The cross section of top quark pair production is measured in the t ̄t→(lνl)(τhντ)b ̄ ̄ ̄b final state, where τh refers to the hadronic decays of the τ lepton, and l is either an electron or a muon. The data sample corresponds to an integrated luminosity of 35.9 fb−1 collected in proton-proton collisions at √s = 13 TeV with the CMS detector. The measured cross section is σt ̄t = 781 ± 7 (stat) ± 62 (syst) ± 20 (lumi) pb, and the ratio of the partial width Γ(t → τντb) to the total decay width of the top quark is measured to be 0.1050 ± 0.0009 (stat) ± 0.0071 (syst). This is the first measurement of the t ̄t production cross section in proton-proton collisions at √s = 13 TeV that explicitly includes τ leptons. The ratio of the cross sections in the lτh and ll final states yields a value Rlτh/ll = 0.973 ± 0.009 (stat) ± 0.066 (syst), consistent with lepton universality.
