Observation of VVZ production at sqrt(s) = 13 TeV with the ATLAS detector

A search for the production of three massive vector bosons, VVZ (V=W,Z), in proton-proton collisions at sqrt(s)=13 TeV is performed using data with an integrated luminosity of 140 fb-1 recorded by the ATLAS detector at the Large Hadron Collider. Events produced in the leptonic final states WWZ→lνlνll (ℓ=e,μ), WZZ→lνllll, and ZZZ→llllll, and the semileptonic final states WWZ→lνqqlℓ and WZZ→lνqqlℓ are analysed. The measured cross section for the pp→VVZ process is 660-90+93(stat)-81+88(syst) fb, and the observed (expected) significance is 6.4 (4.7) standard deviations, representing the observation of VVZ production. In addition, the measured cross section for the pp→WWZ process is 442±94(stat)-52+60(syst) fb, and the observed (expected) significance is 4.4 (3.6) standard deviations, representing evidence of WWZ production. The measured cross sections are consistent with the Standard Model predictions. Constraints on physics beyond the Standard Model are also derived in the effective field theory framework by setting limits on Wilson coefficients for dimension-8 operators describing anomalous quartic gauge boson couplings.