Successful Beam Test of CEPC High-Granularity Calorimeter Prototype at CERN (2023-04-14)

In July 2012, the Higgs boson was discovered simultaneously in the ATLAS and CMS experiments at the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN), revealing the mystery of the origin of fundamental particle masses and marking a major milestone in particle physics research. In September of the same year, Chinese scientists took the lead in proposing the international big science project of the Circular Electron-Positron Collider (CEPC), proposing to build the CEPC in China as a Higgs boson factory, which can also serve as a Z and W boson factory (as shown in Figure 1). Using the Higgs boson as a probe, we conduct in-depth research on the spontaneous symmetry breaking mechanism of the electroweak force and the origin of mass, and explore major scientific issues such as dark matter, the electroweak phase transition in the early evolution of the universe, and the matter-antimatter asymmetry in the universe. The CEPC team has carried out design and key technology research and development, and achieved a series of significant progress.

To improve particle detection performance and physical potential, the CEPC team needs to design and develop a high-performance detector. Considering that the main decay products of the Higgs, Z and W bosons are hadronic final states, the energy resolution performance of the calorimeter is extremely critical. According to physical requirements, the jet energy resolution of the CEPC calorimeter needs to reach 30%/√E(GeV). Compared with the CERN Large Electron-Positron Collider (LEP) and the currently operating LHC experiments, the performance of the calorimeter needs to be nearly doubled, which is a highly challenging performance index. The baseline design scheme of the CEPC calorimeter adopts the high-granularity calorimeter technology based on the Particle Flow Algorithm (PFA) proposed by the CALICE international collaboration, which is an important development direction for future calorimeter detection technology. The CEPC detector team has carried out in-depth international cooperation on relevant technologies within the CALICE collaboration.

The CEPC calorimeter prototype is a sampling calorimeter design based on plastic scintillators (sensitive layers) and absorber layers, in which the plastic scintillators are read out by Silicon Photomultipliers (SiPMs) with ultra-high granularity, which can perform four-dimensional imaging (including three-dimensional space and time) of the shower of high-energy particles.

The CEPC Electromagnetic Calorimeter prototype (ScW-ECAL) consists of 32 layers of sensitive layers and absorber layers (tungsten-copper alloy). The size of the sensitive layer is 21mm×21mm, and the effective granularity of the detection unit is 5mm×5mm, with a total of 6720 electronic readout channels. The preliminary cosmic ray test results show that the position resolution of each layer of the calorimeter is better than 2mm. This prototype is the world's first technical prototype based on the plastic scintillator and SiPM scheme.

The CEPC Hadronic Calorimeter prototype (AHCAL) is composed of 40 layers of sensitive layers and absorber layers (carbon steel). The size of the sensitive layer is 72cm×72cm, and the detection unit is a 4cm×4cm plastic scintillator with a total of 12960 electronic readout channels. The prototype adopts two types of SiPMs from Hamamatsu Photonics K.K. (HPK) in Japan and the New Device Laboratory (NDL) of Beijing Normal University in China. During the development of the prototype, the team mass-produced plastic scintillator sheets using scintillator injection molding technology, developed an automatic packaging machine for the reflective film of detection units, and established a quality management process for mass production by developing a batch testing platform for detection units and SiPMs.

Through the development of the prototype, the team has mastered the key technologies of high-granularity calorimeters, including mass production and testing technology of scintillator detection units, modular design and assembly technology of sensitive layers, high-density integration of front-end readout electronic chips, large-scale SiPM monitoring and calibration, thermal simulation and monitoring technology, etc., accumulating valuable experience and laying a solid foundation for large-scale mass production in the future.

From October 19 to November 2, 2022, Chinese and foreign scientists operated the CEPC high-granularity electromagnetic and hadronic calorimeter prototypes and conducted a two-week high-energy particle beam test experiment on the H8 beam line of the Super Proton Synchrotron (SPS) at CERN, as shown in Figure 3. The team used 10~120 GeV electrons and hadrons, and 160 GeV muons provided by the CERN SPS H8 beam line. This beam test systematically tested the key indicators such as energy linearity and energy resolution of the two prototypes of the CEPC electromagnetic and hadronic calorimeters, and obtained more than 25 million high-energy particle events in total. These data samples will provide an important foundation for the research on the detection performance of the CEPC calorimeter prototype, shower spatial distribution and time evolution, verification of hadronic interaction models, and experimental research on the Particle Flow Algorithm.

Dr. Liu yong from the Institute of High Energy Physics (IHEP) served as the technical coordinator of the team, responsible for docking with CERN technicians in all aspects such as CERN SPS transportation, on-site installation, radiation protection, and beam control, and coordinated with Eva Barbara Holzer, the SPS technical coordinator, and Johannes Bernhard, the person in charge of the beam test area. He also collaborated with Professor Yang Haijun, Professor Zhang Yunlong and Associate Professor Shen Zhongtao to implement the data acquisition plan, ensuring that the team successfully completed all the planned data acquisition tasks.

Researcher Lou Xinchou from IHEP and the CEPC project manager pointed out: "The development of this prototype has reached the international advanced level, and it is the first particle flow calorimeter in China. The success of this beam experiment has prepared a technical foundation for the future CEPC experiment." The development of the CEPC calorimeter prototype has received strong support from the National Key R&D Program of Frontier Research on Major Scientific Infrastructures of the Ministry of Science and Technology and the International Cooperation and Exchange Project of the National Natural Science Foundation of China. The main undertaking units for the development of the calorimeter prototype include the Institute of High Energy Physics of the Chinese Academy of Sciences, the University of Science and Technology of China and Shanghai Jiao Tong University.

Figure 1: Schematic diagram of the CEPC Circular Electron-Positron Collider

Figure 2: CEPC ECAL and AHCAL prototypes at the CERN beam test site

Figure 3: Partial members of the beam test team with the detectors (left) and core team members (right)

Figure 4: Shower events of high-energy particles in CEPC high-granularity calorimeter prototype