### Research areas

- Experimental hadron physics
- Theoretical hadron physics

### Project leaders

### Goals

The excitation spectra of hadrons reflect the underlying strong dynamics complementary to polarisabilities, elastic form factors and parton distributions. The central goal of this project is the reconstruction of partial wave amplitudes and baryon resonance properties from experimental data with minimal model dependence. We are measuring and analyzing different meson-nucleon final states in photoinduced reactions at MAMI and ELSA and in charmonium decays at BESIII. In parallel, we are developing tools for partial wave analyses and for the determination of resonance parameters.

**Photoproduction experiments at MAMI and ELSA**

Several experiments with polarized beam and target were successfully performed at MAMI and are already published. In order to overcome the energy limitation at MAMI (W≲GeV), we have started a participation in the Crystal Barrel experiment at the ELSA accelerator. In spring 2017 the cryostat of our polarized target was transported to ELSA and several runs were successfully performed. We plan to continue these activities in the last funding period. At MAMI we will focus on specific questions which arose during the last years. In particular, we plan to search in photoinduced reactions for evidence for the \(d^*(2380)\) resonance which is presently discussed as potential exotic 6-quark particle (hexaquark).**Baryon resonances in Charmonium decays**

The BESIII experiment has recently multiplied its \(J/\psi\) data set by a factor of 8 and reached the number of 10 billion \(J/\psi \) events. This unique \(J/\psi\) data set gives the opportunity to study excited baryons in a completely different environment with respect to photoproduction. We perform the analysis of the two reaction channels: \(J/\psi \rightarrow p\bar{p}\eta\) and \(J/\psi \rightarrow K_S \Sigma^+ \bar{p}\). First we determine the branching fractions of the final states which show slight deviations from former analyses or are not measured yet. After finishing these two analyses including the partial wave analysis of each channel we plan to also investigate the reaction channel \(J/\psi \rightarrow p\bar{p}\eta'\).**Partial wave analysis**

In order to perform sophisticated partial wave analyses with the available resources, we founded a collaboration with groups in Tuzla and Zagreb. We have developed and published a new EtaMAID isobar model. Modifications have been worked out to bring this MAID model in line with fixed-t dispersion relations. In parallel, a strategy to avoid strong model constraints in single-energy multipole analyses using fixed-t analyticity was developed and published. In the last funding period, we will extend all methods to other channels (\(\pi N\), \(\omega N\), \(K Y\)). In particular the MAID model will be used to obtain a multi-channel fit of all modern datasets. As a final step in all our analyses we plan to extract resonance poles and inelastic residues via a complex continuation of the obtained multipole amplitudes, e.g. using a combination of Pietarinen and Laurent expansions as a least model-dependent background and resonance parametrization.