Evidence of ZZ$\gamma$ production and observation of $4\mathrm{\ell }\gamma$ 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$\mathrm{\ell }\gamma$ ($\mathrm{\ell }$ = e, $\mu$), with an observed (expected) significance of 3.7 (3.1) standard deviations in a fiducial region defined by $p_{\mathrm{T}}^{\gamma }$ $>$ 20 GeV, $\mid {\eta }^{\gamma }\mid$ $<$ 2.4, $\mathrm{\Delta }R(\mathrm{\ell },\gamma )$ $>$ 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${}_{-22}^{+27}$ ab (47.56 $\pm$ 0.04 ab). Additionally, the inclusive production of pp $\to$ 4$\mathrm{\ell }\gamma$ is studied by removing the $m_{\text{Z}\gamma }$ requirement to include final state radiation where one Z boson decays to 2$\mathrm{\ell }\gamma$, yielding an observed (expected) significance of 5.0 (4.2) standard deviations and a measured (predicted) fiducial cross section of 156${}_{-35}^{+39}$ ab (99.97 $\pm$ 0.09 ab).