Measurement of the $t \bar{t}$ pair production cross section in the dilepton decay channel using Lepton+Track selection in 7 TeV p-p collisions with L = 698 pb$^{-1}$

We present measurements of the t ̄ t production cross section obtained using the ATLAS detector at the LHC with pp collisions at √ s = 7 TeV in dilepton final states, from a data sample of L =698 pb − 1 taken in 2011. For dilepton pair selection we have used a ‘Lepton+Tra ck’ approach, where selection requirements on one lepton leg are reduced to a requirement of only a high qu ality Inner Detector track. This proce- dure, while allowing more background in the selected sample , enhances the sensitivity to τ -leptonic decays of the W ± and increases the t ̄ t acceptance by 20% over the conventional dilepton analysis ( where each lepton is identified as an electron or muon). Two modes of the l +track analysis were studied: the inclu- sive and the exclusive mode. The measured t ̄ t cross sections in the inclusive e TL and TL channels are 182 . 9 ± 9 . 3 (stat . ) +21 . 9 − 16 . 7 (syst . ) +8 . 2 − 7 . 3 (lum . ) pb and 171 . 0 ± 8 . 5 (stat . ) +20 . 6 − 17 . 5 (syst . ) +7 . 8 − 7 . 0 (lum . ) pb, respectively. The inclusive e TL and TL combined t ̄ t cross section is 177 . 4 ± 6 . 3 (stat . ) +18 . 4 − 15 . 2 (syst . ) ± 7 . 6 (lum . ) pb. The mea- sured t ̄ t cross sections in the exclusive e TL and TL channels are 160 . 8 ± 22 . 6 (syst . ) +45 . 1 − 33 . 1 (syst . ) +8 . 3 − 6 . 8 (lum . ) pb and 168 . 3 ± 23 . 9 (syst . ) +45 . 8 − 39 . 0 (syst . ) +9 . 0 − 7 . 5 (lum . ) pb, respectively. The exclusive e TL and TL combined t ̄ t cross section is 164 . 5 ± 16 . 4 (syst . ) +39 . 0 − 30 . 5 (syst . ) +8 . 6 − 7 . 1 (lum . ) pb. These t ̄ t cross section measurements are consistent with each other and are in good agreement with the Standard Model prediction of 164 . 6 +11 . 5 − 15 . 8 pb.