Institut de recherche sur les lois fondamentales de l'Univers

Light-by-light scattering is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Then, using Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector in december 2015, the ATLAS Collaboration has reported the first evidence for the quasi-elastic reaction involving 4 photons, published in Nature Physics since August 2017. We describe the different steps that have led to this observation, the principles of the measurement, some results and their consequences.