Photoproduction of mesons off 7Li : properties of hadrons in nuclear matter

The study of hadrons properties has been for long time one of the central interests in strong interactions physics. At low energy (<10 GeV), where the QCD perturbative approach is not valid anymore, hadrons have a very complex behavior. At these energies, the chiral symmetry is broken. However, many models suggest a partial chiral symmetry restoration at high temperature and density. This is indirectly connected to the hotly debated topic of hadrons properties in the nuclear medium. Various theoretical and experimental programs have been pursued through the last decades in view of the investigation of a possible modification of these properties, principally the mass and width of certain mesons and nucleon resonances.
First experimental evidences came from heavy-ion collisions and from pion- induced experiments. An exciting program using meson photoproduction and aiming - among other interesting studies - at the same goal has been initiated at different electron accelerators. The present experiment was conducted during 2005 at the MAMI accelerator in Mainz using a lithium target. It profited from 4π solid angle detector setup providing data of excellent statistical quality and small systematic uncertainties. The present work summarizes the results obtained for different reactions related to the topic of hadron properties in nuclear medium.
The main studied reaction was the double pion photoproduction in view of the σ-meson properties in medium. The coherent π0 photoproduction was then investigated in view of the properties of the
Δ-resonance in-medium. This reaction served also for the extraction of the 7Li mass form factor and rms radius. Finally, the very interesting topic of meson bound states was investigated through the coherent photoproduction of η-mesons.
Concerning the structure of this work, a general introduction and a brief theoretical overview will be given first. Previous results and the motivation to choose the 7Li as a target will then be discussed. The experimental setup will be presented after that in details (properties, electronics, calibrations, identification techniques..). In order to extract the detection efficiency and the cross sections, the experimental setup was simulated using the GEANT package. The simulations will be summarized in Chapter V and developed for each reaction in chapter VI. Results will then be presented as the following:
- The double pion channel (separate analysis for the neutral and the mixed charged channels) :
- Reaction identification and signal to background estimation
- Simulations
- Invariant mass distributions and total cross sections
- Comparison and ratio of Neutral/Mixed charged mass distributions
- The coherent π0 channel :
- Reaction identification and missing energy spectra
- Simulations
- Differential and total cross sections
- Application: extraction of 7Li form factor and mass rms radius
- The coherent η channel :
- The quasi-free channel (Reaction identification, simulations, signal to background estimation, cross sections)
- Coherent reaction identification and simulation
- Missing energy spectra
- Coherent cross section
- Comparison to 3He
Finally, before a conclusion and an outlook are given, the systematic effects will be discussed.