Particle physics research

Photon-photon interactions in the Standard Model and beyond: a new research unit at JGU has obtained funding from the DFG

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

In classical physics, the superposition of light waves leading to interference is a well-known phenomenon. An interaction of light rays in the direction of scattering is, however, conventionally impossible. Conversely, in the subatomic world, which is described by quantum effects, quantum particles of light – called photons – do 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 and beyond. 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 around 3.5 million euros over the next four years. The spokesperson for the research unit is Professor Achim Denig, experimental physicist and the co-spokesperson is Professor Marc Vanderhaeghen, theoretical physicist, both working at JGU’s Institute of Nuclear Physics.

Ill./©: Institute of Nuclear Physics

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

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

Measurements from multiple particle accelerators will provide the full picture

The full title of the new research unit is “Photon-photon interactions in the Standard Model and beyond – Harnessing the discovery potential of MESA at the LHC”. The title conveys the premise that the basis for a better understanding includes not only innovative theoretical calculations, but above all 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 the BES-III experiment in China and will be used for 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 bottom up. The research unit is also part of PRISMA+ research program cluster of excellence at JGU and includes international cooperation partners from Poland, China and CERN.

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