Pluto: a Monte Carlo simulation tool for hadronic physics

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

Spin flip effects in pion-nucleus interactions

We investigate the effect of spin flip in the reaction mechanism of pion nucleus interactions, theoretically and experimentally. We construct the microscopic optical potential in configuration space from first principles, summing particle-hole pair contributions. Subsequently, we solve the coupled-channel Klein-Gordon equation including spin flip and Fermi averaging corrections. We calculate the transition matrix elements for elastic, inelastic and charge exchange scattering. We compare and contrast to previous models with the aid of a code, based on the formalism as described. In addition, we analyze the experimental spectra obtained for the double charge exchange reaction \sp{93}Nb(\pi\sp{+}, \pi\sp{-})\sp{93}Tc at a scattering angle of 5\sp\circ and three beam energies, T\sb{\pi} = 164, 230, 294 MeV, with emphasis on a particular class of states (T\sb{\u3c}) below the double isobaric analog state. The excitation function of the T\sb{\u3c} states cannot be understood without the participation of spin flip in the reaction mechanism. From our theoretical and experimental analysis we conclude that spin flip is an important component of the reaction mechanism in the scattering of pions from nuclei