A milestone in triboson physics: first ATLAS observation of WWγ
The ATLAS Collaboration has reached a new milestone by observing WWγ production in proton-proton collisions at 13 TeV, a rare process in which two W bosons are produced together with a photon. This triboson process, predicted by the Standard Model, provides a sensitive test of the electroweak sector and the gauge structure that governs the weak force, which is carried by the W and Z bosons. The observation comes from the full LHC Run-2 data collected between 2015 and 2018 and marks ATLAS’s first explicit observation of WWγ, complementing earlier results from CMS and joining a family of measured triboson processes at ATLAS.
Why WWγ matters for the Standard Model
Multi-boson production channels like WWγ offer a stringent probe of the Standard Model’s description of gauge boson interactions. Since the weak force operates at the heart of nuclear processes and stellar phenomena, precision measurements of how W bosons and photons interact can reveal subtle effects or deviations that hint at new physics. The WWγ channel is particularly challenging due to overlapping backgrounds that mimic the signal, such as top-quark–related processes and Z-boson production with photons, as well as events where photons are misidentified.
Disentangling signal from background
To isolate the WWγ signal, ATLAS researchers targeted its most distinctive signature: an oppositely charged electron and muon pair, significant missing transverse momentum from neutrinos, and a high-energy photon. In addition, events containing jets originating from b-quarks were rejected to suppress top-quark backgrounds. The analysis benefited from refined lepton, photon, and b-jet identification developed during Run-2, which reduced misidentification rates and improved the purity of the selected sample.
Data-driven techniques and machine learning
A Boosted Decision Tree (BDT) classifier was trained to distinguish subtle differences between signal and background using several kinematic variables. A fit across the signal and control regions was then performed to estimate the WWγ contribution. The BDT’s output in the signal region shows an excess over the background expectation consistent with the WWγ hypothesis, as illustrated by the data-driven fit to the observed events.
Results: precision and implications
The measured cross section for WWγ production is 6.2 ± 0.8 (stat.) ± 0.6 (syst.) femtobarns (fb), in agreement with the Standard Model prediction of 6.1 ± 1.0 fb. This figure represents the most precise WWγ cross-section measurement to date, achieving a relative uncertainty of about 16%. The result not only confirms the predicted rate for this rare process but also provides a crucial data point for testing the gauge structure of the electroweak interaction.
Beyond the Standard Model: EFT constraints
Using an Effective Field Theory (EFT) framework, ATLAS researchers set new constraints on possible new particles or interactions that could modify triboson couplings. While no deviation from the Standard Model was observed in this analysis, the limits contribute to global EFT combinations that tighten the overall bounds on anomalous quartic gauge-boson couplings. As more data from LHC Run-3 becomes available, these measurements will sharpen tests of electroweak theory and potential new physics scenarios.
A look ahead: Run-3 prospects
With Run-3 underway and ongoing upgrades to detectors and analysis techniques, ATLAS and its collaborators anticipate even more precise measurements of WWγ and other triboson processes, such as WWW, γγγ, Wγγ, Zγγ, and VVZ. Each new result helps to map the gauge-boson interaction landscape and to probe the limits of the Standard Model with greater sensitivity.
Context and resources
The WWγ observation adds to a growing program of multi-boson studies that test the electroweak sector’s core predictions. Ongoing and upcoming results from ATLAS, CMS, and other experiments will continue to refine our understanding of the weak force, informing theories of fundamental interactions and guiding future explorations of particle physics.
