Search for the Standard Model Higgs Boson in the $H\rightarrow ZZ^{*}\rightarrow\ell^{+}\ell^{-}q\bar{q}$ final state with the ATLAS detector at the LHC

The search for the Standard Model (SM) Higgs boson is one of the most crucial goals of the LHC physics program. The high center-of-mass energy ($\sqrt{s}$) of the LHC enables not just to search for the SM Higgs boson at low mass, i.e. between $120\ \GeV - 2 \times m_{\Zboson}$, but to extend it to much larger masses, in the range $200\ \GeV - 1\ \TeV$. Although a large portion of last mass range is indirectly excluded at $95\%$ Confidence Level ($CL$) by global fits to SM observables, it is crucial to complement such indirect limits by direct searches. Further, possible extensions to the SM can conspire to allow a heavy Higgs boson to be compatible with existing measurements and latest Higgs boson-candidate discovery. My Ph.D. research activity was focused in the $H \rightarrow \Zboson\Zboson \rightarrow l^{\pm}l^{\pm}q\bar{q}$ analysis in the full Higgs mass range, which has been split in: Low Mass ($120\ \GeV - 2 \times m_{\Zboson}$) and High Mass ($200\ \GeV - 1\ \TeV$), where the crucial work was performed in the inclusion, for the first time into the ATLAS research program, of the study of the $H \rightarrow \Zboson\Zboson(^*) \rightarrow l^{\pm}l^{\pm}q\bar{q}$ decay in the Low Mass range using 2011 data recorded at $\sqrt{s} = 7\ \TeV$, principal subject of this dissertation. Recently, taking the mentioned discover of a particle compatible with the Higgs boson (\emph{July 4th, 2012}) like a strong guide to continue this research using the full available 2012 recorded data at $\sqrt{s} = 8\ \TeV$, and knowing that several Beyond SM models (BSM), compatible with the observed $\sim 125\ \GeV$ resonance ($h1$) and Electroweak (EW) fit, predict a second 'SM-like' heavy Higgs state $h2$, it will be possible to search for SM resonance in the heavy mass region ($400\GeV - 1\ \TeV$) looking for excess with respect to the SM predictions regardless which model could produce such excess. This is our actual research activity into the ATLAS experiment and a view of its state of the art is included into this dissertation, as the novel tools developed during such a studies.