ATLAS first observes W-boson scattering into two photons

The ATLAS Collaboration at CERN used the full 2015–2018 LHC Run-2 dataset to observe two W bosons scattering into two high-energy photons for the first time, accompanied by two widely separated jets. The target is extremely rare: the dominant strong-interaction background is roughly 1,000 times more common, and after selection only about one in six candidate events was expected to be signal. The result reaches 6.2 standard deviations, with a measured production rate of 13.8 ± 3.0 femtobarns, consistent with the Standard Model prediction of 17.1 ± 2.4. It completes the observed set of electroweak processes in which W or Z bosons scatter into combinations of W, Z and photons. No deviation was found, but the result provides a baseline for more precise High-Luminosity LHC tests.