CMS-PAS-SMP-23-001

CMS-PAS-SMP-23-001
Observation of electroweak production of pairs of Z bosons in proton-proton collisions at 13 TeV with the CMS detector
CMS Collaboration
2026-03-20

Abstract: The CMS experiment reports the first evidence of electroweak (EW) production of pairs of Z bosons in association with two jets (jj) in the final state $ \mathrm{ZZjj} \to \ell\ell\nu\nu $ jj, where $ \ell = e, \mu $. The analysis is based on a data sample of proton--proton collisions at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Events are selected by requiring exactly two same-flavor, oppositely charged leptons, large missing transverse momentum, and two jets with a large rapidity separation and large dijet mass. The observed (expected) significance of the signal is 3.1 (2.8) standard deviations. The EW production cross section in a fiducial volume is $ \sigma_{\mathrm{EW}} $(pp $ \to $ ZZjj $ \to \ell\ell\nu\nu $ jj) = $ $ 0.37 $ ^{+0.14}_{-0.12} $ (stat) $ \pm $ 0.06 (syst) $ \mathrm{fb} $, in agreement with the standard model prediction of 0.39 $ \pm $ 0.06 $ \mathrm{fb} $. Limits on anomalous quartic gauge couplings are derived in terms of dimension-8 effective field theory operators. In combination with the previously reported result from the decay channel with four charged leptons, the observed (expected) significance is 5.0 (4.5) standard deviations.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example Feynman diagrams of the signal EW ZZ process (the first three diagrams) and the QCD ZZ background process (the last one).
png pdf	Figure 1-a: Example Feynman diagrams of the signal EW ZZ process (the first three diagrams) and the QCD ZZ background process (the last one).
png pdf	Figure 1-b: Example Feynman diagrams of the signal EW ZZ process (the first three diagrams) and the QCD ZZ background process (the last one).
png pdf	Figure 2: Distributions of the dijet invariant mass (left) and the flattened GNN discriminator (right) in the signal region after the maximum-likelihood fit. The data are shown as black points, while the stacked histograms represent the fitted signal and background contributions. The grey band indicates the total uncertainty in the prediction after the fit, including both statistical and systematic components. All Run 2 data sets are combined.
png pdf	Figure 2-a: Distributions of the dijet invariant mass (left) and the flattened GNN discriminator (right) in the signal region after the maximum-likelihood fit. The data are shown as black points, while the stacked histograms represent the fitted signal and background contributions. The grey band indicates the total uncertainty in the prediction after the fit, including both statistical and systematic components. All Run 2 data sets are combined.
png pdf	Figure 2-b: Distributions of the dijet invariant mass (left) and the flattened GNN discriminator (right) in the signal region after the maximum-likelihood fit. The data are shown as black points, while the stacked histograms represent the fitted signal and background contributions. The grey band indicates the total uncertainty in the prediction after the fit, including both statistical and systematic components. All Run 2 data sets are combined.
png pdf	Figure 3: The $ m_T $ distribution (left) with the expected contribution from a T8 operator and (right) expected and observed likelihood curves for fitting of the $ f_{\text{T8}} $ coefficient.
png pdf	Figure 3-a: The $ m_T $ distribution (left) with the expected contribution from a T8 operator and (right) expected and observed likelihood curves for fitting of the $ f_{\text{T8}} $ coefficient.
png pdf	Figure 3-b: The $ m_T $ distribution (left) with the expected contribution from a T8 operator and (right) expected and observed likelihood curves for fitting of the $ f_{\text{T8}} $ coefficient.

Tables
png pdf	Table 1: Estimated event yields for signal and background processes in the signal region after the maximum-likelihood fit, with associated uncertainties including the statistical and systematic components. The QCD-induced production of Z boson pairs is shown separately for $ \mathrm{q}\overline{\mathrm{q}} $ and $ \mathrm{g}\mathrm{g} $ initial states.
png pdf	Table 2: Expected and observed likelihood limits (95% CL) and the corresponding unitarity limits on couplings for a set of dimension-8 operators. The unitarity bounds are computed following the partial-wave eigenvalue conditions derived in Ref [4].

Summary

In summary, we report first evidence for EW production of Z boson pairs in the final state $\mathrm{ZZjj} \to \ell\ell\nu\nu $, with a significance of 3.1 standard deviations. The measured rate and fiducial cross sections are in agreement with SM predictions. A search for aQGCs shows no evidence of BSM effects and we report limits on operator coefficients in dimension-8 EFT. In combination with the previously reported result from the ZZ decay channel with four charged leptons, the observed significance is 5.0 standard deviations. With the addition of this result, the CMS experiment has observed electroweak production of all pairs of massive gauge bosons: WW, WZ, and ZZ.

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