EicC: Electron-Ion Collider in China

• EicC is a planned high-luminosity, high-precision electron-ion collider designed to probe nucleon and nuclear structure in Quantum Chromodynamics (QCD).
• Its electromagnetic calorimeter features segmented pCsI and Shashlik modules with advanced wavelength-shifting fiber technology for enhanced energy and position resolution.
• The modular ECAL system supports precise particle identification and electron-pion discrimination via refined E/p ratios and detailed shower profile analysis.

The Electron-Ion Collider in China (EicC) is a planned high-luminosity, high-precision lepton-hadron collider facility designed to enable advanced studies of nucleon and nuclear structure in Quantum Chromodynamics (QCD). Its experimental program necessitates sophisticated particle detection systems, with electromagnetic calorimetry playing a pivotal role in electron and photon measurement, particle identification (PID), and the resolution of complex final states.

1. EicC Electromagnetic Calorimeter: Architecture and Segmentation

The electromagnetic calorimeter (ECAL) at EicC is engineered as a multi-component system, each segment optimized for distinct kinematic coverage and detection demands:

• Electron-Endcap Region: Employs high-resolution, pure cesium iodide (pCsI) crystal calorimetry, targeting the most stringent demands in energy and position resolution for scattered electrons.
• Central Barrel and Ion-Endcap Regions: Utilize a cost-effective, high-granularity Shashlik-style sampling architecture. In these regions, alternating layers of 1.5 mm-thick polystyrene-based plastic scintillator and 0.35 mm-thick lead are stacked to a total depth of approximately 48 cm (≈16 X₀), defining the longitudinal containment and sampling performance. The modular design features projective towers with a front-face cross-section of 4 × 4 cm²; in the barrel section, modules are frustum-shaped to ensure projectivity toward the interaction region, while the ion-endcap uses rectangular prisms (Tian et al., 4 Dec 2025).

Each Shashlik module is penetrated by 16 double-clad wavelength-shifting (WLS) fibers (1.2 mm diameter) traversing the entire stack. The rear of the fibers is mirrored for enhanced photon return, and readout is performed at the front face with 6 × 6 mm² silicon photomultipliers (SiPMs).

Table 1: ECAL Segmentation and Materials

Region	Technology	Absorber/Scintillator [thick]	Depth (X₀)
Electron-Endcap	pCsI crystal	—	20
Barrel	Shashlik (Pb/scintillator)	0.35 mm Pb / 1.5 mm plastic	16
Ion-Endcap	Shashlik (Pb/scintillator)	0.35 mm Pb / 1.5 mm plastic	16

2. Optical Design, Light Collection, and Readout

EicC's Shashlik implementation leverages advanced light-collection and optical-enhancement techniques for maximizing photon yield and uniformity:

• Wavelength-Shifting Fiber Matrix: 16 WLS fibers arranged uniformly across each module efficiently capture scintillation photons and provide fine transverse sampling.
• Fiber Endpoint Treatment: The rear ends employ high-reflectivity ESR mirror films (>98%), minimizing photon losses.
• Interlayer Reflectivity Enhancements: ESR films are inserted between scintillator and lead layers to reflect side-escaping photons back towards fibers. The module's outer surfaces are painted with TiO₂-loaded coatings to further improve internal reflectivity (Tian et al., 4 Dec 2025).
• Mechanical Stability: Compression rods guarantee uniform layer contact, suppressing optical gaps that could degrade light collection.
• Photosensor: SiPMs with a photon detection efficiency (PDE) ≥25% in the WLS emission band are employed, directly coupled at the fiber front ends.

3. Performance: Energy and Position Resolution

Extensive Geant4-based simulations were carried out to optimize ECAL performance. For the Shashlik modules in the barrel and ion-endcap, critical metrics include:

• Energy Resolution:

$$\frac{\sigma_E}{E} = \frac{5.0\%}{\sqrt{E \ [\mathrm{GeV}]}}\oplus 1.0\%$$

The stochastic term is dominated by the sampling fraction:

$$f_\mathrm{s} = \frac{1.5\,\mathrm{mm}/43\,\mathrm{cm}}{1.5\,\mathrm{mm}/43\,\mathrm{cm} + 0.35\,\mathrm{mm}/0.56\,\mathrm{cm}} \simeq 0.33$$

and photostatistics is typically $N_\mathrm{pe} \sim 30$/MeV, producing a negligible additional $\sim5\%/\sqrt{E}$ term (Tian et al., 4 Dec 2025).

• Position Resolution:

$$\sigma_x(E) \simeq 5.0\,\mathrm{mm}/\sqrt{E}\oplus 1.0\,\mathrm{mm}$$

Logarithmic weighting reconstruction is used for the hit position, deploying per-tower energy deposits weighted as $w_i = \max\{0, W_0 + \ln(E_i/E_\mathrm{tot})\}$, with $W_0=3.5$ for Shashlik.

• Energy Linearity: The fractional non-linearity remains within ±1% across the 0.1–15 GeV operational range.

4. Particle Identification and Electron-Pion Discrimination

Shashlik modules achieve high-quality electron/hadron separation by exploiting differences in electromagnetic and hadronic shower profiles and leveraging the precise energy-momentum comparison (E/p):

• Separation Variables: The $E/p$ ratio, spatial shower dispersion $D = \sqrt{D_x^2 + D_y^2}$, and cluster profile shape are used for PID cuts.
• Simulated PID performance at 2 GeV/c:
  • Electron efficiency: $\varepsilon_e \approx 99.3\%$
  • Pion misidentification: $\varepsilon_{\pi \rightarrow e} \approx 0.7\%$
  • Pion rejection factor: $\sim 100:1$ for $>99\%$ electron retention (Tian et al., 4 Dec 2025).

The electron/pion discrimination performance is robust due to intrinsic fine sampling and precise spatial resolution from the modular geometry.

5. Calibration, Uniformity, and Operational Considerations

EicC ECAL design addresses calibration and uniformity via:

• Module Calibration: Equalization based on minimum-ionizing particle (MIP) responses, with position-dependent corrections for fiber attenuation.
• Temperature Compensation: SiPM bias voltage is adjusted to correct for thermal drifts, as system performance can be sensitive to $+45$ mV/°C at the SiPM (Semenov et al., 2020).
• Longitudinal Segmentation: Each WLS fiber is read out independently at the module front, enabling flexibility in introducing fine longitudinal sampling or potential PID via shower profile discrimination.
• Channel Uniformity: Simulated and prototyped systems achieve module-to-module response variations $\lesssim2\%$ after calibration (Pari et al., 2018).

6. Comparison to Shashlik Calorimetry in Other Facilities

The EicC Shashlik ECAL design parameters benchmark favorably against other modern shashlik implementations:

Facility/Prototype	Stochastic Term [%/√E]	Constant Term [%]	Granularity	Reference
EicC Barrel / Ion-Endcap	5.0	1.0	4 × 4 cm²	(Tian et al., 4 Dec 2025)
MPD/NICA Barrel	3.0	2.4	5–6 cm towers	(Semenov et al., 2020)
ENUBET/Ultra-Compact Module	15.7	<1	3 × 3 cm²	(Pari et al., 2018)
DarkQuest (ex-PHENIX)	13.5	2.1	5.25 × 5.25 cm²	(Apyan et al., 27 Feb 2025)

EicC achieves improved energy and position resolutions via finer sampling (thin Pb/scintillator layers), optimized WLS fiber coverage, and high-reflectivity interfacial treatments. The modular approach facilitates scalability and maintainability essential for a large-scale collider environment.

7. Future Directions and Advanced Concepts

The EicC calorimeter R&D continues to explore enhancements in timing, radiation hardness, and deep longitudinal segmentation. Notable global trends in shashlik calorimeter evolution include:

• Radiation-Hard Scintillator Media: Deployment of LYSO:Ce, LuAG ceramics, or polysiloxane-based scintillators for harsh radiation environments (Hu et al., 2022, Acerbi et al., 2020).
• Wavelength-Shifting Fiber Technology: Optimization of fiber diameter, cladding, readout interface, and reflective terminations to improve light yield and response uniformity.
• Ultra-Fine Sampling Structures: GRAiNITA-style micro-shashlik concepts offer $\sim$2–3%/√E stochastic terms and $\sim$1% constant term, with extremely fine absorber/scintillator mixing (Barsuk et al., 2023).
• Integration of Deep Learning Algorithms: Adoption of CNNs for position and cluster reconstruction has demonstrated $\sim$30% spatial resolution improvement over CoG methods in Tsinghua/MPD ECal tests (Wang et al., 2019).
• Timing Capabilities: Integration of capillary/quartz fiber readout enables $\mathcal{O}(50)$ ps timing for EM showers, addressing pile-up and event separation at high-luminosity colliders (Wetzel et al., 2023, An et al., 2022).

Ongoing studies involve full detector integration, high-rate operation, radiation damage mitigation, and large-scale calibration protocols.

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The EicC electromagnetic calorimeter system exemplifies state-of-the-art shashlik technology, balancing energy and position resolution, granularity, and scalability to meet demanding collider physics requirements (Tian et al., 4 Dec 2025). Its architecture and optimization strategy are consistent with, and in certain respects surpass, the capabilities of analogous contemporary sampling calorimeters in high-energy and nuclear physics.