Observation of top quark pairs produced in association with a vector boson in pp collisions at √s=8 TeV

Abstract

Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb−1, collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from tt¯W or tt¯Z decays. The tt¯W cross section is measured to be 382 + 117− 102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The tt¯Z cross section is measured to be 242 + 65− 55 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the tt¯W and tt¯Z cross sections.