Evidence of ZZγ Production with the ATLAS Detector

This Letter presents the first evidence of the simultaneous production of two Z bosons and one photon with the ATLAS detector at the Large Hadron Collider. The measurement is performed using the full Run-2 dataset, recorded from 2015 to 2018, of proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 140  fb^{-1}. The fully leptonic final state with four leptons and one photon is analyzed, pp→ZZγ→ℓ^{+}ℓ^{-}ℓ^{'+}ℓ^{'-}γ with ℓ,ℓ^{'}=e or μ. This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of p_{T}^{γ}>20  GeV. Eight events are selected, with a background estimate of 0.92±0.15. This results in an observed (expected) significance of the ZZγ final state of 4.4σ (4.0σ). The measured cross section for pp→ZZγ→ℓ^{+}ℓ^{-}ℓ^{'+}ℓ^{'-}γ in the fiducial region is σ_{ZZγ}=0.144_{-0.051}^{+0.065}(stat)_{-0.004}^{+0.008}(syst)  fb, in agreement with the standard model prediction of σ_{fid}^{SM}=0.130±0.006  fb.