Measurement of the top quark pair cross section with ATLAS in $pp$ collisions at $\sqrt{s} =$ 7 TeV using final states with an electron or a muon and a hadronically decaying $\tau$ lepton

A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the LHC at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb^-1. Events with an isolated electron or muon and a tau lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, sigma_ttbar = 186+/- 13 (stat) +/- 20 (syst) +/- 7 (lum.) pb is in good agreement with the Standard Model prediction.