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Beiträge zur Erforschung der starken Wechselwirkung

English title Investigations of the Strong Interaction
Applicant Krusche Bernd
Number 113511
Funding scheme Project funding (Div. I-III)
Research institution Departement Physik Universität Basel
Institution of higher education University of Basel - BS
Main discipline Nuclear Physics
Start/End 01.01.2007 - 31.12.2007
Approved amount 500'745.00
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Keywords (8)

Struktur des Nukleons; In - Mediumeigenschaften von Nukleonen; Photonukleare Reaktionen; Niederenergie Reaktionen (nukleare Astrophysik); structure of hadrons; hadron-in-medium properties; photoproduction of mesons; proton - antiproton reactions

Lay Summary (English)

Lead
Lay summary
The main topic of the research project is the structure of hadrons and in-medium modifications of their properties. Hadrons are particles which participate in the so-called strong interaction, which is one of the four fundamental forces of nature (gravitation, weak force, electromagnetism, strong force). They are composed of elementary particles called quarks. All hadrons known up to now belong to one of two different classes, the baryons (e.g. protons and neutrons called nucleons) which are composed of three quarks and the mesons (e.g. pions), which are made of quark - antiquark pairs. The structure of hadrons is intimately related to the properties of the strong interaction, in a regime where no perturbative treatment of the theory is possible.

The structure of nucleons is mostly described in the framework of models ('constituent quark models') and the most important step towards an understanding of their structure is the identification of the relevant effective degress-of-freedom. Recently, due to the large progress in the field of lattice QCD calculations and chiral perturbation theory, a much more solid connection between experimental observations and the fundamental properties of QCD starts to emerge. However, up to now the excitation spectrum of the nucleon is far from being understood. Basically all models predict much more excited states than have been observed and even the ordering of some of the lowest lying states is not correctly predicted. Most excited nucleon states have been first observed with hadron induced reactions, in particular elastic scattering of charged pions. It is thus possible that the data base is biased for states that couple only weakly to $\pi N$.

However, the large progress in accelerator and detector technology, now allows to study the electromagnetic excitation of resonances via photon induced reactions with comparable precision, although the cross sections are much smaller. These experiments have developed into two directions:
measurements of photon induced meson production reactions up to high excitation energies and for many different nucleon - meson final states with the aim to identify at least some of the 'missing' states and precise investigations of the properties of low lying states. Our group is involved in both activities at the Bonn ELSA (maximum energy 3.5 GeV) and the Mainz MAMI (maximum energy 850 MeV, upgrade to 1.5 GeV available in2007) accelerators. Both experiments use tagged photon beams and state-of the art electromagnetic calorimeters for the detection of the decay products of the mesons. The availability of polarized photon beams combined with polarized targets opens a whole new field of experiments, which are higly sensitive to the properties of nucleon resonances.

In addition both programs investigate the modifications of hadrons embedded in the nuclear medium via the investigation of meson photoproduction off nuclear targets. In-medium modifications of the mass and widths of hadrons are on one hand side closely related to fundamental properties of QCD like partial chiral symmetry restoration and on the other hand important for the behavior of matter under extreme conditions and thus important for astrophysics.

In parallel to these experimental activities our group is involved in the technical developments of the PANDA detector, which will be the main work horse for the investigation of the strong interaction via proton - antiproton collisions at the planned FAIR facility at GSI in the next decade.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
117601 Beiträge zur Erforschung der starken Wechselwirkung 01.01.2008 Project funding (Div. I-III)
109459 Beiträge zur Erforschung der starken Wechselwirkung 01.01.2006 Project funding (Div. I-III)

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