Search for charged Higgs bosons produced in top-quark decays or in association with top quarks and decaying via 𝐻± →𝜏±⁢𝜈𝜏 in 13 TeV 𝑝⁢𝑝 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± →𝜏±⁢𝜈𝜏, are searched for in 140  fb−1 of proton-proton collision data at √𝑠 =13  TeV recorded with the ATLAS detector. Depending on whether the top quark is produced together with the 𝐻± decays hadronically or semileptonically, the search targets 𝜏 +jets or 𝜏 +lepton final states, in both cases with a 𝜏-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of 80 ≤𝑚𝐻± ≤3000  GeV, upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏) in the range 4.5 pb–0.4 fb. In the mass range 80–160 GeV, assuming the Standard Model cross section for 𝑡⁢¯𝑡 production, this corresponds to upper limits between 0.27% and 0.02% on ℬ⁡(𝑡 →𝑏⁢𝐻±) ×ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏).