CMS-SMP-24-014

CMS-SMP-24-014 ; CERN-EP-2026-102
Evidence of $ \mathrm{Z}\mathrm{Z}\gamma $ production and observation of 4 $ \ell\gamma $ in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
3 April 2026
Submitted to Physical Review Letters
Abstract: 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 $ \mathrm{p}\mathrm{p}\to\mathrm{Z}\mathrm{Z}\gamma\to4\ell\gamma $ ($ \ell = \mathrm{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, $ \|\eta^\gamma\| < $ 2.4, $ \Delta R(\ell,\gamma) > $ 0.5, $ m_{\mathrm{Z}} $ between 60 and 120 GeV, and the invariant mass of either of the two Z bosons combined with the photon ($ m_{\mathrm{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 $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ is studied by removing the $ m_{\mathrm{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).
Links: e-print arXiv:2604.02594 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative tree-level standard model Feynman diagrams with four isolated leptons and a photon in the final state. From left to right, photon emitted from a quark line, associated ZH production, and final-state radiation.
png pdf	Figure 1-a: Representative tree-level standard model Feynman diagrams with four isolated leptons and a photon in the final state. From left to right, photon emitted from a quark line, associated ZH production, and final-state radiation.
png pdf	Figure 1-b: Representative tree-level standard model Feynman diagrams with four isolated leptons and a photon in the final state. From left to right, photon emitted from a quark line, associated ZH production, and final-state radiation.
png pdf	Figure 1-c: Representative tree-level standard model Feynman diagrams with four isolated leptons and a photon in the final state. From left to right, photon emitted from a quark line, associated ZH production, and final-state radiation.
png pdf	Figure 2: Transverse momentum (left) and pseudorapidity (right) of the leading photon passing the loose selection and failing the tight cutoff-based identification in the nonprompt photon application region with four tight leptons. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.
png pdf	Figure 2-a: Transverse momentum (left) and pseudorapidity (right) of the leading photon passing the loose selection and failing the tight cutoff-based identification in the nonprompt photon application region with four tight leptons. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.
png pdf	Figure 2-b: Transverse momentum (left) and pseudorapidity (right) of the leading photon passing the loose selection and failing the tight cutoff-based identification in the nonprompt photon application region with four tight leptons. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.
png pdf	Figure 3: Post-fit mass distributions of the $ \mathrm{Z}\mathrm{Z}\gamma $ system in the triboson $ \mathrm{Z}\mathrm{Z}\gamma $ (left) and inclusive $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ (right) regions, obtained with the cutoff-based and multivariate photon identification, respectively. The last bin includes the events exceeding the upper bound of the x-axis. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.
png pdf	Figure 3-a: Post-fit mass distributions of the $ \mathrm{Z}\mathrm{Z}\gamma $ system in the triboson $ \mathrm{Z}\mathrm{Z}\gamma $ (left) and inclusive $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ (right) regions, obtained with the cutoff-based and multivariate photon identification, respectively. The last bin includes the events exceeding the upper bound of the x-axis. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.
png pdf	Figure 3-b: Post-fit mass distributions of the $ \mathrm{Z}\mathrm{Z}\gamma $ system in the triboson $ \mathrm{Z}\mathrm{Z}\gamma $ (left) and inclusive $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ (right) regions, obtained with the cutoff-based and multivariate photon identification, respectively. The last bin includes the events exceeding the upper bound of the x-axis. The vertical bars represent the statistical uncertainties in the data. The hatched area represents the statistical uncertainties in the simulation.

Tables
png pdf	Table 1: Post-fit event yields in the triboson $ \mathrm{Z}\mathrm{Z}\gamma $ (left) and inclusive 4 $ \ell\gamma $ (right) regions. Uncertainties are statistical only. The row labeled $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ OOA (out-of-acceptance) gives the number of events in the $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ signal sample failing the fiducial selection.

Summary

In summary, this Letter presents the first evidence in CMS for triboson production, $ \mathrm{p}\mathrm{p}\to\mathrm{Z}\mathrm{Z}\gamma\to4\ell\gamma $, where both Z bosons decay leptonically to electrons or muons, in proton-proton collisions at $\sqrt{s}=13$ TeV, collected in 2016--2018 and corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The fiducial cross section, $\sigma_{\text{fid}} = 60^{+27}_{-22}$ ab, is the smallest ever measured at the LHC. In a more inclusive signal region with FSR contributions, $ \mathrm{p}\mathrm{p}\to4\ell\gamma $ production is also observed, with a fiducial cross section of $156^{+39}_{-35}$ ab. The results are in good agreement with standard model predictions.

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