Search for Higgs boson pair production and study of the high granularity timing detector peripheral electronics in the ATLAS experiment

Within the framework of the Standard Model, the Higgs mechanism explains how fundamental particles acquire mass. It achieves this by introducing a self-coupling mechanism for the Higgs boson, allowing the Higgs field to obtain a non-zero vacuum expectation value, thereby leading to the breaking of electroweak symmetry and enabling other particles to acquire mass. Measuring the self-coupling of the Higgs boson provides us with a better understanding of the shape of the Higgs potential, facilitating further investigation into significant issues such as testing the Higgs mechanism, early universe thermodynamic processes, and the stability of the cosmic vacuum. At the Large Hadron Collider (LHC), studying Higgs boson pair production ( 𝐻𝐻 ) allows for the measurement of the Higgs boson self-coupling. Additionally, the presence of anomalous couplings, as proposed by many new physics hypotheses beyond the Standard Model, can alter the Higgs potential and its self-coupling strength, thereby increasing the production rate of 𝐻𝐻 events. It is possible to discover Higgs boson pair production within the data collected thus far. Exploring Higgs boson pair production is a significant research focus of the ATLAS experiment, which has the potential to deepen our fundamental understanding of the Higgs boson potential and its self-coupling. The first part of this thesis is to search for non-resonant production of Higgs boson pairs in the final state of two 𝑏−jets and two 𝜏-leptons (𝑏𝑏̄𝜏+𝜏−) through the 𝐻𝐻 ggF and VBF production modes by using the 140 fb−1 Run-2 data collected by the ATLAS detector at the centre-of-mass energy of 13 TeV, thus the Higgs boson self-coupling modifier 𝜅𝜆 can be measured. This analysis gives the best measurement of this quantity in the 𝐻𝐻 final state. This search is performed based on the previous round of the analysis. Notable improvements in experiment sensitivity are achieved through various updates, including more precise study of the key backgrounds, the inclusion of dedicated VBF SR, improvement of BDT strategy, and etc. No significant excess above the expected background from Standard Model processes is observed. The observed (expected) 95% confidence level (CL) upper limit on 𝜇_𝐻𝐻 is 5.9 (3.3) times the Standard Model prediction. The observed limit on 𝜇_𝐻𝐻 is looser than the expected one as a result of a mild excess in the high-mHH region of lephad SLT channel. The corresponding observed (expected) 95% confidence intervals for the self-coupling modifier 𝜅𝜆 and the quartic coupling modifier 𝜅2𝑉 are respectively −3.1 < 𝜅𝜆 < 9.0 (−2.5 < 𝜅𝜆 < 9.3) and −0.5 < 𝜅2𝑉 < 2.7 (−0.2 < 𝜅2𝑉 < 2.4). Although we have achieved quite good measurement of the Higgs boson self-coupling, there is still room for improvement towards more precise regime. This goal can be realized by collecting more collision data. The HL-LHC project is established, targeting for the extension of LHC operability and an increase of the collision rate, which means the ability of producing much more 𝐻𝐻 events. In order to deal with the challenges posed by the HL- LHC, especially the adverse effects of the pile-up, the ATLAS detector has to be upgraded (called ATLAS Phase-2 upgrade) to properly handle the flood of incoming data and increased irradiation level. A high granularity timing detector (HGTD) is therefore proposed for the ATLAS upgrade. With the timing information provided by the HGTD, the tracks that are spacially overlapped can still be distinguished. The PEB board plays an important role in the HGTD peripheral electronics system, and it serves as a bridge that connects the HGTD modules and the off-detector systems. Its design faces many difficulties, such as high complexity, huge data throughput, working in an irradiation environment, and limited installation space. The second part of this thesis is to design a prototype before the final PEB design. The design requirement is: under the circumstances of key chips scarcity, all key functions of PEB should be integrated into the prototype for verification to ensure the normal progress of HGTD peripheral electronics research and development. It aims to verify the following five aspects: system configuration, data transmission, power supply distribution, monitoring network, clock and fast command distribution. Among them, the verification of data transmission is more complicated and needs to consider versatile link (lpGBT + VTRx+) communication, full data path communication, E-port data rate setting, etc. In response to the design requirement raised above, it was decided to adopt a modular design, that is, to integrate the key chips onto pluggable daughter boards. This design allows key chips to be replaced, and even if a key chip fails, it will not cause the waste of other chips in the system. Functionally, this design is a simplified PEB, but it can achieve all the above functions to be verified by using all kinds of key chips. Based on the joint test system, we tested the above-mentioned key functions one by one, which all meet the specified requirements and work as expected. The modular PEB demonstrates full operational capability in terms of hardware design and verifies the feasibility of key functions, which gives us more confidence on the future PEB board design. Certainly, the modular PEB can not only be used as a PEB-related electronics test and verification platform, but has also been used by many HGTD sub-groups (such as CERN, Nikhef and KTH) for different tasks, greatly accelerating the progress of these tasks.