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 t¯tW or t¯tZ decays. The t¯tW cross section is measured to be 382+117−102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The t¯tZ 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 t¯tW and t¯tZ cross sections.