Characterisation of the HGCROC3 read-out chip for the future HGCAL, innovative calibration techniques at trigger level and study of the ZZ/ZH→bbττ processes with the CMS detector at the LHC

This Thesis presents the study of the production of two Z bosons (ZZ) and a Z boson in association with a Higgs boson (ZH) in the final state with two b quarks and two τ leptons (bbττ), using proton-proton collision data recorded at a centre-of-mass energy of $13\,\text{TeV}$. The dataset analysed corresponds to an integrated luminosity of $137\,\text{fb}^{-1}$ collected during Run II (2016-2018) by the CMS experiment at the LHC. The non-resonant ZZ/ZH production, as predicted by the Standard Model (SM), is theoretically and experimentally well-established in high-resolution fully leptonic channels. Given the close similarity in the experimental signature, the SM ZZ/ZH processes in the bbττ channel serve as cross-validation for the strategy employed in the search of Higgs boson pair (HH) production in the same final state. The expected upper limits at 95% confidence level are found to be 1.8 and 3.6 times the SM predictions for the ZZ and ZH production cross section, respectively. The resonant ZZ/ZH search is motivated by numerous theories beyond the SM (BSM) predicting the existence of spin-0 or spin-1 high-mass particles possibly decaying into ZZ/ZH. Different mass hypotheses are investigated, ranging from $200\,\text{GeV}$ to $4\,\text{TeV}$, and expected upper limits on the resonance cross section are extracted for each mass and spin assumption. The Run III (2022-2025) data-taking is currently underway and presents opportunities to improve the detection performance. The Level-1 (L1) trigger system plays a key role in the event detection, combining data from the CMS sub-detectors to perform a real-time selection of interesting events. This Thesis proposes an innovative Machine Learning (ML) method for the calibration of Layer-1 calorimeter Trigger Primitives, the basic constituents of L1 objects. The ML-based calibration provides enhancements in energy resolution and selection efficiency, offering a scalable solution to a wide range of calibration contexts. After Run III, the LHC will undergo a major upgrade towards the High-Luminosity LHC (HL-LHC), opening new horizons for discoveries and precision physics. In order to maintain its excellent performance, the CMS is planning a series of upgrades of the sub-detectors, including the replacement of the endcap calorimeters with the High-Granularity CALorimeter (HGCAL). This Thesis focuses on the HGCal Read-Out Chip (HGCROC3), the front-end chip designed to read-out the six million channels of the future HGCAL. Along with cutting-edge specifications in terms of noise, charge and time measurement, the HGCROC3 requires high radiation tolerance. Extensive test-bench characterisation and irradiation testing with X-rays, heavy ions, and protons demonstrated robust performance while also highlighting design vulnerabilities. These findings guided the development of an improved version of the chip, ensuring reliable operation under the challenging HL-LHC environment.