2035–2045 window, with projected cost near $5$ billion USD, inclusive of contingency (Group, 2022, Group, 2019).

6. Comparative Assessment and Theoretical Context

CEPC achieves percent to sub-percent precision in Higgs couplings and electroweak parameters, matching or exceeding FCC-ee in some scenarios and complementing HL-LHC for flavor and CP-violation observables (Ruan, 2014). Its unique strengths arise from the clean $e^+e^-$ environment, recoil-mass analysis, and high-statistics multi-mode runs.

Limitations on Higgs self-coupling $\lambda_{HHH}$ are indirect ($\mathcal{O}(1)$ fractional precision) (Ruan, 2014). Top-quark couplings, critical for probing vacuum stability and new physics, are accessible via future CEPC upgrades ($e^+e^-\to t\bar{t}H$ threshold) and associated precision Higgs/WW measurements (An et al., 2018).

The CEPC’s comprehensive physics program extends from coupling determination and rare decay searches to effective field theory and new physics scale sensitivity, with a detector and accelerator suite tailored for robust, low-background operation and upgrade potential (Group, 2019, Ai et al., 27 Dec 2024).

7. Outlook and Strategic Impact

CEPC’s design and technology suite have reached the Technical Design Report stage, with key accelerator and detector R&D validated through beam tests and simulation (Li et al., 2 Apr 2024, Group, 2022). Its international collaboration framework includes multiple institutes and is positioned for further global partnership.

The program’s challenges include achieving system-level precision (alignment, calibration, systematic errors), scaling up fine-granularity detector component production, and advancing theory–experiment integration through Monte Carlo simulation fidelity and NNLO+EW corrections (Group, 2019).

Beyond the Higgs factory, CEPC infrastructure supports future upgrades to the energy frontier (SppC), synergy with other electron-positron collider initiatives (FCC-ee, ILC), and leadership in precision electroweak and flavor physics (An et al., 2018, Group, 2022).