Photon-photon interactions in the Standard Model and beyond: New research unit at JGU granted DFG funding

A pure quantum effect as the key to a better understanding of the subatomic world / New research program in Mainz bundles a wide range of expertise

The logo of the new research group showing the five subprojects in the shape of
The logo of the new research group showing the five subprojects in the shape of a so-called Feynman diagram of photon-photon interactions.

In classical physics, the superposition of light waves resulting in interference is a well-known phenomenon. An interaction of light rays in the sense of a scattering is, however, classically impossible. Conversely, in the subatomic world, which is described by quantum effects, the quantum particles of light - known as photons - do indeed interact with each other. Moreover, photon-photon interactions play a crucial role in the Standard Model of particle physics. A better understanding of this pure quantum effect is the key to gaining important new insights both within the Standard Model as well as beyond it. This photon-photon interaction is the focus of a new research unit at Johannes Gutenberg University Mainz (JGU). Funding for the research unit has just been approved by the German Research Foundation (DFG); the DFG will initially provide roughly 3.5 million Euros over the next four years. The spokesperson of the research unit is Professor Achim Denig, an experimental physicist and the co-spokesperson is Professor Marc Vanderhaeghen, a theoretical physicist, both of whom work at JGU's Institute of Nuclear Physics.

The light-by-light scattering effect was theoretically predicted by Euler and Heisenberg in 1936, but the effect has only recently been experimentally confirmed at CERN's Large Hadron Collider (LHC). It is still true that photons do not interact directly with each other in the quantum world. The scattering is caused by the exchange of virtual particles which, according to Heisenberg's uncertainty principle, can appear briefly in the vacuum - for example through the interaction with quarks, which are subject to the strong interaction. This so-called "hadronic light-by-light scattering", along with other hadronic effects, provides significant contributions to a theoretical prediction of precision observables within the Standard Model. It is important to consider that a calculation of these effects is complex and therefore usually limited in its accuracy. "The aim of our research unit is to overcome the existing limitations for describing photon-photon interactions. This will have far-reaching consequences for how we perceive subatomic matter and for precision tests of the Standard Model - for example with regard to the anomalous magnetic moment of the muon," points out Professor Achim Denig. "Photon-photon interactions are thus the key to a whole range of discoveries in the field of hadron and particle physics. The study of this interaction could potentially lead to the detection of new particles that are beyond the Standard Model - such as axion-like particles that are considered the most promising candidates for dark matter."

Measurements at several particle accelerators will provide a complete picture

The full title of the new research unit is "Photon-photon interactions in the Standard Model and beyond - Exploiting the discovery potential from MESA to the LHC". The title conveys the premise that the foundation for a better understanding includes not only innovative theoretical calculations but especially measurements at particle accelerators. The research unit will combine measurements at the lowest energy levels and highest intensities using the new MESA accelerator (JGU) with high-energy experiments at the LHC in Geneva. The results of measurements at medium energies will be equally important. These latter measurements will be obtained at the MAMI Mainz accelerator and at the BES-III experiment in China and will be used for the theoretical calculations.

The new research unit integrates the extensive, Mainz-based expertise in these research areas to establish a unique collaboration between experimental and theoretical physicists. The resulting synergies will lead to a better understanding of photon-photon interactions at all levels and from the ground up. The research unit is also part of the PRISMA+ cluster of excellence research program at JGU and includes international cooperation partners from Poland, China and CERN.