First evidence of VBS in semileptonic decays with $WVjj$ $\rightarrow$ $lvqqjj$ final state and optimization of the CMS electromagnetic calorimeter for Run III

Two main contributions are presented in this thesis: the first evidence in the search for electroweak (EW) vector boson scattering (VBS) in semileptonic decays with $WVjj$ $\rightarrow$ $lvqqjj$ finalstate, and the optimization of the CMS electromagnetic calorimeter (ECAL) trigger and localreconstruction for Run III of LHC.The VBS search uses a data set of proton-proton collisions at 13 TeV, corresponding to an integrated luminosity of 137 fb $^{-1}$ collected with the CMS detector during 2016-2018 (Run II).VBS is important because of its strong link with the EW symmetry breaking mechanism (theHiggs mechanism) in the Standard Model. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with a large pseudorapidity separation and a large dijet invariant mass, and a signature consistent with the hadronic decay of a W/Z boson. Events are separated into two categories: either the hadronically decaying W or Z boson is reconstructed as one large-radius jet, or it is identified as a pair of jets with dijet invariant mass close to the bosons mass. The signal strength is measured by fitting the distributions of machine learning based multivariate discriminators, implemented to separate the signal from the backgrounds in each category. The observed EW signal strength is $\mu$$_{EW}$ = 0.86$\frac{+0.23}{-0.21}$ = 0.86 ± 0.12 (stat) $\frac{+0.19}{-0.17}$ (syst), corresponding to a signal significance of 4.4 standard deviations (5.1 expected). The result reported in this thesis corresponds to the first evidence of vector boson scattering in a semileptonic channel at the LHC.The large total integrated luminosity delivered by LHC to the CMS detector and the increasing level of simultaneous interactions (pileup) has induced a dynamic and challenging environment for the performance of the ECAL detector during Run II. Ageing effects due to radiation damage have affected both the crystal transparency and the signal pulse shape. The performance of ECAL trigger has been evaluated using these data, and the energy reconstruction algorithms have been adapted to cope with the even more challenging conditions posed by the start of Run III in 2022.The ECAL trigger on-detector hardware employs fast digital algorithms to precisely measure the energy and timing information of ECAL energy deposits. This thesis describes the optimization of the digital filter weights which improves the robustness of the energy estimation against pileup. Moreover a novel hardware configuration is explored for the first time to help reject anomalous signals or tag out-of-time energy deposits.The reconstruction of electrons and photons in CMS depends on topological clustering of the energy deposited by an incident particle in different crystals of the electromagnetic calorimeter(ECAL). The presence of upstream material causes electrons and photons to start showering before reaching the calorimeter. Due to the 3.8 T CMS magnetic field, the energy is spread in several clusters around the primary one. It is essential to recover the energy contained in these satellite clusters in order to achieve the best possible energy resolution for physics analyses.Historically, satellite clusters have been associated to the primary cluster using a purely topological algorithm which does not attempt to remove spurious energy deposits from additionalpileup interaction. The performance of this algorithm is expected to degrade during LHC Run III because of the larger average pileup levels and the increasing levels of noise due to the ageing of the ECAL detector. New methods have been investigated in this thesis to exploit stateof-the-art deep learning architectures like Graph Neural Networks (GNN) and Transformer algorithms. These more sophisticated models, implemented for the first time in ECAL, may improve the energy resolution up to 10% and are more resilient to pileup and noise, helping to preserve the electron and photon energy resolution achieved during LHC Runs I and II.