Search for charged Higgs Bosons in Multi-Lepton and Dijet final states via Vector Boson Fusion with the ATLAS Detector

This thesis reports on two independent studies conducted using $140 fb^{-1}$ of proton--proton collision data at a center-of-mass energy of $\sqrt{s}=13$~TeV, collected with the ATLAS detector during Run 2 of the Large Hadron Collider. The first study focuses on the performance and calibration of quark/gluon jet discrimination techniques, which are essential tools for a wide range of physicsanalyses at the LHC. Two approaches are explored: a track-based tagger and a boosted decision tree combining multiple jet substructure observables. The tagging efficiencies are measured for jets with transverse momentum in the range of 500~GeV to 2000~GeV. Differences between data and simulation are quantified at various working points, and corresponding correction factors are derived to enhance the precision of future physics analyses. The second study presents a combined analysis of searches for singly and doubly  charged Higgs bosons, $H^{\pm}$ and $H^{\pm\pm}$, produced through vector boson  fusion processes. These searches target decay channels involving massive vector bosons,  specifically focusing on fully leptonic final states containing electrons or  muons. The analysis sets new limits on the product of the production cross-section  and branching fraction for charged Higgs boson masses in the range of 200~GeV to  3000~GeV. The results are interpreted within the framework of the Georgi-Machacek  model, establishing the most stringent constraints to date on its parameter space  across the probed mass range.