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 pₓ^γ> 20 GeV. Eight events are selected, with a background estimate of 0. 920. 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 σₙₙ⏒ = 0. 144 -₀. ₀₅₁^+0. 064 (stat. ) -₀. ₀₀₅^+0. 007 (syst. ) fb, in agreement with the predicted Standard Model one, σ₅₈₃. ^SM = 0. 143 -₀. ₀₀₄^+0. 007 fb.
Aad et al. (Thu,) studied this question.