Speaker
Description
The discovery of the Higgs boson filled the last missing block of the Standard Model. After this, the precision measurement of this particle became a frontier scientific problem, because it can be used as a probe to study physics beyond the Standard Model. Such scientific target requires unprecedented jet energy resolutions. For the electromagnetic calorimeter (ECAL), the required boson mass resolution (BMR) and energy resolution are 4% and $\left ( 30 \text{–} 40 \right ) \% / \sqrt{E \ \mathrm{[GeV]}}$ respectively. Such requirements can be met with sampling ECAL designs, viz. SiW- and ScW-ECAL, which can achieve an energy resolution of $\left ( 15 \text{–} 20 \right ) \% / \sqrt{E\ \mathrm{[GeV]}}$. Such value is sufficient for most physical processes; yet for $H \to \gamma \gamma$ process, the width of the invariant mass strongly depends on the energy resolution of photons. Hence, homogeneous ECAL concepts based on crystals and SiPMs have been proposed, and are expected to further improve BMR and energy resolution to 3% and $\left ( 3 \text{–} 4 \right ) \% / \sqrt{E \ \mathrm{[GeV]}}$ respectively.
The novel design of ECAL for CEPC features long BGO crystal bars (typically $1 \times 1 \times 40 \, \mathrm{cm}^3$). For consecutive layers, they are aligned orthogonally with read-out on both ends of the crystal bars, hence improving granularity, reducing the number of read-out channels and minimising the amount of dead material in layers. For the Technical Design Report (TDR), the mechanical structure has also been developed. Recently, the overall structure is still being optimised.
Currently, the performance of SiPMs and crystals are being studied. Specifically, the linearity and dynamic range of SiPM response have been obtained with optical simulation and laser test, the uniformity of BGO crystal bars has been obtained with radiation source scanning, and the time resolutions of long crystal bars have been studied. Furthermore, beam test experiments have been carried out at CERN and DESY respectively, providing valuable data for studying the performance of the crystal modules. Further studies of the intrinsic properties of SiPMs and crystals are still ongoing at present.
