| A measurement of the production cross sections of top quark pairs in association with a W or Z boson is presented. The measurement uses 20.3 fb-1 of data from proton-proton collisions at ✓s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. Four different final states are considered: two opposite-sign leptons, two same-sign leptons, three leptons, and four leptons. The t t¯ W and t t¯ Z cross sections are simultaneously extracted using a maximum likelihood fit over all the final states. The t t¯ Z cross section is measured to be 176+58-52 fb, corresponding to a signal significance of 4.2sigma. The t t¯ W cross section is measured to be 369+100-91 fb, corresponding to a signal significance of 5.0sigma. The results are consistent with next-to-leading-order calculations for the tt¯W and tt¯Z processes. |
