# Institute's Profile

Research work of the Institute for Nuclear Physics at the Johannes Gutenberg-University Mainz puts its focus on understanding the phenomenon and interactions of hadrons, hence on mesons and baryons. According to present knowledge, these objects are many-body states of quarks and gluons bound through strong interaction. In the naïve quark model mesons consist of a quark-antiquark pair whereas baryons are composed of three quarks. It is known, however, that the mass of the hadrons is far larger than that of the constituent quarks, which can be described qualitatively by the dynamics of strong interactions. A quantitative approach has so far not been possible. The quantum field theoretic description of strong interactions, quantum chromo dynamics (QCD), is characterized by a weak coupling-constant at small quark distances or large momentum-transfer (so-called asymptotic freedom) and was successfully tested in high-energy physics experiments in this regime. This condition forms a sharp contrast to the understanding of strong interactions at large distances or small momentum-transfer. Here, QCD has a strong coupling-constant due to the self-energy of the color-charged gluons and a perturbative description of the QCD is not possible. This regime of confinement which leads to theoretically unexplainable hadron binding, is based on effective field theories and, recently, with increasing success, also on ab-initio-calculations according to Lattice Gauge Theory.

At the Institute for Nuclear Physics precision measurements and precision calculations are performed in order to test the theory of strong interactions at weak momentum-transfer. To do so, a large number of clearly defined observables accessible to measurement are studied. These include form factors, polarizability, polarisation observables, as well as observables of the flavour structures of hadrons and their symmetry and there mass spectrum.

Such investigation not only help understand QCD at low energies but are also important for the standard model of particle physics, in general, seeing that insufficient knowledge on the subject of strong interaction cuts down on important precision tests of the electro-weak Standard Model, thus limiting the search for extensions of the Standard Model. Some examples for important precision variables of this area include the anomalous magnetic myons plus the running electromagnetic fine-structure constant, the theoretical knowledge of which is limited because of the hadronic vacuum polarization. Also, the extraction of CKM parameters at the B-factories is significantly limited due to hadronic uncertainties.