Measurement of ZZγ production with the ATLAS detector Anke Ackermann1 1 Kirchhoff-Institute for Physics, Heidelberg University Abstract The Standard Model of particle physics predicts the rare production of triboson final states, offering a unique probe of gauge boson selfinteractions and sensitivity to anomalous quartic gauge couplings. We present the measurement of ZZγ production, utilizing the large dataset collected by the ATLAS detector during Run 2. The analysis focuses on the fully leptonic final state, pp → ZZγ → ℓ+ ℓ− ℓ′+ ℓ′− γ with ℓ, ℓ′ = e or µ. Evidence for the ZZγ production has been observed with eight candidate events and an expected background of less than one event. The measured cross-section of σZZγ = 0.144 ± 0.058 (stat.) ± 0.006 (syst.) fb is consistent with Standard Model predictions. To achieve this result, we develop a novel method to estimate the dominant background from non-prompt photons produced within jets, using a jet ratio technique to extrapolate from a high-statistics control region. This talk will summarize the analysis strategy, background estimation, and cross-section measurement, highlighting the potential for future analysis to probe new physics in the electroweak sector. 1