Measurements of the Properties of the Higgs-Like Boson in the Four Lepton Decay Channel with the ATLAS Detector

On July 4, 2012, experimental physicists from both the A Toroidal LHC Apparatus (ATLAS), and the Compact Muon Solenoid (CMS) experiments at the Large Hadron Collider(LHC), reported evidence for the existence of a particle consistent with the Higgs boson at the level of 5 standard deviations, with a mass around 125 GeV. In March 2013, in the light of the updated ATLAS and CMS results, CERN announced that the new particle was indeed a Higgs boson. Having analyzed two-and-a-half times more data than was available for the discovery announcement in July, the confidence of observation has risen to 10 standard deviations. The experiments were also able to show that the properties of the particle as well as the ways it interacts with other particles were well-matched with those of a Higgs boson, which is expected to have spin 0 and even(+) parity . This dissertation presents Higgs property measurements using the “golden” channel $H \to ZZ^{*} \to l^+ l^- l^{\prime +} l^{\prime -}$, where l, l′ = e, μ. A clear excess of events over the background is observed at mH = 124.3 GeV in the combined analysis of the two datasets with a significance of 6.6 standard deviations, corresponding to a background fluctuation probability of 2.7 × 10−11 . The mass of the Higgs-like boson is measured to be $m_H = 124.3^{+0.6}_{-0.5} (stat.) {}^{+0.5}_{-0.3} (syst.)$ GeV. The signal strength (the ratio of the observed cross section to the expected Standard Model (SM) cross section) at this mass is found to be μ = 1.7 ± 0.5(stat.) ± 0.4(syst.). My key contribution has been the analysis of the spin-parity quantum numbers, one of the essential measurements to establishing the new boson as the Higgs boson. This analysis is performed on events that are most consistent with decays of the new boson. It is performed on the events with resonstructed four-lepton invariant mass, m4l , satisfying 115 GeV < $m_{4l}$ < 130 GeV. The Higgs-like boson is found to be compatible with the SM expectation of J P = 0 + when comapred pairwise with J P = 0−, 1+ , 1−, 2+ and 2−. In 2013, two of the theoretical physicists who predicted the existence of the Standard Model Higgs boson, Peter Higgs and Franois Englert, were awarded the Nobel Prize in Physics. Physicists have now to pursue their measurements to determine if this Higgs particle corresponds indeed to the SM Higgs boson or if it is part of a new physics scenario.