Scientists with the ATLAS experiment at CERN report the first clear observation of a rare electroweak process that produces two isolated photons together with two jets. Using the full Run‑2 dataset of proton–proton collisions at a centre‑of‑mass energy of 13 TeV, corresponding to 140 fb⁻¹ of data, they see the electroweak γγjj signal with a significance of 6.2 standard deviations. The measured fiducial cross section is 13.8 +3.0/−2.6 femtobarns, consistent within uncertainties with the Standard Model prediction of 17.1 ± 2.4 femtobarns.

The paper separates two ways the γγjj final state can appear. The signal of interest is the pure electroweak contribution, which includes diagrams where electroweak bosons scatter off one another. These vector‑boson scattering-like events are sensitive to the self‑interaction of the gauge bosons. A larger background comes from mixed QCD–electroweak processes where the strong force produces the jets and photons in a different way. Events where photons come from hadron decays (for example π0 → γγ) are treated as non‑prompt background and are reduced by selecting photons that are isolated from other hadronic activity.

To reach these results the team selected events with two tightly identified, isolated photons and two jets in a topology expected for vector‑boson scattering: jets in opposite hemispheres and a large invariant mass between the two jets. Photons were reconstructed in the electromagnetic calorimeter and were required to pass tight identification and isolation criteria. The analysis relies on detailed Monte Carlo simulations to model the signal and backgrounds. The electroweak signal was simulated with MadGraph5_aMC@NLO interfaced to Pythia for parton showering, and alternative samples from Sherpa were used to check modelling. Non‑prompt backgrounds were estimated with a data‑driven method.