Local Thermal and Chemical Equilibration and the Equation of State in Relativistic Heavy Ion Collisions

Abstract

Thermodynamical variables and their time evolution are studied for central relativistic heavy ion collisions from 10.7 to 160 AGeV in the microscopic Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model exhibits drastic deviations from equilibrium during the early high density phase of the collision. Local thermal and chemical equilibration of the hadronic matter seems to be established only at later stages of the quasi- isentropic expansion in the central reaction cell with volume 125 fm${}^3$. distributions at all collision energies for $t\ge 10fm/c$ with a unique Baryon energy spectra in this cell are approximately reproduced by Boltzmann rapidly dropping temperature. At these times the equation of state has a simple form: $P\cong (0.12-0.15)\epsilon$. At 160 AGeV the strong deviation from chemical equilibrium is found for mesons, especially for pions, even at the late stage of the reaction. The final enhancement of pions is supported by experimental data.

17 Pages, LaTex, 8 eps figures. Talk given at SQM'98 conference, 20-24 July 1998, Padova, Italy, submitted to J. Phys. G

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• Microscopic Models for Ultrarelativistic Heavy Ion Collisions
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• Equation of State, Spectra and Composition of Hot and Dense Infinite Hadronic Matter in a Microscopic Transport Model
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