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Manuscript Title: VNI 3.1, MC-simulation program to study high-energy particle collisions in QCD by space-time evolution of parton-cascades and parton-hadron conversion.
Authors: K. Geiger
Program title: VNI-3.1
Catalogue identifier: ADFZ_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 104(1997)70
Programming language: Fortran.
Computer: IBM RS-6000.
Operating system: IBM-AIX, Sun-OS, LINUX.
RAM: 2000K words
Word size: 32
Keywords: Monte carlo simulation, Event generator, Kinetic theory qcd, Parton cascades, Parton coalescence, Hadronic final states, Particle physics, Elementary, Event simulation, Particle collisions, Many body kinetics, Space-time dynamic, Parton-evolution, Cluster formation.
Classification: 11.2.

Nature of problem:
In high-energy particle collisions certain phase-space regions can be populated by a large number of quanta, such that statistical correlations among them (e.g., in space, momentum, or color) become of essential importance. Examples are deep-inelastic lepton-hadron scattering and hadron-hadron collisions in the region of very small Bjorken-x, or, collisions involving heavy nuclei in the central rapidity region. In these cases the produced particles evolve in a complicated non-equilibrium environment created by the presence of neighboring ones. The 'deterministic' quantum evolution of particle states due to self- interactions (depending only on the particle itself), receives a new 'statistical' kinetic contribution due to mutual interactions (depending crucially on the local density). The theoretical basis for addressing the solution for the dynamics of such particle systems is a quantum- kinetic formulation of the QCD equations of motion, an approximation that combines field-theoretical aspects associated with the renormalization group (including well-known resummation techniques) with aspects of transport theory associated with non-equilibrium multi- particle dynamics (including important quantum effects beyond the classical level).

Solution method:
The solution of the underlying quantum-kinetic equations of motion for non-equilibrium multi-particle QCD by Monte-Carlo simulation of collisions allowing for a variety of combinations of beam and target particles. To simulate the real-time evolution of the collision system in position space and momentum space on the basis of the equations of motions, the procedure is three-fold: i) the construction of the initial state including the decomposition of the beam and target particles into their partonic substructure, (ii) the evolution of parton cascades including multiple scatterings, emission- and fusion-processes, and (iii) the self-generated conversion of partons into hadrons using a phenomenological model for parton-coalescence into pre-hadronic clusters and subsequent decay into final-state particles.

For very high collision energy (sqrt(s) >> 10 TeV in hadronic collisions, and sqrt(s) >> 5 TeV/nucleon in nuclear collisions) numerical inaccuracies due to repeated Lorentz boosts, etc., may accumulate to cause problems. Although the most concerned parts of the program use double precision, for extreme energies the code would require a conversion in full to double precision format (which is planned in the near future).

Running time:
The CPU time for a typical simulation is strongly dependent on the type of beam and target, the magnitude of collision energy, as well as on the time interval Delta t chosen to follow an event in its real-time evolution. Examples are (for Delta t = 35 fm): a) e+ + e- at sqrt(s) = 100 GeV: 10000 events/hour; b) p + pbar at sqrt(s) = 200 GeV: 5000 events/hour; c) p + Au at sqrt(s) = 200 GeV/nucleon: 100 events/hour; d) Au + Au at sqrt(s) = 200 GeV/nucleon: 1 event/hour; All of the above quotes are approximate, and refer to a typical 133 Mhz or 166 Mhz processor on a modern Power-Workstation or Power-PC.