Study of the production cross section of the Higgs boson decaying into four leptons and study of muon isolation in the ATLAS experiment at the LHC

The main subject of the thesis is the measurement of the production cross section of the Higgs boson decaying to two Z bosons in the four-lepton final state (H → ZZ* → 4l, l = e, μ), using 36.1 fb-1 of data recorded during 2015 and 2016 with the ATLAS experiment installed at the Large Hadron Collider (LHC) at CERN. The inclusive fiducial cross section in the H → ZZ* → 4l decay channel is measured to be 3.62 ± 0.53 (stat) ± 0.25 (syst) fb, in agreement with the Standard Model prediction of 2.91 ± 0.13 fb. Several differential fiducial cross sections are measured for observables sensitive to the Higgs-boson production and decay, including kinematic distributions of the jets produced together with the Higgs boson. Good agreement is found between the data and the predictions of the Standard Model. The extracted cross-section distributions are also used to constrain anomalous Higgs-boson interactions with Standard-Model particles. The second part of the thesis is dedicated to the muon isolation. Isolation variables, based on information from the tracker (track-based isolation) and the calorimeters (calorimeter-based isolation) measure the ambient activity surrounding a particle in the detectors and are a powerful tool to reject background objects. Using 36.1 fb-1 of data recorded by ATLAS in 2015 and 2016, the variable distributions are compared with simulation. Working points are then defined as sets of cuts applied to the isolation variables. Using the Z boson decays to a pair of muons, the signal efficiency of the working points is calculated. Scale factors are then derived as ratio of efficiencies measured in data over simulation. The good agreement leads to scale factors close to one.