Evidence of $ZZ\gamma$ 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, $pp\to ZZ\gamma \to {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}{\mathrm{\ell }}^{\mathrm{\prime }+}{\mathrm{\ell }}^{\mathrm{\prime }-}\gamma$ with $\mathrm{\ell },{\mathrm{\ell }}^{\mathrm{\prime }}=e$ or $\mu$. This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of $p_{\mathrm{T}}^{\gamma }>20$ GeV. Eight events are selected, with a background estimate of $0.92\pm 0.15$. This results in an observed (expected) significance of the $ZZ\gamma$ final state of $4.4\sigma$ ($4.4\sigma$). The measured cross-section for $pp\to ZZ\gamma \to {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}{\mathrm{\ell }}^{\mathrm{\prime }+}{\mathrm{\ell }}^{\mathrm{\prime }-}\gamma$ in the fiducial region is ${\sigma }_{ZZ\gamma }=0.144_{-0.051}^{+0.064}{\text{ (stat.)}}_{-0.005}^{+0.007}\text{ (syst.) fb}$, in agreement with the predicted Standard Model one, ${\sigma }_{\text{fid.}}^{\text{SM}}=0.143_{-0.004}^{+0.007}$ fb.