Search for Pair Production of Higgs Bosons in the Four B Quark Final State With the ATLAS Detector

Abstract

This thesis presents a search for Higgs boson pair production, in the \bbbb~ final state. This search uses the full 2015 and 2016 datasets collected by the ATLAS Collaboration at center of mass energy $\sqrt{s}=13$ \TeV, corresponding to $3.2 \pm 0.2$\,\ifb\ in 2015 and $32.9 \pm 1.1$\, \ifb\ in 2016 of proton-proton collision data. Improvements with respect to the previous analysis come from the a larger dataset, detailed background estimation and additional signal regions. Search sensitivity is particularly enhanced for resonance signals between $2500$ \GeV and $3000$ \GeV. The data is found to be compatible with the Standard Model predictions and no signs of new physics have been observed. The results are interpreted in the context of the bulk Randall-Sundrum warped extra dimension model with a Kaluza-Klein graviton $k/\bar{M}_{\rm Pl} = 1.0$ or $2.0$ decaying to two Higgs bosons, and the Type 2 two-Higgs doublet model (2HDM) where the neutral heavy CP-even scalar decays to two Higgs bosons.