Search for charged Higgs bosons produced in top-quark decays or in association with top quarks and decaying via $ {H}^{\pm }\to {\tau }^{\pm }{\nu }_{\tau }$ in 13 TeV $ pp$ collisions with the ATLAS detector

Charged Higgs bosons produced either in top-quark decays or in association with a top quark, subsequently decaying via $ {H}^{\pm }\to {\tau }^{\pm }{\nu }_{\tau }$, are searched for in $ 140\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collision data at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ recorded with the ATLAS detector. Depending on whether the top quark is produced together with the $ {H}^{\pm }$ decays hadronically or semileptonically, the search targets $ \tau +\text{jets}$ or $ \tau +\mathrm{lepton}$ final states, in both cases with a $ \tau $-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of $ 80\le {m}_{{H}^{\pm }}\le 3000\text{}\text{}\mathrm{GeV}$, upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction $ \mathcal{B}({H}^{\pm }\to {\tau }^{\pm }{\nu }_{\tau })$ in the range 4.5 pb–0.4 fb. In the mass range 80–160 GeV, assuming the Standard Model cross section for $ t\overline{t}$ production, this corresponds to upper limits between 0.27% and 0.02% on $ \mathcal{B}(t\to b{H}^{\pm })\times \mathcal{B}({H}^{\pm }\to {\tau }^{\pm }{\nu }_{\tau })$.