Search for charged Higgs bosons decaying via $H^{\pm }\to {\tau }^{\pm }{\nu }_{\tau }$ in the $\tau$+jets and $\tau$+lepton final states with 36 fb${}^{-1}$ of $pp$ collision data recorded at $\sqrt{s}=13$ TeV with the ATLAS experiment

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 36.1 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 $H^{\pm }$ decays hadronically or leptonically, the search targets $\tau$+jets and $\tau$+lepton final states, in both cases with a hadronically decaying $\tau$-lepton. No evidence of a charged Higgs boson is found. For the mass range of $m_{H^{\pm }}$ = 90-2000 GeV, upper limits at the 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction $$\mathrm{B}$(H^{\pm} \to \tau^{\pm}\nu_{\tau})$ in the range 4.2-0.0025 pb. In the mass range 90-160 GeV, assuming the Standard Model cross-section for $t\bar{t}$ production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction $$\mathrm{B}$(t\to bH^{\pm}) \times $\mathrm{B}$(H^{\pm} \to \tau^{\pm}\nu_{\tau})$.