Measurement of the top-quark mass and the tt̄Z cross section in ATLAS: the tt̄ dilepton way

A measurement of the top-quark mass and the $tt̄Z$ cross section is performed using 4.6/fb and 20.3/fb of data from proton-proton collisions collected with the ATLAS detector at the LHC. The top-quark mass is measured in the tt̄ eμ channel using the mean value of the $m_{\mathrm{T}2}$ variable with the calibration curve method to obtain the first top-quark mass measurement in the dilepton channel using the ATLAS detector: $m_{\mathrm{top}} = 175.2 ± 1.6\: (\text{stat.}) ± 3.1\: (\text{syst.})$ GeV. Improvements in the computation of the systematic uncertainty, the measurement method and the inclusion of the tt̄ ee and μμ channels result in a significant increase in the precision of the measurement, leading to a measured top-quark mass of $m_{\mathrm{top}} = 173.7 ± 0.8\: (\text{stat.}) ± 1.8 (\text{syst.})$ GeV. The use of the $m_{\text{T2perp}}$ variable is also studied, providing an additional mass measurement with similar uncertainty: $m_{\mathrm{top}} = 173.3 ± 0.7\: (\text{stat.}) ± 1.7\: (\text{syst.})$ GeV. A tt̄Z cross-section measurement is performed in the $tt̄Z → 4ℓ$ channel using a likelihood fit to five signal regions and one $ZZ$ control region used to determine the normalization of the main background from data. Seven events are selected in data and, from the result of the likelihood fit, a $tt̄Z$ cross section of $σ = 0.32^{+0.18}_{-0.14}\: (\text{stat.})^{+0.12}_{-0.05} (\text{syst.})$ pb is measured, which is compatible with the Standard Model $tt̄Z$ prediction for proton-proton collisions at 8 TeV.