Search for Charged Higgs Bosons in the $H^{\pm} \to \tau^{\pm} \nu_{\tau}$ Final State with $139\, \mathrm{fb}^{-1}$ of $\mathrm{pp}$ Collision Data at $\sqrt{s} = 13$ TeV with the ATLAS Experiment

This dissertation uses $139\, \mathrm{fb}^{-1}$ of $\mathrm{pp}$ collision data collected at a center of mass energy of $\sqrt{s} = 13$ TeV by the ATLAS detector to search for charged Higgs bosons decaying to a tau lepton and a neutrino ( $H^{\pm} \to \tau^{\pm} \nu_{\tau}$ ) in association with a top quark. Two subchannels are defined based on the leptonic decay ( $\tau+\ell$ ) or hadronic decay ( $\tau + \text{jets} $ ) of the associated top quark. Parameterized Neural Networks are investigated and optimized to improve signal separation from background in both subchannels across a wide $m_{H^{\pm}}$ range of 80 GeV to 3000 GeV. Expected exclusion limits at 95% confidence on $\sigma( \mathrm{pp} \to tb H^{\pm})\times \mathrm{\cal{B}}(H^{\pm} \to \tau \nu)$ are compared to previous observed limits.