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

Abstract

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.