The History of BUU Codes at Gießen

Our interest in transport calculations started with a sabbatical visit of Ulrich Mosel at Michigan State University during spring/summer of 1986. During this time also the Giessen PhD student Wolfgang Bauer spent some time there, under the guidance of George Bertsch, but working for a degree at Giessen. While George had written one of the very first BUU codes Wolfgang Bauer rewrote this code and during Mosel's visit at MSU the first calculations of particle production, in this case photons, were done and later published. With Wolfgang Bauer the very first Giessen BUU code returned to Giessen.

From then on a number of very good young scientists have worked on it. Wolfgang Cassing joined the effort at this point. First, Koji Niita (Japan) rewrote again large parts of the code and implemented pion production, photon production and a good description of the nuclear ground state, Angel de Paoli and Gustavo Batko (both Argentina) used it to calculate etas and kaons produced in heavy-ion collisions and Gyuri Wolf (Hungary) did a series of important papers on dilepton production which provided some of the motivation for the construction of HADES, the dilepton spectrometer at GSI. Very early on, a parallel development to construct a manifestly covariant BUU code was started. Giessen PhD students Volker Koch and Bernhard Blaettel wrote the first such code and used it to analyse BEVALAC cata on flow and particle production (1990). Klaus Weber in his thesis (1992) did work on a covariant description of the momentum dependence of the mean field at high energies, where the Walecka model becomes too repulsive. Tomoyuki Maruyama (Japan) followed up on this and did the first numerical implementation of an explicit momentum dependence that corrects this deficiency of the Walecka model at high energies. Giessen PhD student Andreas Lang performed the first thermal analysis of heavy-ion reactions with this code; he analyzed the degree of equilibration during heavy-ion collisions and showed that full equilibration is reached on in the latest stages of a collision when the nuclear density has already decreased below its equilibrium value. Another Giessen PhD student, Stefan Teis, then did a very complete analysis of pion and eta production in heavy-ion collisions measured by the TAPS group at GSI (1997). He also analyzed the antiprotons and very-high energy pions produced in such reactions. At about this time, diploma student Alexander Hombach did the very first application of a BUU calculation to photon-induced reactions on a nucleus, analyzing the eta-production data obtained by the TAPS group at MAMI. Alexander Hombach, in his PhD thesis, did a corresponding detailed analysis of flow observables in such collisions. Teis, together with another PhD student, Martin Effenberger, rewrote parts of the code and implemented significantly more resonance excitations into the code. At roughly the same time (1998), PhD student Jochen Geiss, applied the earlier version of the code to an analysis of ultrarelativistic heavy-ion collisions, in particular to the problem of production of strangeness and charmonium in such collisions; the interest in these questions is still alive today.

Besides the original field of application, which were heavy ion collisions, the code was then already applied to elementary collisions on a nucleus, as e.g. of photon- and pion-induced reactions. In the years 2000-05, Thomas Falter worked on the description of high-energy non-resonant electron-induced reactions. Together with Kai Gallmeister a picture for the time-dependence of fragmentation was established. Alexei Larionov has been working on the description of heavy ion collisions in the SIS energy region and above; as a major topic, he studied the influence of medium-dependent cross sections. Together with Markus Wagner he investigated strangeness production in heavy ion collisions. Jürgen Lehr included off-shell nucleons in his code version and worked on both photon- and electron- induced processes in the resonance region. The last results based on the the original Effenberger-Teis version were published by Pascal Mühlich in the year 2007. He worked on pion- and photon- induced processes both in the resonance and high energy region, thereby focusing on possible signals for changes of meson properties within the medium.

Oliver Buss during ​his PhD (2004-08) did a complete rewrite of the code and transformed it to a modular, modern FORTRAN version. Since this point we label the code by "GiBUU".

While O.Buss was rewriting the basic ingredients as e.g. the propagation routines, the first person to join the new code structure was T.Falter with his high-energetic electron-induced reactions. Later, also A.Larionov and K.Gallmeister joined the initiative. A.Larionov took care of the implementation of strangeness production and improved the baryon-baryon cross sections. Furthermore, he included a relativistic mean field in the propagation, which can now be used alternatively to the Skyrme mean field. In a joined effort, O.Buss and K.Gallmeister implemented the local ensemble method. This algorithm was not included in the precursor Effenberger-Teis version and improved the speed of the full ensemble simulations. K.Gallmeister was also a major player in the speed-up of the core routines and the basic Makefile design. Furthermore, he managed to replace FRITIOF by a modified PYTHIA version. In 2006, Tina Leitner joined the project and implemented her model for neutrino-nucleon interactions and shared the work-load in the implementation of the off-shell potential. Lately, also Theo Gaitanos (nuclear fragmentation), David Kalok (nucleon spectral functions), Birger Steinmüller (ground state properties) and Janus Weil (dileptons and vector meson production, gfortran compatibility) contributed to the new code.

In April 2008, the first source code release was published on this website under the GPL license.