Observation of the Higgs boson in the WW channel and search for Higgs boson pair production in the bb̄bb̄ channel with the ATLAS detector

This dissertation presents the observation and measurement of the Higgs boson in the H→ WW*→ ℓνℓν channel at √s = 7 TeV and √s = 8 TeV and a search for Higgs pair production in the HH→ bb̄bb̄ channel at √s = 13 TeV with the ATLAS detector in pp collisions at the Large Hadron Collider. First, the discovery of a particle consistent with the Higgs boson in 4.8 fb−1 at √s = 7 TeV and 5.8 fb−1 at √s = 8 TeV is discussed. Then, the measurement of the Higgs boson signal strength and cross section in both the gluon fusion and vector boson fusion (VBF) production modes using 20.3 fb−1 of √s = 8 TeV data combined with 4.8 fb−1 of 7 TeV data is shown. The combined signal strength is measured to be µ = 1.09−0.21+0.23. The total observed significance of the H → WW∗ process is observed to be 6.1σ (with 5.8 σ expected). Advanced methods for background reduction and estimation, particularly in same-flavor lepton final states, are shown. The VBF signal strength is measured to be µVBF = 1.27−0.45+0.53 with an observed significance of 3.2σ (with 2.7 σ expected). In the VBF channel, a selection requirement based method, the precursor to the final multivariate technique used for the result, is detailed. Finally, a search for Higgs pair production in the bb̄bb̄ final state with 3.2 fb−1 at √s = 13 TeV is presented. A particular focus is placed on a tailored signal region for resonant production of Higgs pairs at high masses. No significant excesses are observed, and upper limits on cross sections are placed for spin-2 Randall Sundrum gravitons (RSG) and narrow spin zero resonances. The cross section of σ(pp → GKK* → hh → bbbb) with k/MPl=1 is constrained to be less than 70 fb for masses in the range 600 < mGKK* < 3000 GeV. The cross section upper limits for σ(pp → H → hh → bbbb) range from 30 to 300 fb in the mass range of 500 < mH < 3000 GeV. Higgs boson; LHC; ATLAS; WW; Di-Higgs; Beyond the Standard Model Physics; HH; Boosted jets;