#Higgs field theory update

The multivariate electron identification algorithm produced for this thesis is now the default for CMS analyses with data from the second LHC run. As a result, the identification of electrons remains very performant, even at a significant increase in the number of proton-proton interactions per bunch crossing.

#Higgs field theory update

Therefore, this thesis includes a rigorous update and optimization of the multivariate electron selection based on the Boosted Decision Tree (BDT) algorithm, applied within the CMS collaboration since the beginning of LHC data-taking. Among these two types of leptons, electrons are more challenging to measure. Beyond that, this thesis presents studies of the WWZ processes sensitivity to anomalous quartic gauge couplings in an effective field theory framework.Triboson production is a rare process that requires high electron and muon identification efficiencies. All results are in agreement with the standard model predictions. Combined with other analyses covering the remaining massive triboson final states, this measurement contributed to observing the production of three massive gauge bosons with a significance of 5.67 standard deviations (2.90 standard deviations for on-shell vector bosons). However, this plays a minor role in searches for new physics at energy scales beyond the electroweak scale, where triboson production has the advantage of probing a more massive final state.The analysis presented in this thesis establishes evidence for the WWZ final state with a significance of 3.35 standard deviations (1.31 standard deviations if the vector bosons are required to be on-shell). The VBS process better isolates the vector boson self-couplings, whereas triboson production is mainly affected by diagrams without any vector boson self-interaction. Additionally, triboson production is an increasingly important background to both direct searches for new physics and standard model precision measurements.While trilinear gauge couplings are also accessible via diboson production, probing quartic couplings requires the measurement of Vector Boson Scattering (VBS) or triboson production. The process is sensitive to the trilinear and quartic gauge couplings, which might be modified by physics beyond the standard model. The measurement considers 137 inverse femtobarn of collision data recorded during the second LHC run (2015 - 2018).The production of three massive gauge bosons is a so-far unobserved phenomenon predicted by the standard model of particle physics. This process has relatively little reducible background because of the fully reconstructed Z boson, and a higher production cross-section than processes with more Z bosons. The thesis work focuses on the production of two W bosons and one Z boson (WWZ), decaying to either electrons or muons.

This thesis reports on the measurement of the production of three massive gauge bosons in proton-proton collisions at 13 TeV with the CMS experiment at the CERN Large Hadron Collider (LHC).