Hint of the Standard Model Higgs boson its its decay to $H \to ZZ^{*} \to 4l$

The Standard Model (SM) Higgs boson may be searched for at the Large Hadron Collider (LHC) in various decay channels, the choice of which is determined by the signal rates and the signal-to-background ratios in various mass regions. This dis- sertation presents the search for the SM Higgs boson in the mass range from 110 to 600 GeV/c^2 in the golden channel – H → ZZ^(*) → l^+l^−l′^+l′^− , where l, l′ = e,μ. It is one of the most promising experimental searches and is characterized by high signal-to-background ratios in the low-mass Higgs region where m_H < 2m_Z. In this low-mass region, one of the Z bosons decays on-shell ensuring high efficiency (i.e., H → ZZ*). In the high-Higgs-mass region (m_H > 2m_Z), the channel performs well, with both Z bosons decaying on-shell; this allows the search range to be extended to 600 GeV/c^2 (i.e., H → ZZ). 4.8-4.9 fb^−1 of data at √s = 7 TeV collected by the ATLAS detector from the 2011 pp collision run is used in the search that is presented. While a direct discovery of a Standard Model Higgs boson has not been made with the present analysis, exclusion limits are set on possible Higgs masses, and evidence points strongly to a low-mass Higgs near 125 GeV/c^2.