CMS reports first evidence of electroweak ZZ production with two jets at 13 TeV

CMS has reported the first evidence of electroweak production of Z-boson pairs with two jets in proton-proton collisions at 13 TeV, a rare Standard Model process that probes vector-boson scattering dynamics. The result comes from 138 fb^-1 of LHC data and focuses on the ZZjj -> llnunu jj final state, where one Z boson decays to charged leptons and the other to neutrinos.

The analysis measures a fiducial electroweak cross section of 0.37 +0.14/-0.12 (stat) +0.06/-0.06 (syst) fb, compared with a Standard Model prediction of 0.39 ± 0.06 fb. CMS reports an observed significance of 3.1 standard deviations, with 2.8 expected.

The measurement

The new paper was posted to arXiv on June 18, 2026. It is the latest CMS step in a program aimed at isolating electroweak ZZ production, which is difficult to measure because it is much rarer than strong-interaction ZZ plus jets production.

The llnunu jj channel uses events with exactly two same-flavor, opposite-charge leptons, large missing transverse momentum, and two jets separated by a large rapidity gap and with a large dijet invariant mass. That topology is the signature CMS uses to identify electroweak vector-boson-scattering-like production.

The final state is experimentally challenging because one of the Z bosons is invisible to the detector. CMS therefore relies on the visible lepton pair, the missing transverse momentum, and the jet system to separate signal from background.

Why the two-jet topology matters

Electroweak ZZ production with two jets is important because it tests how the electroweak force behaves when gauge bosons scatter off one another. The forward-jet, large-gap event structure helps distinguish that process from more common backgrounds.

In this kind of analysis, the jet pair is not a side detail. It is the key handle that lets CMS pick out a very rare interaction in a busy proton-proton environment at the Large Hadron Collider.

Limits on new physics

CMS also uses the same measurement to set limits on anomalous quartic gauge couplings in a dimension-8 effective field theory framework. Those limits are a standard way to look for deviations from the Standard Model in interactions among electroweak bosons.

The collaboration says the result strengthens constraints on the electroweak sector while remaining consistent with the Standard Model expectation. No conflict with the Standard Model is reported in the measurement itself.

Combined result

The new llnunu jj channel is not the first CMS ZZjj study. CMS previously reported evidence for electroweak production of two Z bosons plus two jets in the fully leptonic ZZ -> llll channel.

When CMS combines the new result with that earlier four-charged-lepton measurement, the evidence for electroweak production of Z-boson pairs rises to 5.0 standard deviations, with 4.5 expected. That reaches the conventional observation threshold for the combined ZZ measurement.

What comes next

For the moment, the new channel itself remains an evidence-level result rather than a standalone observation. The combined CMS picture is stronger, but the llnunu jj mode still leaves room for future updates as more data are analyzed.

The most immediate follow-up questions are whether CMS will publish a fuller breakdown of systematic uncertainties, whether the collaboration will release a companion summary, and how the new limits compare with other CMS and ATLAS constraints on anomalous quartic gauge couplings.

The broader physics significance is straightforward: each new ZZjj measurement tightens the experimental test of vector-boson scattering and the electroweak sector of the Standard Model. This result adds a new channel to that program and improves the available constraints from the LHC.