### Research areas

- Experimental hadron physics
- Theoretical hadron physics
- Lattice gauge theory

### Project leaders

### Associated project leader

### Goals

Project P1 aims for an improved determination of the Standard Model prediction of the anomalous magnetic moment of the muon, \((g-2)_\mu\), and of the running fine structure constant, \(\alpha_{\rm QED}(M_Z^2)\). Hadronic contributions to \((g-2)_\mu\) will be determined directly via ab-initio calculations in lattice QCD. Furthermore, accurate measurements of the hadronic cross section provide crucial experimental input for the determination of the hadronic vacuum polarization using dispersion theory. Theoretical studies in lattice QCD will focus on reducing the uncertainties in the estimate of the hadronic vacuum polarization and on providing an estimate for the hadronic light-by-light scattering contribution. Furthermore, an experimental program designed to search for dark sector particles at the future MESA accelerator is being prepared. The focus is on the construction of the focal plane detector of the MAGIX experiment at MESA.

**Lattice QCD calculations of the hadronic vacuum polarization contribution**

The main goal in the third funding period is the calculation of the hadronic vacuum polarization contribution in QCD with dynamical up, down and strange quarks, with overall precision at the percent level and better. This necessitates the inclusion of strong and electromagnetic isospin-breaking effects, precise determinations of the contributions from quark-disconnected diagrams and a comprehensive investigation of finite-volume effects in conjunction with the long-distance behaviour of the vector correlator.**Hadronic light-by-light scattering in lattice QCD**

Determinations of the hadronic light-by-light scattering contribution require the calculation of four-point functions of the electromagnetic current, which will be combined with the results of a semi-analytical

calculation of a kernel function.**Measurements of the hadronic cross section**

The focus of the ISR program in the third funding period will be on an improved BESIII measurement of the channel \(e^+e^- \to \pi^+\pi^-\). Moreover, an inclusive R measurement is planned with the goal to improve the accuracy by a factor of two, which will have a strong impact on the theoretical prediction of \((g-2)_\mu\) and \(\alpha_{\rm QED}(M_Z^2)\).**Investigations of the charmonium spectrum**

The hadronic cross section program also opens the avenue for detailed investigations of the charmonium spectrum. A dedicated energy scan around the \(\chi_{c2}\) resonance is planned in order to observe the two-photon production of this resonance.**Preparation of the internal target experiment MAGIX at MESA**

For the focal plane detector of the MAGIX experiment at MESA, a time projection chamber with an open field cage and GEM readout will be designed and constructed.