CERN Sees High-Energy Light Interacting With Itself For The First Time

ATLAS Experiment © 2007 CERN/Claudia Marcelloni

A crucial prediction of the quantum theory of light has been confirmed by CERN. Physicists at the European Institution saw for the first time a high-energy photon-on-photon interaction, where two particles of light “hit” each other and change directions.

The process, called light-by-light scattering, is one of the oldest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism. As reported in Nature Physics, the team looked through more than 4 billion events recorded in 2015 and found 13 candidate photon interactions.

The phenomenon had escaped observation for decades, with this detection only possible thanks to the incredible (and increased) capabilities of the ATLAS experiment. Previous calculations suggested that the detector could find evidence of photon interactions during heavy ion collision events, where the nuclei of lead atoms are smashed together.

Normally, heavy ions are used to study some of the most extreme conditions in the universe, some of which have not existed since the Big Bang. Looking for them is hard work, since not many lead atoms merge when the ion beams collide. Due to this, the researchers looked for other things they can keep an eye out for during these experiments and realized that the photons surrounding the heavy ions can suddenly interact and bounce off each other.

“Finding evidence of this rare signature required the development of a sensitive new ‘trigger’ for the ATLAS detector,” Peter Steinberg, ATLAS Heavy Ion Physics Group Convener, said in a statement. “The resulting signature – two photons in an otherwise empty detector – is almost the diametric opposite of the tremendously complicated events typically expected from lead nuclei collisions. The new trigger’s success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it.”

The detection has a confidence level of 4.4 sigmas or a confidence level of 99.9989 percent, almost at the golden standard of five sigmas, the threshold for a particle physics discovery. This number will be refined next year, when the Large Hadron Collider begins the next run of heavy ion collisions. The researchers expect to have a lot more collisions than in the previous run.

"This is a milestone result: the first direct evidence of light interacting with itself at high energy," Dan Tovey, ATLAS Physics Coordinator, added. "This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED."

The new data will certainly improve precision and might even spot an example of new and unexpected physics.

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