Abstract:
Direct photon production in central Pb+Pb
collisions at
CERN-SPS energy is calculated within the
relativistic
microscopic transport model UrQMD, and within
distinctly
different versions of relativistic
hydrodynamics. We find
that in UrQMD the local momentum
distributions of the
secondaries are strongly elongated along the
beam axis
initially. Therefore, the pre-equilibrium
contribution
dominates the photon spectrum at transverse
momenta above
$\approx 1.5$ GeV. The hydrodynamics
prediction of a strong
correlation between the temperature and
radial expansion
velocities on the one hand and the slope of
the transverse
momentum distribution of direct photons on
the other hand
thus is not recovered in UrQMD. The rapidity
distribution of
direct photons in UrQMD reveals that the
initial conditions
for the longitudinal expansion of the photon
source (the
meson ``fluid'') resemble rather
boostinvariance than
Landau-like flow.
Duke University * Arts & Sciences * Physics * Faculty * Staff * Grad * Researchers * Reload * Login
Copyright (c) 2001-2002 by Duke University Physics.