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 ${\text{fb}}^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with the $H^{\pm }$ decays hadronically or semi-leptonically, the search targets $\tau$+jets or $\tau$+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 $80\le m_{H^{\pm }}\le 3000$ 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 $t\bar{t}$ production, this corresponds to upper limits between 0.27% and 0.02% on $\mathcal{B}(t\to b{H}^{\pm })\times \mathcal{B}(H^{\pm }\to {\tau }^{\pm }{\nu }_{\tau })$.