Double Drell-Yan measurement, heavy Higgs searches in the four leptons final state and forward muons performance with the ATLAS detector.

In this thesis, physics studies are presented using data collected by the ATLAS detector at CERN on 2012 corresponding to L = 20.3 fb$^{-1}$, at the LHC $\sqrt{s} = 8$ TeV. The studies performed refer to the identification of events in final states with two Z bosons decaying to four leptons (electrons, muons). Furthermore, the effect of QED radiative corrections in the $Z \rightarrow \ell \ell$ process and the muon reconstruction efficiency in the forward region of the ATLAS detector have been studied. The first study is the cross-section of the Double Drell-Yan process $Z+Z\rightarrow4\ell$. The cross-section is measured to be $~0.74^{+0.51}_{-0.44}(\text{stat.})^{+0.21}_{-0.14}(\text{syst.})$ fb. Due to the large uncertainties a 95\% CL upper limit in the cross-section is set of $1.54$ fb. This result is converted to a 95\% CL lower limit on the phenomelogical factor $\sigma_{eff}$ of $0.40$ mb. The second study is the search for an additional heavy Higgs boson in the $H \rightarrow ZZ \rightarrow 4 \ell$ final state. The search is performed in the mass range between 140 - 1000 TeV. No significant excess in data with respect to the background expectations are observed, and upper limits are set on the production cross-sections for all the mass points. The cross-section $\times$ branching ratio limits are extracted independently for the gluon-gluon fusion (ggF) and vector-boson fusion (VBF) production mechanisms. Additionally, the effect of non-collinear QED radiative corrections is examined in the H $\rightarrow ZZ \rightarrow 4\ell$. QED Final State Radiation (FSR) of leptons originating from Z bosons is reconstructed focusing on non-collinear radiation from the lepton. The photons are reconstructed with high efficiency and purity looking at $Z \rightarrow \mu\mu$ and $Z\rightarrow ee$ events in the 2012 data. At the ATLAS Run-I 2012 $H\rightarrow 4\ell$ mass measurement, 2 out of 30 Higgs candidates in the signal window (120-130 GeV) had their final invariant mass corrected for non-collinear FSR effects. Lastly, the reconstruction performance of muons in the forward pseudorapidity region of the detector is examined in the 2012 data $Z\rightarrow\mu\mu$ decays. The study focuses on the region $2.5 < |\eta| < 2.7$. The reconstruction efficiency is at the level of $>90\%$. Efficiency scale factors are extracted which are used to correct the simulation of the muon behaviour at this part of the detector, in order to match data observations.