My nuclear research is focussing on the study of hadron properties and the search for so-called missing baryon resonances. The group is involved with experiments carried out at Jefferson Laboratory (JLab) in Virginia, USA, and at the Electron Accelerator Facility (ELSA) in Bonn, Germany (University of Bonn). I am the author and spokesperson of an experiment at JLab on the "Measurement of Double-Polarization Observables in Double-Pion Photoproduction using the CLAS Spectrometer" (E06-013). In addition, I am one of the lead authors of a proposal on the "Measurement of the Helicity Difference in &pi&eta Photoproduction using the Crystal Barrel Detector at ELSA" (ELSA 07/2005).
There has been no doubt since the discovery of the neutron in 1932 that atomic nuclei are composed of neutrons and protons (called nucleons). It turned out later that Proton and Neutron are only representatives of a large family of particles which are called Hadrons. These particles are strongly interacting. The strong force for example binds the atomic nucleus.
The mass spectrum of hadron resonances is clearly organized according to flavor content, spin and parity. Along with the observation of cross section scaling in deep inelastic scattering of leptons, this provides evidence that hadrons are built of elementary quarks and gluons, which form the basic degrees of freedom of a fundamental quantum field theory: Quantum Chromodynamics (QCD). For intermediate and long-distance phenomena such as hadron properties, the full complexity of QCD emerges, including nonlinearity and confinement, and is a strong obstacle to understanding hadronic phenomena at a fundamental level. Since the QCD Lagrangian cannot be solved in the low-energy regime and for bound states, quark models have been developed in order to predict the properties of hadronic states. Thus, the primary goals of hadron physics are to determine the relevant degrees of freedom at different scales, to relate them to each other, and ultimately to the parameters and fundamental fields of Quantum Chromodynamics.
A long-standing question in hadron physics is whether the large number of so-called missing baryon resonances really exists, i.e. experimentally not established baryon states which are predicted by constituent quark models. Nearly all existing data on non-strange production of baryon resonances result from pion-nucleon scattering experiments. However, quark models predict strong couplings of these states to "photon + p" as well as single-meson final states like N &eta, N &eta', N &rho, N &omega, but also, especially at higher energies, to two-meson final states such as N &pi &pi or N &pi &eta. At center-of-mass energies below 1.7 GeV, the single-pion production channel dominates both the pion and photoabsorption cross sections. As the c.m. energy increases towards 2.0 GeV and beyond, the two- and even three-meson final states become more dominant, and it is in this important energy region that the masses and partial widths of baryon resonances are poorly determined. Thus, photoproduction experiments investigating final states like Delta &pi, Delta &eta, and Delta &omega exhibit a large discovery potential for these missing states.