The ATLAS Collaboration reports its first observation of WWγ production – a rare process involving the simultaneous production of two W bosons and a photon.

The weak force is one of the four fundamental forces of nature. Carried by the W and Z bosons, the influence of the weak force extends from the subatomic to the astronomical, driving the formation of heavy elements and shaping the nuclear processes that power stars. The W and Z bosons were first observed by the UA1 and UA2 experiments at CERN in the early 1980s, and continue to serve as vital probes of the Standard Model.

Among the most sensitive tests of the Standard Model are multi-boson production processes, where two or more force carriers are produced together. These interactions are precisely predicted by theory, and any deviation in their strengths or rates could signal the presence of new physics phenomena.

In a new analysis of the full LHC Run-2 dataset (collected 2015–2018), ATLAS researchers identified events consistent with WWγ production, with a statistical significance of 5.9 standard deviations. This confirms a previous result from the CMS Collaboration and marks the first such observation by ATLAS.

Despite the large dataset available, identifying this process posed significant challenges. WWγ production closely resembles more common background processes, such as top-quark-pair production with a photon or Z-boson production associated with photons. Background processes with misidentified photons further complicate the analysis.

To better isolate the WWγ signal, the ATLAS team focused their search on the most characteristic signature of WWγ decay, containing an oppositely-charged electron and muon, missing transverse momentum from undetected neutrinos and a high-energy photon. They also excluded events containing jets of particles originating from b-quarks (b-jets) to suppress background top-quark processes. Leveraging the developments in lepton, photon, and b-jet identification achieved with the Run-2 data, they were able to reduce the rate of particle misidentification and improve signal purity.

A Boosted Decision Tree (BDT) was trained to identify subtle differences between signal and background events, using kinematic features. A statistical fit was then performed across the signal region and background control regions. Figure 1 shows the BDT output distribution in the signal region. The data exhibit an excess over the background prediction, well described by the WWγ signal.

The measured cross section is 6.2 ± 0.8 (stat.) ± 0.6 (syst.) fb, in agreement with the Standard Model prediction of 6.1 ± 1.0 fb. This result represents the most precise measurement of the WWγ cross section to date, with a relative uncertainty of about 16%.

Using the framework of Effective Field Theory (EFT), physicists also set new constraints on possible new particles or interactions beyond the LHC’s direct reach. These results will provide inputs to the global EFT combinations, helping to tighten the overall limits on anomalous interactions.

This new observation by the ATLAS Collaboration marks another triboson process observed at ATLAS, joining WWW, γγγ, Wγγ, Zγγ, WZγ and VVZ production. With LHC Run 3 underway and more data on the horizon, these measurements will become increasingly precise, offering crucial tests of the Standard Model’s predictions for gauge-boson interactions.

Learn more

• Observation of W+W-γ production in proton-proton collisions at 13 TeV with the ATLAS detector and constraints on anomalous quartic gauge-boson couplings (arXiv:2509.14070, see figures)
• CMS Collaboration: Observation of 𝑊⁢𝑊⁢𝛾 Production and Search for 𝐻⁡𝛾 Production in Proton-Proton Collisions at 13  TeV
• Lepton Photon 2025 presentation by Diego Baron: Measurement of rare electroweak processes including vector boson scattering and triboson in ATLAS