Observation of $VVZ$ production at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

The ATLAS Collaboration reports the first observation of triple-vector-boson production $pp\to VVZ$ (with $V=W,Z$) at $\sqrt{s}=13$ TeV using 140 fb$^{-1}$ of data. The combined $VVZ$ cross section is measured as $660^{+93}_{-90}( ext{stat.})^{+88}_{-81}( ext{syst.})$ fb with a 6.4σ observed significance, while the $WWZ$ component is measured at $442\pm 94\text{(stat.)}^{+60}_{-52}\text{(syst.)}$ fb with 4.4σ observed significance, both consistent with SM predictions. A seven-channel, seven-region BDT-based signal extraction is performed via a simultaneous fit to seven signal-discriminant distributions and five control-region distributions, incorporating data-driven and MC-based background estimates. Beyond SM tests, the analysis constrains beyond-SM quartic gauge-boson couplings through an effective-field-theory framework with dimension-8 operators, providing 95% CL limits on Wilson coefficients $f_{M2}$, $f_{M3}$, $f_{M4}$, and $f_{M5}$, including unitarity-validated limits derived with energy clipping. The results strengthen the experimental scrutiny of the SM electroweak sector and yield competitive bounds on anomalous quartic gauge interactions, comparable to those from related channels such as $W\gamma jj$.

Abstract

A search for the production of three massive vector bosons, $VVZ (V=W, Z)$, in proton-proton collisions at $\sqrt{s} = 13$ TeV is performed using data with an integrated luminosity of $140$ fb$^{-1}$ recorded by the ATLAS detector at the Large Hadron Collider. Events produced in the leptonic final states $WWZ \to \ellν\ellν\ell \ell$ ($\ell=e, μ$), $WZZ \to \ellν\ell\ell \ell\ell$, $ZZZ \to \ell\ell \ell\ell \ell\ell$, and the semileptonic final states $WWZ \to qq \ellν\ell \ell$ and $WZZ \to \ellνqq \ell \ell$, are analysed. The measured cross section for the $pp \rightarrow VVZ$ process is $660^{+93}_{-90}(\text{stat.})^{+88}_{-81}(\text{syst.})$ fb, and the observed (expected) significance is 6.4 (4.7) standard deviations, representing the observation of $VVZ$ production. In addition, the measured cross section for the $pp \rightarrow WWZ$ process is $442 \pm 94 (\text{stat.})^{+60}_{-52}(\text{syst.})$ fb, and the observed (expected) significance is 4.4 (3.6) standard deviations, representing evidence of $WWZ$ production. The measured cross sections are consistent with the Standard Model predictions. Constraints on physics beyond the Standard Model are also derived in the effective field theory framework by setting limits on Wilson coefficients for dimension-8 operators describing anomalous quartic gauge boson couplings.

Figures (5)

• Figure 1: Representative Feynman diagrams for the production of three massive vector bosons, including diagrams with (a) mono boson vertices, diagrams sensitive to (b) triple and (c) quartic gauge boson couplings, and (d) the Higgsstrahlung process, where $H^{(*)}$ denotes an on-shell or off-shell Higgs boson.
• Figure 2: Comparison of the observed numbers of events to the predicted yields after fitting for all SRs and CRs. The bottom panel shows the ratio of the data and SM predictions. The uncertainty band includes both statistical and systematic uncertainties obtained by the fit.
• Figure 3: Comparison of the BDT distribution between data and predictions for the (a) 3$\ell$-1j, (b) 3$\ell$-2j-inV, (c) 3$\ell$-2j-outV, (d) 4$\ell$-DF, (e) 4$\ell$-SF-outZ, (f) 4$\ell$-SF-inZ, and (g) $5\ell$ SRs. The bottom panel shows the ratio of the data and SM predictions. The uncertainty band includes both statistical and systematic uncertainties obtained by the fit.
• Figure 4: The combined 3$\ell$+jets and 4$\ell$ channel log-likelihood curves for the Wilson coefficients (a) $f_{M2}$, (b) $f_{M3}$, (c) $f_{M4}$, and (d) $f_{M5}$. Expected and observed log-likelihood curves are shown, no unitarisation is applied.
• Figure 5: The clipping scans for the Wilson coefficients (a) $f_{M2}$, (b) $f_{M3}$, (c) $f_{M4}$, and (d) $f_{M5}$ are shown together with the unitarity limits as a function of the clipping parameter $\sqrt{\hat{s}_c}$, where the EFT contribution in events with $m_{max}(V_i V_j) > \sqrt{\hat{s}_c}$ is set to zero.