Search for Higgs-like resonances in the ZZ-> llqq decay channel with the ATLAS experiment at LHC

The search for the Standard Model (SM) Higgs boson is one of the most cru- cial goals of the LHC physics program. The high center-of-mass energy (?s) of the LHC enables not just to search for the SM Higgs boson at low mass, i.e. be- tween 120 GeV ? 2 à- mZ , 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 mea- surements and latest Higgs boson-candidate discovery. My Ph.D. research activity was focused in the H ? ZZ ? l±l±qq ? analysis in the full Higgs mass range, which has been split in: Low Mass (120 GeV?2à-mZ) 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 ? ZZ(?) ? l±l±qq ? decay in the Low Mass range using 2011 data recorded at ?s = 7 TeV, principal subject of this dissertation. Recently, taking the mentioned discover of a particle compatible with the Higgs boson (July 4th, 2012) like a strong guide to continue this research using the full available 2012 recorded data at ?s = 8 TeV, and knowing that several Beyond SM models (BSM), compatible with the observed ? 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.