Observation of the Higgs boson in the $WW^*$ channel and search for Higgs boson pair production in the $b\bar{b}b\bar{b}$ channel with the ATLAS detector

This dissertation presents the observation and measurement of the Higgs boson in the $H\to WW^*\to\ell\nu\ell\nu$ channel at $\sqrt{s} = 7$ TeV and $\sqrt{s} = 8$ TeV and a search for Higgs pair production in the $HH\to b\bar{b}b\bar{b}$ channel at $\sqrt{s} = 13$ 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 $4.8 \textrm{ fb}^{-1}$ at $\sqrt{s} = 7$ TeV and $5.8 \textrm{ fb}^{-1}$ at $\sqrt{s} = 8$ 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 $20.3 \textrm{ fb}^{-1}$ of $\sqrt{s} = 8$ TeV data combined with $4.8 \textrm{ fb}^{-1}$ of $7$ TeV data is shown. The combined signal strength is measured to be $\mu = 1.09^{+0.23}_{-0.21}$. The total observed significance of the $H\to WW^*$ process is observed to be $6.1\sigma$ (with $5.8 \sigma$ 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 $\mu_{\rm VBF} = 1.27^{+0.53}_{-0.45}$ with an observed significance of $3.2\sigma$ (with $2.7 \sigma$ 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 $b\bar{b}b\bar{b}$ final state with $3.2 \textrm{ fb}^{-1}$ at $\sqrt{s} = 13$ 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 scalar resonances. The cross section of $\sigma(pp \to G^*_{kk} \to hh \to b\bar{b}b\bar{b})$ with $k/\bar{M}_{\rm Pl}=1$ is constrained to be less than $70 \textrm{ fb}$ for masses in the range $600 < m_{G^*_{kk}} < 3000$ GeV. The cross section upper limits for $\sigma(pp \to H \to hh \to b\bar{b}b\bar{b})$ range from $30$ to $300 \textrm{ fb}$ in the mass range of $500 < m_{H} < 3000$ GeV.