Science makes an enlightening discovery that could rewrite what we know of physics

Light beams usually pass through each other without any effect. According to electrodynamics, they can overlap in the same space without interacting. That is why the kind of light battles imagined in films would not happen in reality. But quantum physics predicts something different: a rare process called light-on-light scattering, where photons can interact by briefly creating other particles.

These particles are called virtual particles. They appear for a short time and then vanish, but they still leave measurable effects. Jonas Mager from TU Wien (Vienna University of Technology) explained: “Even though these virtual particles cannot be observed directly, they have a measurable effect on other particles. If you want to calculate precisely how real particles behave, you have to take all conceivable virtual particles into account correctly. That's what makes this task so difficult – but also so interesting.”

When photons scatter, they can temporarily turn into an electron and a positron, which then interact before disappearing again. Things become more complicated when heavier particles are involved, such as mesons. These are made of a quark and an antiquark and are subject to strong nuclear forces.

The TU Wien team has shown that one type of meson, the tensor meson, plays a bigger role than previously thought. Mager said: “We have now been able to show that one of them, the tensor mesons, has been significantly underestimated. Through the effect of light-light scattering, they influence the magnetic properties of muons, which can be used to test the Standard Model of particle physics with extreme accuracy.” Earlier calculations treated tensor mesons too simply, but the new work shows their contribution is stronger and even opposite in sign compared to past assumptions.

The Standard Model is the leading theory in particle physics that explains the fundamental particles making up matter and how they interact through three forces: electromagnetic, weak, and strong nuclear forces. It includes quarks, leptons, force-carrying bosons, and the Higgs boson, which gives particles mass. Developed in the 1970s, it has successfully predicted many experimental results, though it does not fully explain gravity, dark matter, or dark energy.

Within it, a muon is a fundamental subatomic particle similar to an electron, but about 207 times heavier and highly unstable. Produced when cosmic rays strike Earth’s atmosphere, muons decay in microseconds and help scientists test the Standard Model of Particle Physics with extreme precision.

This connects to a bigger challenge in particle physics. The anomalous magnetic moment of the muon is one of the most precise tests of the Standard Model. To calculate it, scientists must include all possible contributions from hadronic light-by-light scattering. Short-distance constraints from quantum chromodynamics (QCD) are important here. Previous models matched these constraints only partly.

The TU Wien study shows that tensor mesons can help fill this gap. In holographic QCD, their infinite tower of excited states contributes specifically to the symmetric longitudinal short-distance constraint. Numerically, they add a sizeable positive effect from the low-energy region below 1.5 GeV, a smaller one from the mixed region, and almost none from high energies. This could explain the remaining difference between dispersive and lattice results for the full hadronic light-by-light contribution.

A meson is a subatomic particle made of one quark and one antiquark bound together by the strong nuclear force. Examples include pions and kaons. A tensor meson is a special type of meson distinguished by its spin-2 quantum state, mathematically described as a symmetric rank-2 tensor. Unlike ordinary scalar or vector mesons, tensor mesons have more complex angular momentum properties and play an important role in advanced quantum chromodynamics and light-scattering calculations.

Anton Rebhan from TU Wien explained their method: “The tensor mesons can be mapped onto five-dimensional gravitons, for which Einstein's theory of gravity makes clear predictions. We now have computer simulations and analytical results that fit well together but deviate from certain previous assumptions. We hope that this will also provide new impetus to accelerate already planned specific experiments on tensor mesons.”

The findings, published in Physical Review Letters, help reduce uncertainties in muon magnetic moment calculations. By clarifying the role of tensor mesons, the study strengthens confidence in theoretical predictions and supports ongoing efforts to test whether the Standard Model is complete or if new physics lies beyond it.

Source: Vienna University of Technology, Physical Review Letters

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