Measurements of ttbar pairs produced in association with electroweak gauge bosons using the ATLAS detector

The large centre-of-mass energy available at the proton-proton collider LHC allows for the copious production of top-quark pairs in association with electroweak gauge bosons (W / Z / gamma) at high transverse momenta. The tt_bar+Z and tt_bar+W production cross sections are simultaneously measured using a combined fit in several analysis regions. The measurement of the tt_bar+Z cross section is used to set constraints on effective field theory operators which modify the tt_barZ vertex. The ttgamma measurements are performed in single-lepton and dilepton final states in a fiducial volume. The differential cross-sections are measured as a function of several photon kinematic variables.