The quantum electrodynamic process of photon–photon scattering has for the first time been confirmed experimentally to a high degree of certainty. CERN’s ATLAS collaboration, which involves hundreds of physicists from around the world, made the breakthrough after analysing a large dataset of candidate scattering events using a neural network. Their discovery could fuel new research into a variety of theories beyond the Standard Model of particle physics.

In classical electrodynamics, photons cannot interact with each other because they have no charge. At the same time, however, quantum electrodynamics predicts that two photons can scatter off each other by exchanging virtual charged fermions or W bosons. Some theorized extensions to the Standard Model predict that these scattering events are sensitive to as-yet unconfirmed particles, including axions and magnetic monopoles.

To test these theories, physicists at CERN’s Large Hadron Collider (LHC) have attempted to induce photon–photon scattering by firing heavy ions towards each other at relativistic speeds. As they pass closer and closer to each other, the ions exchange an increasing number of virtual photons. If scattering occurs between any two of these photons, the ion pair will lose a small amount of energy and emit a pair of real photons. These light flashes will then hit opposite sides of the detector, revealing the characteristics of the original scattering event.

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