[Paper Review] PFA Oriented ECAL Optimization for the CEPC

H. Zhao, M. Q. Ruan+4 more|arXiv (Cornell University)|Dec 27, 2017

Particle physics theoretical and experimental studies10 references3 citations

TL;DR

This paper optimizes the electromagnetic calorimeter (ECAL) for the Circular Electron Positron Collider (CEPC) using Geant4 simulations and the Arbor particle flow reconstruction algorithm. It proposes a reduced 25-layer ECAL with 1 mm silicon sensors and 5–10 mm transverse cells, achieving 1.64% Higgs boson mass resolution in di-photon decays and maintaining low photon shower overlap (2%) at Z→τ⁺τ⁻ events, balancing high performance with reduced channel count and power consumption.

ABSTRACT

The design and optimization of the Electromagnetic Calorimeter (ECAL) are crucial for the Circular Electron Positron Collider (CEPC) project, a proposed future Higgs/Z factory. Following the reference design of the International Large Detector (ILD), a set of silicon-tungsten sampling ECAL geometries are implemented into the Geant4 simulation, whose performance is then scanned using Arbor algorithm. At single particle level, the photon energy response at different ECAL longitudinal structures is analyzed. At bi-particle sample, the separation performance with different ECAL transverse cell sizes is investigated and parametrized. The overall performance is characterized by a set of physics benchmarks, including $ννH$ events where Higgs boson decays into a pair of photons (EM objects) or gluons (jets) and $Z oτ^+τ^-$ events. Based on these results, we proposed an optimized ECAL geometry for the CEPC project.

Motivation & Objective

• To optimize the ECAL geometry for the CEPC detector to meet stringent physics performance requirements while minimizing readout channels and power consumption.
• To improve photon energy resolution and Higgs boson mass resolution in H→γγ decays using longitudinal and transverse ECAL parameters.
• To enhance shower separation performance for jets and tau decays, especially in Z→τ⁺τ⁻ events, to support precise electroweak measurements.
• To reduce the number of readout channels by increasing transverse cell size and reducing longitudinal layers without degrading core physics performance.
• To provide a physics-optimized ECAL design that balances performance, cost, and power efficiency for the CEPC as a future Higgs and Z factory.

Proposed method

• Simulates CEPC_v1 detector geometry using Geant4 with a simplified, defect-free ECAL structure to isolate geometry effects.
• Employs the Arbor particle flow reconstruction algorithm to evaluate photon energy resolution and shower separation performance.
• Scans longitudinal parameters (absorber thickness, layer number, sensor thickness) using single-photon and H→γγ events to assess energy resolution and mass resolution.
• Evaluates transverse cell size impact on shower separation using di-photon and Z→τ⁺τ⁻ events, with overlap probability as a key metric.
• Uses Higgs→gg events to assess jet energy resolution via invariant mass resolution of ννHiggs final states.
• Cross-validates results across multiple physics benchmarks: H→γγ, Z→τ⁺τ⁻, and H→gg, ensuring consistency across performance metrics.

Experimental results

Research questions

• RQ1 What is the optimal number of longitudinal layers and silicon sensor thickness in the ECAL to maintain high photon energy resolution?
• RQ2 How does increasing the transverse cell size affect the separation of nearby electromagnetic showers, particularly in Z→τ⁺τ⁻ events?
• RQ3 What ECAL geometry achieves the required 1.64% Higgs boson mass resolution in H→γγ decays while minimizing readout channels?
• RQ4 Can reducing the number of longitudinal layers from 30 to 25 be compensated by increasing silicon sensor thickness to 1 mm without performance loss?
• RQ5 What is the maximum transverse cell size that maintains low photon shower overlap (≤2%) in high-multiplicity environments like Z→τ⁺τ⁻ decays?

Key findings

• The optimized ECAL achieves a single-photon energy resolution of 15.9%/√E ⊕ 0.95%, matching CALICE test beam results and improving upon the CEPC_v1 baseline.
• A Higgs boson mass resolution of 1.64% is achieved in H→γγ decays using the optimized geometry, representing a 20% improvement over CEPC_v1.
• Reducing the number of longitudinal layers from 30 to 25, with silicon sensors thickened from 0.5 mm to 1 mm, maintains equivalent performance in photon energy and mass resolution.
• Increasing the transverse cell size from 5 mm to 20 mm raises the photon shower overlap probability in Z→τ⁺τ⁻ events from ~2% to ~20%, degrading PFA performance.
• The Higgs boson mass resolution in H→gg events remains below 3.75% for cell sizes up to 10 mm, rising to 3.93% at 20 mm, indicating a sharp performance degradation beyond 10 mm.
• The recommended ECAL geometry—25 layers, 1 mm silicon sensors, 3.36 mm tungsten plates, and 5–10 mm transverse cells—meets all physics benchmarks while minimizing channel count and power consumption.