Development of Radiation Resistant Pixel Detectors for the Luminosity Frontier and Measurement of the Higgs Boson Production via Vector Boson Fusion with the CMS Experiment at the LHC

The Large Hadron Collider (LHC) is the largest circular accelerator ever build, allowing collisions at a center-of-mass energy of 13 TeV. The Phase-2 of the LHC, known as High Luminosity LHC (HL-LHC), is going to start in 2027. With HL-LHC, the Compact Muon Solenoid (CMS) experiment will gather an integrated luminosity up to 4000 fb^{-1} in 10 years, making it possible to study rare events of the Standard Model (SM) or to search for processes beyond it. The CMS experiment will be upgraded between 2025 and 2027 to cope with the higher luminosity: especially in the regions near the collision point, unprecedented requirements in terms of radiation resistance and granularity need to be met. The first part of this Thesis focuses on the upgrade of the CMS silicon tracker, whose inner section will be made of pixel detectors. The characteristics of the new tracker will be extremely important in the future analysis to be carried out in CMS during Phase-2. For the new Phase, pixel sensors of new conception have been considered, in which the electrodes (p+ and n+) penetrate deep into the silicon from the same side of the sensor: these new pixels are referred as `3D' for their characteristic of having columnar implants as deep as the active thickness of the sensor, while the more conventional planar `2D' pixels have superficial implants of small thickness. Thanks to this structure, 3D sensors can have excellent performance even with high radiation damage, making them suitable for the use in the inner layers of the future CMS tracker. Due to the cutting edge technology needed to produce these sensors, their use for a large scale experiment has only recently become feasible. However, the production processes are more complex than those of planar sensors, and this affects costs and production effciency. Therefore, 3D sensors have been taken into consideration only for the inner layers of the tracker, while planar sensors will be used in the other layers. In this Thesis, a complete characterisation of 3D and planar pixel detectors is presented. The studies are performed at the INFN and CERN laboratories and in several test beam experiments at DESY. My work was crucial for the characterisation of the detectors both before and after irradiation, to verify that both the sensor and the readout chip are able to resist the high fuences expected at the HL-LHC with a minimum loss of performance. The studies I made demonstrated that planar pixel detectors reach a hit detection effciency of over 99% at a bias voltage of 600 V after an irradiation corresponding the fuence expected after ten years of operations of HL-LHC. 3D pixel detectors have not been tested to these fuences yet (new test beams in the near future will target their characterisation), but are expected to reach similar effciencies with far lower bias voltages, around 150 V. Having high efficiencies at relatively low bias voltages leads to a lower power consumption and reduces the susceptibility to sparking issues with respect to planar sensors. Both of these features are invaluable in the inner tracker environment. Among the studies presented in this Thesis, the spatial resolution of 3D and planar pixel detectors was thoroughly evaluated. Non-irradiated 3D and planar pixel detectors have shown remarkable spatial resolution, down to 2 µm or 5 µm depending on the pixel pitch. The results presented in this Thesis will contribute significantly to the choice of the pixel sensors to be used in the future CMS Inner Tracker. The second part of this Thesis focuses on the measurement of the Vector Boson Fusion (VBF) Higgs production mechanism in the H->WW decay channel. A particle consistent with the SM Higgs boson was observed in 2012 by the CMS and ATLAS collaborations at the LHC. After the discovery, precision on the measure- ment of this new particle properties and interactions has progressed as more data were collected. Currently, all production processes have been observed in one or more decay channels or via combination of several decay channels, with no significant deviations with respect to the SM prediction. However, the VBF mechanism, being at the heart of the electroweak symmetry breaking, needs to be studied with ever-improving analysis techniques while waiting for additional data to reduce the statistical uncertainty. Such a rare process is sensitive to new physics phenomena and allows to set stringent constraints on the compatibility of the Higgs boson itself with the SM. The cross section for the VBF mechanism in proton-proton collisions at a center of mass energy of 13 TeV has been measured by CMS in several Higgs decay channels. The H->WW decay, thanks to its large branching ratio, is ideal for the observation of this production process. The most recent CMS analysis in the H ! WW decay channel, however, mainly focused on the measurement of the global production cross section: the analysis was not optimized with respect to the VBF production mode. In this Thesis, a multivariate analysis was implemented in order to enhance the sensitivity of the measurement of the VBF mechanism in the H->WW decay channel. In particular, a Deep Neural Network (DNN) was developed in order to isolate the signal events from the background, which is mainly composed by top quarks events, non-resonant WW and gluon fusion Higgs boson production mechanism. The DNN yields four scores for each event, corresponding to the degree of compatibility either with the signal or one of the main backgrounds. These scores are then combined and used in the fitting procedure. This innovative approach was necessary because one of the main backgrounds of this analysis is another Higgs production process, therefore making it diffcult to tackle this analysis in a simple signal versus background paradigm. This study is based on the whole Run-2 dataset, collected from 2016 to 2018 with the CMS experiment. The VBF Higgs production mechanism is observed with a significance of 3.6 standard deviations, resulting in the first evidence of this production mechanism in the WW decay channel with the CMS experiment. Moreover, the measured cross section is compatible with the Standard Model within one standard deviation. This work established an analysis strategy that will be used for the LHC Run-3 and possibly beyond it.