Measurement of VH(H→WW∗) process in ATLAS and development of radiation hard LGAD

This doctoral thesis investigates two key topics in particle physics, addressing the fundamental unsolved questions of nature and the technical challenges of next generation collider experiments. The research focuses on the measurement of Higgs boson production in association with vector bosons and with the Higgs boson decaying into W boson pairs in the ATLAS experiment, as well as the development of the radiation-resistant Low-Gain Avalanche Detector (LGAD) for the high-granularity timing detector upgrade, covering conceptual design, simulation, fabrication, and characterization. The thesis begins with a summary of the theoretical framework of particle physics, including the Standard Model and an overview of the Large Hadron Collider (LHC) and the ATLAS experiment. It also discusses the high-luminosity LHC upgrade and the High-Granularity Timing Detector (HGTD) for the second phase of the ATLAS upgrade. Following this, the thesis summarizes the properties of the Higgs boson, including the Higgs mechanism, production and decay modes, and observations at the LHC. The study delves into the measurement of Higgs boson production in association with vector bosons, with the Higgs boson decaying into W boson pairs using 139 fb−1 data collected by the ATLAS detector from 2015 to 2018. This part includes physical motivation, data and Monte Carlo sample overview, background estimation, systematic uncertainties, statistical treatment, and results. By using a global fitting to constrain the backgrounds, the measurement precision has been significantly improved. The statistical significance for 𝑊 𝐻, 𝑍𝐻, and their combined 𝑉 𝐻 signals are 1.5 (3.3), 4.6 (3.1), and 4.6 (4.7) standard deviations, respectively. The corresponding cross-sections for 𝑊𝐻 are 0.13++0.08 −0.07 (statistical error)+0.05 −0.04 (systematic error) pb, 𝑍𝐻: 0.31++0.09 −0.08 (statistical error)±0.03 (systematic error) pb, and 𝑉 𝐻: 0.44 ± 0.10 (statistical error)+0.06 −0.05 (systematic error) pb, which are consistent with the theoretical prediction from the Standard Model. In addition, the thesis discusses silicon detectors and their applications in particle physics experiments. It investigates a new type of silicon detector technology, Low Gain Avalanche Diodes (LGAD), for the HGTD of the ATLAS upgrade. It discusses the development of LGAD technology, timing performance, radiation resistance, gain layer degradation, different types of LGAD designs, and potential applications. The thesis showcases the collaborative research between USTC and the Institute of Microelectronics (IME) on the radiation-resistant LGAD fabrication process and production. To meet the requirements of the high-granularity timing detector upgrade, the author used Synopsys’ TCAD EDA software for LGAD process simulation, established process parameters and optimized them, and provided a relatively optimized structural design in conjunction with guidelines from electric simulations. The designed devices were manufactured in collaboration with IME in an 8-inch wafer fabrication line, and a relatively mature LGAD production process was developed. The fabricated LGAD samples were characterized using various methods together with LGAD samples from other producers, including high-voltage probe testing, Strontium-90 beta radiation sources, infrared laser TCT, and particle beam testing at DESY in Hamburg, Germany, and CERN’s PS/SPS facilities. Key parameters such as breakdown voltage, leakage current, charge collection effciency, and timing resolution were obtained. The pre-irradiation timing resolution is better than 35 ps and collected charge is larger than 10 fC. By comparing the test results before and after neutron irradiation, the radiation resistance of the devices was calibrated. The author further developed and optimized the carbon-doping process, resulting in devices with leading radiation resistance performance. The neutron removal cross section reached 1.23 × 10−16 cm2, a world-leading level. After the highest irradiation dose required by HGTD, 2.5 × 1015 cm−2 of 1 MeV neutron equivalent fluences, the devices can still achieve a timing resolution better than 50 ps and a collected charge of 5 fC, significantly outperforming the corresponding products from Hamamatsu, Japan, and fully meeting the requirements of the HGTD project. The author also developed an LGAD manufacturing process based on a 6-inch silicon wafer processing platform at the USTC center for micro- and nanoscale research and fabrication (NRFC). By fully utilizing the conditions and equipment of the micronano processing platform and going through multiple explorations, the author successfully developed AC-coupled LGAD (AC-LGAD)，also called Resistive Silicon Detector (RSD). The resulting devices achieved a timing resolution close to 30 ps and a spatial resolution better than 5 µm measured using the center of gravity reconstruction method. These results are agree with the expectations of the design. This work lays the foundation for the future development of new semiconductor detectors. The thesis systematically summarizes the author’s work in LGAD technology development, manufacturing, characterization, and the application of LGADs for the HGTD of the ATLAS upgrade.