One of the most promising paths to identifying new physics is the measurement of fundamental quantities for which the Standard Model (SM) provides extremely precise predictions, allowing for the detection of potential discrepancies. Such an indirect search strategy is sensitive to new physics contributions at energy scales far larger than those directly accessible with the current generation of accelerators. In this context, our group is heavily involved in the “Muon g-2 Theory Initiative”, a global effort to achieve unprecedented precision in both the theoretical prediction and the experimental measurement of the anomalous magnetic moment of the muon (a_µ). Direct measurements performed at Fermilab have pushed experimental accuracy to 0.12 ppm, making a_µ one of the most precisely measured quantities in history. However, the SM predictions suffer from a significant discrepancy between the experimental inputs required to compute hadronic contributions. We are, therefore, fully committed to the precise measurement of the cross sections for the most important channels entering the prediction.

Our current focus is on the pion vector form factor — the cross section of the process e⁺e^– → π⁺π^– — for which we aim for an accuracy of O(0.5%).

New physics may lie within the MeV-GeV energy range and interact very feebly with the SM, thus escaping previous searches. The large amount of data collected by the BESIII experiment provides an excellent basis to look for the direct production of such particles in e⁺e^– annihilations. Presently, we are involved in the search for dark photons, axion-like particles, and the X17 (a particle with a mass of 17.6 MeV the ATOMKI collaboration claims to have observed in nuclear transitions of Helium and Beryllium).

In classical electrodynamics photons cannot interact with each other. However, quantum electrodynamics, the quantum field theory of the electromagnetic interaction, enables the interaction of two photons via the exchange of virtual particles. Thus, two photons can scatter, but they can also fuse into neutral hadrons, particles composed of quarks and gluons. The production cross section depends on transition form factors (TFF), which are functions of the virtualities of the two photons and parameterize the internal structure of the hadron. For real photons the TFFs are directly related to the radiative decay width of the mesons.

At e+e- colliders, two-photon collisions occur when in the scattering process both beam particles emit virtual photons. The virtuality of each photon is measured as the momentum transfer of the respective scattered beam particle. In contrast to the more prominent direct annihilation process, the photons carry predominantly small virtualities, and the produced hadrons can be (pseudo-)scalar, axial, and tensor mesons.

In our group, we study the momentum transfer dependence of TFFs of different mesons. At the BESIII experiment, momentum transfers between 0.1 and approximately 5 GeV² are accessible. Furthermore, as the cross section for two-photon collisions drops rapidly with increasing momentum transfers, the large data sets acquired with the BESIII detector allow for investigations of the TFFs over a wide range of both momentum transfers. The current focus of our activities is on the measurement of the TFFs of π⁰, η, η’, f1(1285), and f2(1270).

The coupling of photons to these hadrons is of particular importance for the Standard Model prediction of the Light-by-Light scattering contribution to the anomalous magnetic moment of the muon. The results of our measurements serve as important benchmarks or as direct input to data-driven approaches for calculations organized within the g-2 Theory Initiative.

Quarks and gluons are the fundamental degrees of freedom of the strong interaction. Confinement of the strong colour-charge, however, prevents us from directly observing and studying quarks in the experiment. Instead, quarks form colour-neutral bound states called hadrons. In the 1960’s, a whole zoo of new hadrons was discovered. They could all be described by the quark-model introduced by Gell-Mann and Zweig as either mesons, quark anti-quark pairs of opposite colour, or baryons, bound-states of three quarks carrying the three different colours. Nonetheless, already back then, it was pointed out that more complicated hadrons from a larger number of quarks or gluons was possible.

Today, thanks to high-statistics experiments – among them BESIII – a new particle zoo has emerged, with hadrons whose properties are inconsistent with our expectations for quark anti-quark mesons or three-quark baryons being discovered on a regular basis. It is the task of hadron spectroscopy to find these new, exotic hadrons, study their properties and understand the nature of the newly discovered particles.