Nat. Phys. 13, 852–858 (2017)

Scattering of photons by other photons, without the assistance of nonlinear processes in a material, is prohibited in the classical realm. However, in extreme conditions where large field strengths dictate that a quantum electrodynamical description of interactions is required, light-by-light scattering is possible, although it remains difficult to observe using even the most powerful lasers. Now the ATLAS Collaboration, which includes some 3,000 researchers and 182 institutions from around the world, has used the Large Hadron Collider (LHC) and the ATLAS detector to provide evidence for light-by-light scattering from collisions of lead ions with one another. The ATLAS detector is a cylindrical particle detector at the LHC facility in Geneva. Extreme electromagnetic field strengths can result from the collisions of relativistic lead ions and the team determined 13 potential light-by-light scattering events in a dataset recorded using the ATLAS detector in 2015. The team reports the steps taken to eliminate false positives due to various background events such as electron and quark pair production, or cosmic-ray muons. Events with 4.4 standard deviations of statistical significance over background were determined and the interaction cross-section was found to be in good agreement with the standard model predictions.