Multiboson and VBS measurements in ATLAS and CMS

Multiboson and Vector Boson Scattering Measurements in ATLAS and CMS

Overview

The paper "Multiboson and VBS measurements in ATLAS and CMS" (2604.14753) synthesizes recent experimental advances in the study of multiboson production and vector boson scattering (VBS) at the LHC, focusing on results from the ATLAS and CMS collaborations. It reports on precision measurements of diboson and triboson production, newly established CP- and polarization-sensitive observables, and systematic probes of the electroweak sector, including the stringent constraints placed on anomalous gauge couplings within the effective field theory (EFT) formalism. The work establishes the empirical foundation for a high-precision program in electroweak physics leveraging the LHC Run 2 and first Run 3 datasets.

Precision Diboson Measurements

ATLAS' measurement of $ZZ\to\ell\ell\nu\nu$ at $\sqrt{s}=13$~TeV utilizes the full Run 2 dataset, achieving a fiducial cross section $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$~fb, with consistency between fiducial and inclusive cross sections and SM NNLO predictions. The analysis provides differential results in kinematic variables up to high transverse momenta, directly informing constraints on neutral anomalous triple gauge couplings (TGCs) using both vertex-based and SMEFT approaches.

In $W\gamma$ production, ATLAS performs a double-differential cross section measurement in $W$ decay angles, extracting polarization fractions compatible with the SM and providing new sensitivity to the $W$ spin-density matrix.

Figure 1: Left: Measured double-differential cross section in $W$ decay angles $\phi_\ell$ and $\theta_\ell$. Right: Differential cross section as a function of a CP-sensitive neural network observable.

A novel neural-network-based observable is defined to separate linear and quadratic CP-odd SMEFT contributions to the $WW\gamma$ vertex, more than doubling sensitivity relative to classical angular observables and resulting in improved limits on CP-odd dimension-6 operators. These results critically test higher-order QCD and electroweak corrections at the highest available precision.

Vector Boson Scattering: Discovery and Precision Era

VBS is characterized by the production of two vector bosons in association with forward jets, probing gauge cancellations essential for unitarity and providing a natural EFT-sensitive topology. The ATLAS observation of electroweak $\sqrt{s}=13$0 production in semileptonic final states achieves a combined signal strength modifier $\sqrt{s}=13$1, with observed significance of $\sqrt{s}=13$2, an unambiguous establishment of the process in these high-statistics topologies. The analysis leverages a combination of RNN-based multivariate discriminants across all lepton flavors.

Figure 2: Left: Two-dimensional fit for VBS semileptonic signal strength; Right: Combined VBS signal strength modifiers for seven CMS channels.

CMS' global combination across seven VBS channels (fully leptonic and semileptonic) consolidates experimental control, showing all measured signal strengths compatible with the SM and enhancing the robustness of subsequent SMEFT and anomalous quartic gauge coupling (aQGC) searches. Notably, the missing piece in experimental VBS coverage, electroweak $\sqrt{s}=13$3 production, is observed in CMS by combining the $\sqrt{s}=13$4 and $\sqrt{s}=13$5 channels, reaching a combined significance of $\sqrt{s}=13$6. Signal extraction here is powered by GNN discriminants.

Figure 3: Distribution of the GNN discriminator in CMS $\sqrt{s}=13$7 VBS after maximum-likelihood fit.

First VBS measurements at $\sqrt{s}=13$8 TeV with the enlarged Run 3 dataset mark a shift towards high-precision studies, with $\sqrt{s}=13$9 ($$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$0) and $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$1 ($$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$2) channels providing the first probes of VBS topologies at the new energy frontier. The increased center-of-mass energy and luminosity extends discovery and exclusion reach, especially in the high-$$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$3 and high-$$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$4 regions targeted by BSM-sensitive analyses.

Triboson Production and Quartic Gauge Coupling Sensitivity

Triboson production directly constrains quartic gauge couplings via multi-boson emission, complementing VBS. ATLAS reports observation-level sensitivity for $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$5 ($$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$6) production with a measured cross section of $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$7~fb and significance of $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$8, as well as evidence for $$\sigma_\text{fid}(ZZ \to \ell\ell\nu\nu) = 21.0 \pm 1.0$$9 at $W\gamma$0. Limits are placed on dimension-8 operators relevant to aQGC.

CMS presents the simultaneous measurement of $W\gamma$1 and $W\gamma$2 contributions at both $W\gamma$3 and $W\gamma$4~TeV, exploiting the separation of nonresonant and resonant contributions. At $W\gamma$5~TeV, evidence for triboson production ($W\gamma$6) is achieved, with a combined inclusive signal strength $W\gamma$7, compatible with the SM.

Implications and Outlook

The systematic establishment of all major multiboson and VBS channels, and the transition towards measurement-dominated uncertainties, has profound implications for the global SMEFT and aQGC program. The precision and differential reach allow for:

• Robust constraints on BSM physics: High-statistics differential distributions, especially in kinematic regions sensitive to higher-dimension operators, tightly constrain a range of EFT operators not accessible via direct searches or inclusive Higgs measurements.
• Direct probes of gauge and CP structure: Polarization and CP-sensitive observables, leveraging advanced ML techniques, enable detailed mapping of TGC and QGC structures and provide first-class constraints on CP-odd interactions at the weak scale.
• Future developments: The expected increase in luminosity at the HL-LHC will further reduce statistical uncertainties and extend sensitivity in the high-energy tails, making precise multi-variable SMEFT fits feasible and significantly augmenting the discovery potential for subtle deviations from the SM.

Conclusion

The multiboson and VBS program at the LHC has reached comprehensive experimental coverage, with robust precision measurements and the routine use of advanced analysis techniques. All major VBS channels are established, triboson production is observed or evidenced, and the experimental platforms (ATLAS, CMS) are well-positioned for precision phase studies. The interplay with EFT and aQGC interpretations will provide a stringent, model-independent avenue for probing the electroweak sector and potential new physics at the multi-TeV scale (2604.14753).