A Mainz team led by Prof. Dr. Harvey Meyer of the Institute of Nuclear Physics at Johannes Gutenberg University Mainz (JGU) has presented the first direct calculation of an important process in the subatomic world. This so-called hadronic light-to-light scattering plays an important role in exploring the limits of the Standard Model of Elementary Particle Physics, which summarizes our current understanding of the microcosm.

Quantum electrodynamics as part of the standard model describes the interaction of light quanta with matter. Among other things, it predicts that there is a scattering of light quanta, the photons, on other photons. This prediction is in clear contrast to the classical theory valid on macroscopic scales, according to which light rays pass each other without interaction. In quantum theory, this scattering occurs through the mediation of so-called virtual particles, which can be generated briefly from the vacuum due to Heisenberg’s uncertainty principle.

The Mainz physicists have looked at a special contribution to this light-to-light scattering, in which the virtual particles are so-called hadrons, which are subject to the strong interaction. In contrast to the contribution of virtual electrons, the hadronic contribution cannot be calculated using known analytical methods. “Hadrons are made up of quarks,” explains Prof. Meyer. “But the quarks cannot be separated from the hadrons. This interplay, which is also known as quark-hadron duality, together with the strength of the interaction, is what makes the problem so difficult.”

The research unit therefore calculated the hadronic light-to-light scattering using methods of lattice quantum chromodynamics (lattice QCD) by means of large-scale computer simulations. Using the “Clover” supercomputer at the Helmholtz Institute Mainz (HIM), the scientists were able to perform the enormous amount of computational operations in a relatively short time. “This study contains the equivalent of around one million hours of computing time on a typical PC processor – that’s over 100 years,” explains Dr. Georg von Hippel, one of the co-authors of the study. “In the future, we will need much more computing time to achieve our actual goal.”

The current study, which has been published in the renowned journal Physical Review Letters, is only a milestone for the Mainz-based scientists, whose research is funded by the “Precision Physics, Fundamental Interactions and Structure of Matter” (PRISMA) cluster of excellence. “The direct calculation of light-to-light scattering in lattice QCD is groundbreaking,” says PRISMA spokesperson Prof. Dr. Hartmut Wittig, “but our real goal is the precise clause of all hadronic contributions to the anomalous magnetic moment of the muon.” The magnetic moment is a property of the muon (a heavier cousin of the electron) that can be measured experimentally with high accuracy and predicted with theoretical precision. The hadronic light-to-light scattering provides a small but important contribution to the theoretical prediction, which has so far only been known with little precision.

Comparing theory and experiment makes it possible to explore the limits of the standard model of Elementary Particle Physics. And it is precisely in this comparison that a small but clear discrepancy between theoretical prediction and experimental measurement has been passing for years. “It is still too early to say that this is an indication of new physics,” says Meyer, “even if the instructions tend to point in that direction. But before the physics community is prepared to lean so far out of the window, all uncertainties must first be brought under control.”

Until now, a major source of uncertainty has been precisely the contribution of hadronic light-to-light scattering. With the help of the Technics Department techniques now demonstrated by the Mainz scientists, it should be possible in the future to predict this contribution with greater accuracy and thus test the limits of the standard model with greater precision.