Search for Charged Higgs Bosons in $H^+\rightarrow W^+h$ Decays with the ATLAS Detector

Various theories predicting an extended Higgs sector predict also the existence of at least one set of charged Higgs bosons. The main production mode of these new particles depends on their mass. For charged Higgs boson masses larger than the sum of the top and the bottom quark mass, the dominant production mode is expected to be in association with a top quark and a bottom quark ($tbH^{+}$). In the alignment limit of the two Higgs-Doublet model (2HDM), heavy charged Higgs bosons with m(H$^+$)$>$m(t)+m(b) decay almost exclusively via $H^{+}\rightarrow tb$. However, in other models such as the N2HDM or in Higgs triplet models e.g Georgia-Machacek model, significant branching ratios for $H^{+}\rightarrow W^{+}h$ are possible. The latter decay mode is so far uncovered by searches at the LHC. This thesis presents a search for charged Higgs bosons in $H^{+}\rightarrow W^{+}h\rightarrow \ell\nu bb$ decays in final states containing multiple jets, missing transverse energy and a charged lepton. The ATLAS Run 2 dataset, corresponding to a integrated luminosity of 139 $fb^{-1}$ of proton-proton collisions recorded at a centre of mass energy $\sqrt{s}= 13 TeV$, is used. Boosted decision trees (BDTs) are used to solve the jet combinatorics, thus reconstructing the charged Higgs boson decay. Event selection criteria are developed to define signal and control regions. In addition, data driven methods are explored aiming to improve the data-to-simulation agreement in the signal and control regions. The final search sensitive is evaluated in terms of expected upper limits on the cross-section time branching ratio for the $pp\rightarrow tbH^+\rightarrow tb Wh$ process by performing a maximum likelihood fit. The expected upper limits range from 0.1215 pb, for a charged Higgs boson mass of 400 GeV to 0.0179 pb for a charged Higgs boson mass of 1600 GeV.