Searches for resonant Higgs boson pair production in the $b\bar{b}WW^{(*)}$ decay channel for the boosted single-lepton final state at ATLAS

This thesis presents searches for a heavy scalar resonance ($X$) decaying into a pair of scalar bosons in the $b\bar{b}WW^{(*)}$ decay channel for the single-lepton final state and boosted topologies using the full Run 2 ATLAS proton-proton collision dataset. The scalar boson pair can either be two Standard Model Higgs bosons ($HH$) or an additional heavy scalar in conjunction with a Standard Model Higgs boson ($SH$). Due to the interplay between the masses of the scalars, various topologies, extending from fully resolved to fully boosted, are possible. Each of these topologies have their possibilities and challenges. Thus, having orthogonal cuts to define regions, where one or the other topology dominates is crucial to optimise each topology independently. Moreover, the definitions of signal, validation and control regions play a key role in any analysis.
A statistical framework, based on profile likelihood fits, is used to set expected 95% C.L. exclusion limits on the $HH$ production cross section, $\sigma(pp\to X\to HH)$, and on the cross section of $SH$ production times its branching ratio to the $b\bar{b}WW$ decay channel, $\sigma (pp\to X\to SH) \times \mathcal{BR}(SH\to b\bar{b}WW)$, for the $HH$ and $SH$ scenarios, respectively. It is also used to perform an iterative procedure of simultaneous single-bin fits to constrain the leading backgrounds in the control regions. The expected 95% C.L. exclusion limits are set by fitting the shape of the sum of the mass of the visible decay products of $X$ and the missing transverse energy ($m_\text{vis+met}$) in the blinded signal regions, with only the statistical uncertainties included. For the $HH$ samples, the best limit is expected for $m_X = 5.0\,\mathrm{TeV}$ at $2.8\,\mathrm{fb}$, which is in the same order of magnitude as the limits obtained by the boosted 0-lepton final state of the $b\bar{b}WW^{(*)}$ analysis and by the boosted $b\bar{b}b\bar{b}$ analysis. For the $SH$ samples, the best limit is expected for $m_X = 3.0\,\mathrm{TeV}$ and $m_S = 240\,\mathrm{GeV}$ at $0.87\,\mathrm{fb}$.