Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism, and is forbidden by classical physics theories (such as Maxwell’s theory of electrodynamics).
Direct evidence for light-by-light scattering at high energy had proven elusive for decades, until the Large Hadron Collider (LHC) began its second data-taking period (Run 2). Collisions of lead ions in the LHC provide a uniquely clean environment to study light-by-light scattering. Bunches of lead ions that are accelerated to very high energy are surrounded by an enormous flux of photons. When two lead ions pass close by each other at the centre of the ATLAS detector, but at a distance greater than twice the lead-ion radius, those photons can interact and scatter off one another without any further interaction between the lead ions, as the reach of the (much stronger) strong force is limited to the radius of a single proton. These interactions are known as ultra-peripheral collisions.
Yesterday, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS collaboration reported the observation of light-by-light scattering with a significance of 8.2 standard deviations. The result uses data from the most recent heavy-ion run of the LHC, which took place in November 2018. This new measurement opens the door to further study of the light-by-light scattering process, which is not only interesting in itself as a manifestation of an extremely rare QED phenomenon, but may be sensitive to contributions from particles beyond the Standard Model. It paves the way for a new generation of searches for hypothetical light and neutral particles.
Read more on the ATLAS website.