### Research areas

- Experimental hadron physics
- Theoretical hadron physics

### Project leaders

### Associated project leader

### Goals

Project M1 aims to quantify the hadronic light-by-light (HLbL) contribution to the anomalous magnetic moment of the muon \(a_\mu\), as well as study the quark structure of pseudo-scalar, scalar, axial-vector, and tensor mesons. There is a strong interplay between theory and experiment in this project. The experimental part involves the measurement of (spacelike) photon-photon fusion processes at BESIII, the measurement of timelike meson transition form factors (TFFs) in \(e^+e^-\) annihilation at BESIII, and the Dalitz decays of \(\pi^0, \eta, \eta^\prime\) mesons measured at A2/MAMI. The theoretical part focusses on dispersion techniques necessary to interpret the meson TFFs, the production of multi-meson states from photon-photon fusion processes or meson Dalitz decays, in order to quantify their contribution to \( a_\mu\).

The highlights in the third funding period will be:

**Dispersive formalism for hadronic light-by-light scattering**To further support the experimental program at BESIII, the coupled-channel dispersive formalisms will be extended to the virtual unequal mass \(\gamma^\ast \gamma \to \pi^0 \eta\) process, as well as to the double virtual case \(\gamma^\ast \gamma^\ast \to \pi \pi, K \bar K, \pi \eta\). Subsequently, this input will be incorporated into the dispersive formalisms to estimate the HLbL contribution to\(a_\mu\).

**Light-by-light sum rules and its application to charmonium states**We plan to extend the light-by-light sum rules to the radiative decays of quarkonium states. Such analysis allows for exact relations between the \(X \to \gamma Y\) and \(Y \to \gamma X\) radiative decays for \(Y = 1^{--}\) quarkonium states, and provide an estimate of the strength originating from the contribution of states beyond \(q \bar q\).

**Lattice QCD calculations of the HLbL forward amplitudes**The direct lattice QCD program for the HLbL contribution to \(a_\mu\) is described in Project P1. In project M1 we plan a second-generation computation of the HLbL forward scattering amplitudes on the lattice, and will confront them with the improved dispersive input from the \(\pi \pi\) and \(\pi \eta\) channels.

**Spacelike meson TFF program**

We plan to analyze the \(\gamma^*\gamma \to \pi^0 \pi^0\) and \(\gamma^*\gamma \to \pi^0 \eta\) channels as well as the \(f_1\)(1285) resonance. With the increased BESIII data set, which will be available in the 3rd FP, it is also possible to increase the sensitivity for the double-tag measurement of the neutral pion transition form factor, which will allow to distinguish between different hadronic models.**Timelike meson TFF program**The BESIII analysis on the timelike \(\pi^0, \eta, \eta^\prime\) FFs will be brought to publication. Also, a publication on the high statistics A2/MAMI measurement of the timelike pion TFF using the pion Dalitz decay, for which the data was collected in 2018, is foreseen.

**Hardware Project: Construction of a crystal Zero Degree Detector (cZDD) for BESIII**

The installation of a cZDD tagger at BESIII is foreseen for the first half of the third funding period, and will open the avenue for photon and electron tagging at very small polar angles.