Evidence of ZZ$\gamma$ production and observation of $4\ell\gamma$ in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Evidence of the production of two Z bosons and a photon in proton-proton collisions is reported for the first time in CMS. The analysis uses data collected by the CMS experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The first evidence for the process pp $\to$ ZZ$\gamma$ $\to$ 4$\ell\gamma$ ($\ell$ = e, $\mu$), with an observed (expected) significance of 3.7 (3.1) standard deviations in a fiducial region defined by $p_\mathrm{T}^\gamma$ $\gt$ 20 GeV, $\lvert\eta^\gamma\rvert$ $\lt$ 2.4, $\Delta R(\ell,\gamma)$ $\gt$ 0.5, $m_\text{Z}$ between 60 and 120 GeV, and the invariant mass of either of the two Z bosons combined with the photon ($m_{\text{Z}\gamma}$) larger than 100 GeV, is reported. The measured (predicted) fiducial cross section is 60$^{+27}_{-22}$ ab (47.56 $\pm$ 0.04 ab). Additionally, the inclusive production of pp $\to$ 4$\ell\gamma$ is studied by removing the $m_{\text{Z}\gamma}$ requirement to include final state radiation where one Z boson decays to 2$\ell\gamma$, yielding an observed (expected) significance of 5.0 (4.2) standard deviations and a measured (predicted) fiducial cross section of 156$^{+39}_{-35}$ ab (99.97 $\pm$ 0.09 ab).