Search for a bosonic decay of a charged Higgs at the LHC with the ATLAS detector

This thesis describes the search for a charged Higgs boson ($H^{\pm}$) decaying to a pseudoscalar particle ($A$) and a $W$ boson in top-quark pair events. The analysis selects events with one electron and two muons in the final state and three or more jets, at least one of which is $b$-tagged. The signal model considers an $H^{+}$ that is produced in top quark decays, while the $A$ decays to muons. The mass range for the $H^{\pm}$ is between 100 GeV and 160 GeV, and that of the $A$ is between 15 GeV and 75 GeV. This study uses 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS detector during Run 2 of the LHC to search for an excess in the $\mu^+\mu^-$ invariant mass spectrum. No significant excess above Standard Model predictions is observed in the signal region. The largest excess is found at a dimuon mass of 24 GeV and corresponds to a significance of 1.24$\sigma$. Upper limits are set at the 95$\%$ confidence level on the product of the branching ratios $t\rightarrow bH^\pm$, $H^\pm\rightarrow W^\pm A$ and $A \rightarrow \mu^+\mu^-$. The observed (expected) limits vary from $0.9$ $(1.6)$ $\times 10^{-6}$ to $6.9$ $(9.9)$ $\times 10^{-6}$, depending on the masses of the $H^{\pm}$ and $A$.