Properties of the Higgs boson in the 4 lepton final state at the LHC with the ATLAS experiment: mass, limits on the high mass contribution and on the total width

The theme of the analyses presented in this Thesis is the measurement of the Higgs boson properties in the H$\rightarrow$ZZ$\rightarrow$4l decay channel with the ATLAS experiment at the LHC. A detailed overview on the electron calibration process is first presented. In this regard, the track-cluster combination algorithm is found to improve the energy resolution of low $E_{T}$ electrons by exploiting both track and cluster information into a maximum likelihood fit. The improvement in resolution is approximately 18-20% for J/$\Psi$ dielectron decays, and of the order of 3% for Z$\rightarrow$ee events. In addition, the E-p combination algorithm has also been applied to the H$\rightarrow$ZZ$\rightarrow$4l channel with electrons in the final state resulting in a non-negligible gain on the invariant mass distribution (4-5%). Secondly, the Higgs mass and its total width are evaluated in the H$\rightarrow$ZZ$\rightarrow$4l channel. The Higgs mass is measured in the 4l decay channel with particular interest on the beneficial effects brought by the improved electron calibration and the track-cluster combination. The mass on the full 2011 and 2012 datasets is worked out with a 2-dimensional fit on the invariant mass of the 4 lepton final state, $m_{4l}$, and on a boosted decision tree (BDT)-based output conceived against the main ZZ irreducible background and constructed on variables that are sensitive to the Higgs boson spin-parity state. Regarding the Higgs width, results are based on a relatively recent approach aimed at indirectly constraining the Higgs boson width by exploiting the 4l high-mass region where the Higgs boson acts as a propagator. The Higgs production cross section in the on-shell $m_{4l}$ region, where the Higgs boson is a resonance, depends on the total Higgs width, whereas this is not the case for the high mass $m_{4l}$ (off-shell). Limits on the Higgs width can be therefore set when merging the off-shell results with the on-shell ones. A limit of ∼ 6.7 times $\Gamma_{SM}$ is obtained in the four lepton channel. Secondly, by combining with the on-shell measurement and using all the decay channels in the analysis, i.e. ZZ$\rightarrow$4l, ZZ$\rightarrow$2l2$\nu$ and WW$\rightarrow$l$\nu$l$\nu$, the results lead to an observed (expected) 95% C.L. upper limit on the Higgs boson total width of 22.7 (33.0) MeV (4.2 MeV is the Standard Model predicted Higgs width at $m_{H}$=125 GeV). The last section of the thesis is devoted to the evaluation of the Higgs width at $\sqrt{s}$=14 TeV in the high luminosity scenario (High Luminosity LHC), 300 fb$^{−1}$ and 3000 fb$^{−1}$, by employing the same techniques exploited in the previous Run 1 analysis at $\sqrt{s}$=8 TeV.