Measurement of the top-quark mass and the t<span style=

Abstract

A measurement of the top-quark mass and the t<span style="text-decoration: overline">t</span>Z cross section is performed using 4.6 fb^-1 and 20.3 fb^-1 of data from proton-proton collisions collected with the ATLAS detector at the LHC. The top-quark mass is measured in the t<span style="text-decoration: overline">t</span> eµ channel using the mean value of the m_T2 variable with the calibration curve method to obtain the first top-quark mass measurement in the dilepton channel using the ATLAS detector: m_top=175.2 ± 1.6 (stat.) ± 3.1 (syst.) GeV.<br /> Improvements in the computation of the systematic uncertainty, the measurement method and the inclusion of the t<span style="text-decoration: overline">t</span> ee and µµ channels result in a significant increase in the precision of the measurement, leading to a measured top-quark mass of m_top=173.7 ± 0.8 (stat.) ± 1.8 (syst.) GeV. The use of the m_T2perp variable is also studied, providing an additional mass measurement with similar uncertainty: m_top=173.3 ± 0.7 (stat.) ± 1.7 (syst.) GeV.<br /> A t<span style="text-decoration: overline">t</span>Z cross-section measurement is performed in the t<span style="text-decoration: overline">t</span>Z → 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 t<span style="text-decoration: overline">t</span>Z cross section of s_{t<span style="text-decoration: overline">t</span>Z} = 0.32^+0.18_-0.14 (stat.) ^+0.12_-0.05 (syst.) pb is measured, which is compatible with the Standard Model prediction for proton-proton collisions at 8 TeV.