Evidence of ZZ$γ$ production and observation of 4γ in proton-proton collisions at √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 → ZZ$γ$$→$ 4$γ$ ($$ = e, $μ$), with an observed (expected) significance of 3.7 (3.1) standard deviations in a fiducial region defined by $p_T^γ> 20 GeV, η^γ<$ 2.4, $ΔR(,γ)> 0.5, mZ between 60 and 120 GeV, and the invariant mass of either of the two Z bosons combined with the photon (mZγ) larger than 100 GeV, is reported. The measured (predicted) fiducial cross section is 60⁺²⁷₋₂₂ ab (47.56 ± 0.04 ab). Additionally, the inclusive production of pp → 4γ is studied by removing the mZγ requirement to include final state radiation where one Z boson decays to 2γ, 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 ± 0.09 ab).