Conceptual Design of a Novel Highly Granular Crystal Electromagnetic Calorimeter for Future Higgs Factories

TL;DR

The paper tackles the need for unprecedented jet energy resolution at future Higgs factories by proposing a high-granularity crystal ECAL based on orthogonally arranged long crystal bars read out at both ends by SiPMs. It develops a dedicated digitisation framework and a CyberPFA-based reconstruction software to evaluate module- and system-level performance. The module-level results show an electromagnetic energy resolution of $1.12\%/\sqrt{E(\mathrm{GeV})}\oplus0.24\%$ with linearity within $\pm0.5\%$ for $3\mathrm{\,GeV}$ to $100\mathrm{\,GeV}$ electrons, substantially surpassing the design goal of $\leq 3\%/\sqrt{E(\mathrm{GeV})}\oplus1\%$. The study demonstrates feasibility and identifies critical R&D areas, including dynamic range management, calibration, radiation hardness, and exploring alternative crystal materials like BSO for cost and performance optimizations.

Abstract

Next-generation high-energy electron-positron colliders, operating as Higgs factories, require an unprecedented jet energy resolution for precision measurements of Higgs and Z/W bosons. To address this challenge, a conceptual design is presented for a novel high-granularity crystal electromagnetic calorimeter that combines the superior intrinsic energy resolution of a homogeneous calorimeter with the fine segmentation required for particle-flow reconstruction. The crystal electromagnetic calorimeter design is based on orthogonally arranged long scintillating crystal bars read out by silicon photomultipliers (SiPMs) at both ends. Key design specifications were established through comprehensive simulation studies. Critical technical considerations, including crystal choices, photosensors, electronics, mechanical support, and radiation damage, are discussed. A dedicated digitisation framework was developed to realistically model effects from the crystal, SiPMs, and readout electronics. The performance of a single calorimeter module was evaluated using simulated electron showers. Simulation results for a single module demonstrate an excellent electromagnetic energy resolution of $1.12\%/\sqrt{E(\mathrm{GeV})}\oplus0.24\%$ and an energy linearity within $\pm0.5\%$ for electrons from 3 GeV to 100 GeV. The performance significantly exceeds the design requirement of $\leq 3\%/\sqrt{E(\mathrm{GeV})}\oplus1\%$. The results establish the feasibility of the proposed high-granularity crystal calorimeter concept and point to a promising pathway toward the precision calorimetry required for future high-energy electron-positron collider experiments.

Figures (10)

• Figure 1: The schematics of the typical module of the high-granularity crystal ECAL. Long scintillating crystal bars are arranged orthogonally between adjacent layers to provide longitudinal segmentation and effective transverse granularity. Each crystal bar is wrapped in a reflective foil to enhance light collection efficiency and response uniformity, and is read out by two SiPMs, one coupled to each end. The front-end electronics and the cooling system are placed on the four sides of the module.
• Figure 2: The schematics of the preliminary modular geometry design of the high-granularity crystal ECAL for the CEPC. The cylindrical ECAL barrel (left) comprises 480 modules, and the disk-shaped endcap (right) contains 112 modules.
• Figure 3: Dependence of the stochastic term in EM energy resolution on MIP response and energy threshold per crystal. The purple line indicates the best resolution obtained from the original energy deposition. The red dashed line represents the target upper limit for the stochastic term of the crystal ECAL resolution.
• Figure 4: A two-dimensional response map of a crystal ECAL module for perpendicularly incident muons at different positions on the module front face. The response is highest at the four corners, followed by the edges, with the lowest response observed in the central region.
• Figure 5: Energy resolution under different degrees of crystal centre-to-end response reduction. The study employs nine crystal ECAL modules, with electrons randomly incident on the central one.