Evidence of $ZZγ$ production with the ATLAS detector

This paper 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→⁺⁻'⁺'⁻γ with , ' = 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 pT^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 $4.4σ$ ($4.4σ$). The measured cross-section for pp→ ZZγ→⁺⁻'⁺'⁻γ in the fiducial region is σZZγ = 0.144 -0.051+0.064 (stat.) -0.005+0.007 (syst.) fb, in agreement with the predicted Standard Model one, σfid.SM = 0.143 -0.004+0.007 fb.