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Manuscript Title: DRAGON: Monte Carlo generator of particle production from a fragmented fireball in ultrarelativistic nuclear collisions
Authors: Boris Tomásik
Program title: DRAGON
Catalogue identifier: AEDK_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 180(2009)1642
Programming language: C++.
Computer: PC Pentium 4, though no particular tuning for this machine was performed.
Operating system: Linux; the program has been successfully run on Gentoo Linux 2.6, RedHat Linux 9, Debian Linux 4.0, all with g++ compiler. It also ran successfully on MS Windows under Microsoft Visual C++ 2008 Express Edition as well as under cygwin/g++.
RAM: 100 Mbytes
Supplementary material: Sample output files from the test run, provided in the distribution, are available.
Keywords: ultrarelativistic heavy-ion collisions, particle production, fragmentation, event-by-event fluctuations, correlations, Monte Carlo generator.
PACS: 25.75.-q, 25.75.Dw, 25.75.Gz, 25.75.Ld, 25.75.Nq.
Classification: 11.2.

Nature of problem:
Deconfined matter produced in ultrarelativistic nuclear collisions expands and cools down and eventually returns into the confined phase. If the expansion is fast, the fireball could fragment either due to spinodal decomposition or due to suddenly arising bulk viscous force. Particle abundances are reasonably well described with just a few parameters within the statistical approach. Momentum spectra integrated over many events can be interpreted as produced from an expanding and locally thermalised fireball. The present Monte Carlo model unifies these approaches: fireball decays into fragments of some characteristic size. The fragments recede from each other as given by the pre-existing expansion of the fireball. They subsequently emit stable and unstable hadrons with momenta generated according to thermal distribution. Resonances then decay and their daughters acquire momenta as dictated by decay kinematics.

Solution method:
The Monte Carlo generator repeats a loop in which it generates individual events. First, sizes of fragments are generated. Then the fragments are placed within the decaying fireball and their velocities are determined from the one-to-one correspondence between the position and the expansion velocity in the blast wave model. Since hadrons may be emitted from fragments as well as from the remaining bulk fireball, first those from the bulk are generated according to the blast wave model. Then, hadron production from the fragments is treated. Each hadron is generated in the rest frame of the fragment and then boosted to the global frame. Finally, after all directly produced hadrons are generated, resonance decay channels are chosen and the momenta and positions of final state hadrons are determined.

Running time:
Generation of 104 events can take anything between 2 hours to a couple of days. This depends mainly on the size and density of fragments. Simulations with small fragments may be very slow. At the beginning of a run there is a period of up to 1 hour in which the program calculates thermal weights due to statistical model. This period is long if many species are included in the simulation.