- Measurement of low-mass e+e- pair production in 2AGeV C-C collisions with HADES (2007)
- The search for a modification of hadron properties inside nuclear matter at normal and/or high temperature and density is one of the more interesting issues of modern nuclear physics. Dilepton experiments, by providing interesting results, give insight into the properties of strong interaction and the nature of hadron mass generation. One of these research tools is the HADES spectrometer. HADES is a high acceptance dilepton spectrometer installed at the heavy-ion synchrotron (SIS) at GSI, Darmstadt. The main physics motivation of HADES is the measurement of e+e- pairs in the invariant-mass range up to 1 GeV/c2 in pion- and proton-induced reactions, as well as in heavy-ion collisions. The goal is to investigate the properties of the vector mesons rho, omega and of other hadrons reconstructed from e+e- decay pairs. Dileptons are penetrating probes allowing to study the in-medium properties of hadrons. However, the measurement of such dilepton pairs is difficult because of a very large background from other processes in which leptons are created. This thesis presents the analysis of the data provided by the first physic run done with the HADES spectrometer. For the first time e+e- pairs produced in C+C collisions at an incident energy of 2 GeV per nucleon have been collected with sufficient statistics. This experiment is of particular importance since it allows to address the puzzling pair excess measured by the former DLS experiment at 1.04 AGeV. The thesis consists of five chapters. The first chapter presents the physics case which is addressed in the work. In the second chapter the HADES spectrometer is introduced with the characteristic of specific detectors which are part of the spectrometer. Chapter three focusses on the issue of charged-particle identification. The fourth chapter discusses the reconstruction of the di-electron spectra in C+C collisions. In this part of the thesis a comparison with theoretical models is included as well. The conclusion and final remarks are given in chapter five.
- Pluto: a Monte Carlo simulation tool for hadronic physics (2007)
- Pluto is a Monte-Carlo event generator designed for hadronic interactions from Pion production threshold to intermediate energies of a few GeV per nucleon, as well as for studies of heavy ion reactions. The package is entirely based on ROOT, without the need of additional packages, and uses the embedded C++ interpreter of ROOT to control the event production. The generation of events based on a single reaction chain and the storage of the resulting particle objects can be done with a few lines of a ROOT-macro. However, the complete control of the package can be taken over by the steering macro and user-defined models may be added without a recompilation of the framework. Multi-reaction cocktails can be facilitated as well using either mass-dependent or user-defined static branching ratios. The included physics uses resonance production with mass-dependent Breit-Wigner sampling. The calculation of partial and total widths for resonances producing unstable particles is performed recursively in a coupled-channel approach. Here, particular attention is paid to the electromagnetic decays, motivated by the physics program of HADES. The thermal model supports 2-component thermal distributions, longitudinal broadening, radial blast, direct and elliptic flow, and impact-parameter sampled multiplicities. The interface allows angular distribution models (e.g. for the primary meson emission) to be attached by the user as well as descriptions of multi-particle correlations using decay chain templates. The exchange of mass sampling or momentum generation models is also possible. The first feature allows for consistent coupled-channel calculations, needed for a correct description of hadronic interactions. For elementary reactions, angular distribution models for selected channels are already part of the framework, based on parameterizations of existing data. This report gives an overview of the design of the package, the included models and the user interface.