Evidence of $ZZγ$ production with the ATLAS detector

Abstract: 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, $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ with $\ell, \ell' = e$ 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 $p_{\mathrm{T}}^γ> 20$ GeV. Eight events are selected, with a background estimate of $0.92\pm0.15$. This results in an observed (expected) significance of the $ZZγ$ final state of $4.4σ$ ($4.4σ$). The measured cross-section for $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ in the fiducial region is $σ{ZZγ} = 0.144 _{-0.051}^{+0.064} \text{ (stat.)} _{-0.005}^{+0.007} \text{ (syst.) fb}$, in agreement with the predicted Standard Model one, $σ{\textrm{fid.}}^{\textrm{SM}} = 0.143 _{-0.004}^{+0.007}$ fb.