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# GiBUU model

The BUU equation, which can be derived from the so-called Kadanoff-Baym-equations, describes the time evolution of the Wigner transform of the real-time Green’s function. This Wigner transform represents a generalization of the classical phase-space density. We get for each particle species one such equation. All are coupled through the gain and loss terms which represent scattering processes and the mean fields being included in the Hamiltonians.

The GiBUU model includes 61 baryonic and 31 mesonic states. The necessary parameters (e.g. pole masses, life times in vacuum, branching ratios) are based on the Manley analysis and the PDG compilation. The BUU equation is solved applying a test-particle ansatz in a full ensemble scheme which guarantees locality in the scattering processes of the test-particles. Resonances are explicitly propagated, in particular off-shell. Hence an off-shell potential according to Effenberger et al. is introduced which influences the time-development of the spectral functions. The loss and gain terms include besides particle decays also two and three-body reaction channels. The low-energy two-body reaction rates are to a large extent given by resonance excitations. Whereas at higher center-of-mass energies (above 2 GeV for meson-baryon and above 2.6 GeV baryon-baryon scattering) an enhanced version of Pythia is implemented to describe the reaction processes. The Hamiltonian of the nucleon and baryonic resonances includes a momentum-dependent Skyrme-like potential. For the pion, we consider a low-energy potential based on the Delta-hole model and on pionic atom phenomenology. Also Coulomb distortions are taken into account. The nuclear ground state is treated within a local Thomas-Fermi approximation. For this the nuclear density profiles are parametrized according to elastic electron-scattering data and Hartree-Fock nuclear-many-body calculations. The disintegration of excited nuclear residues is simulated on the basis of the Statistical Multifragmentation Model. The GiBUU code is written modular in Fortran 2003 and is being developed in a multi-user environment.

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