Phys. Rev. Lett. 123, 052001 (2019)

Maxwell’s electrodynamics forbids light-by-light scattering, but scattering one photon off another is allowed in quantum electrodynamics. However, light-by-light scattering is challenging to measure in experiments — even with current state-of-the-art lasers. An alternative is to use lead–lead collisions at the Large Hadron Collider, where evidence for elastic light-by-light scattering has already been seen, albeit with limited certainty.

Now, the ATLAS Collaboration has analysed a larger dataset from ultra-peripheral lead–lead collisions, where electromagnetic interactions dominate, confirming elastic light-by-light scattering with a significance of 8.2σ over the background-only hypothesis. The update relied on refined analysis techniques, including improved photon identification based on neural networks. These findings constrain models of physics beyond the standard model — for example those predicting axion-like particles.