Abstract
The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All measurements are consistent with next-to-leading-order calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.
Highlights
The ATLAS detectorThe ATLAS detector [14] consists of four main subsystems: an inner tracking system, electromagnetic (EM) and hadronic calorimeters, and a muon spectrometer
(b) models of physics beyond the SM
The other regions considered in the opposite-sign dilepton channel have lower sensitivity and are used as control regions; event counting is used in the 2 -noZ-3j region, while the scalar sum of the jet transverse momenta (HThad) is used as a discriminant in the 2 -Z-3j and 2 -Z-4j regions
Summary
The ATLAS detector [14] consists of four main subsystems: an inner tracking system, electromagnetic (EM) and hadronic calorimeters, and a muon spectrometer. Hadron calorimetry is provided by a steel/scintillator-tile calorimeter, segmented into three barrel structures in the range |η| < 1.7, and two copper/LAr hadronic endcap calorimeters that cover the region 1.5 < |η| < 3.2. The solid angle coverage is completed with forward copper/LAr and tungsten/LAr calorimeter modules, optimised for EM and hadronic measurements respectively, and covering the region 3.1 < |η| < 4.9. The muon spectrometer measures the deflection of muon tracks in the range |η| < 2.7 using multiple layers of high-precision tracking chambers located in toroidal magnetic fields of approximately 0.5 T and 1 T in the central and endcap regions of ATLAS, respectively. The muon spectrometer is instrumented with separate trigger chambers covering |η| < 2.4
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