Study of diboson production (Z or W) in the ATLAS experiment from their leptonic decays

In this dissertation, a study of ZZ and ZZ* production in proton-proton collisions at the Large Hadron Collider (LHC) at CERN is presented. The data analyzed in this study were recorded by the ATLAS experiment and correspond to integrated luminosities of 4.6 $\rm{fb^{−1}}$ at a centre-of-mass energy of 7 TeV and of 20.3 $\rm{fb^{−1}}$ at a centre-of-mass energy of 8 TeV. We select events consistent with fully leptonic ZZ decays, in particular to electrons and muons. In view of the forthcoming increase of the instantaneous luminosity of LHC, the ATLAS Collaboration foresees upgrades of the detector. In this doctoral thesis, an upgrade of the Muon Spectrometer is also presented. ZZ and ZZ* events are required to have four isolated leptons with high transverse momentum. The major backgrounds to the ZZ signal are events from Z+jets, top-quark production and other diboson processes. The background from Z+jets and top-quark contribution is estimated from data, and negligible contributions from diboson processes are estimated from Monte Carlo simulation. In 7 TeV data, we observe 66 ZZ and 84 ZZ* events with $0.9 \pm 1.1$ and $9.1 \pm 2.3$ background events, respectively. The production cross section measurement is performed with a maximum likelihood method in a restricted phase-space region for both ZZ and ZZ* decays and is then used to derive the total cross section for ZZ events produced with Z bosons in the mass range 66 to 116 GeV. The total ZZ production cross section is measured in events where the Z bosons decay to electrons or muons or one of them to neutrino, and is found to be $\sigma^{tot}_{ZZ} = 6.7 \pm 0.7(stat.) ^{+0.4}_{-0.3}(syst.) \pm 0.3(lumi.) \rm{pb}$, which is consistent with the Standard Model (SM) prediction of $5.89^{+0.22}_{-0.18} \rm{pb}$ calculated at next-to-leading order QCD. Using the 8 TeV data sample, we observe 305 ZZ events with $20.4 \pm 2.9$ background events. The total cross section of ZZ production is measured to be $\sigma^{tot}_{ZZ} = 7.1 ^{+0.5}_{-0.4}(stat.) \pm 0.3(syst.) \pm 0.2(lumi.) \rm{pb}$. The total cross section is defined by requiring both Z bosons to have an invariant mass between 66 and 116 GeV. This result is consistent with the SM expectation of $7.2^{+0.3}_{-0.2} \rm{pb}$. The anomalous Triple Gauge Couplings are zero in the SM and the ZZZ and ZZ$\gamma$ vertices are forbidden at tree level. Any deviation from their SM prediction would be an indirect indication for New Physics. The differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral Triple Gauge Boson Couplings in ZZ production. We extract exclusion limits on the four couplings at 95% confidence intervals, using a frequentist method, superseding the previous ATLAS results on these limits. Finally, a project relative to the upgrade of the ATLAS Muon Spectrometer is presented. New precision and trigger chambers are being installed in the detector during the first Long Shutdown of LHC. The incorporation of these chambers in the ATLAS Geometry Model, via Detector Description, is described. An increase of ∼ 1% in the acceptance of the Barrel Muon Spectrometer is expected by the inclusion of these detection elements.