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Our model consists of a number A of identical balls of radius rhc having mass
m. They perform classical non-relativistic hard-sphere scatterings, conserving
energy, momentum and angular momentum. Initially the A balls are placed
randomly within a sphere of radius R = R0A1/3, and the initial velocities
are chosen as a superposition of thermal (Maxwell-Boltzmann) and collective
motion. We use a spherically symmetric Hubble-like flow field for the initial
collective motion:
v(r) = -v0f r/R (1)
where v0f is a model parameter, v0f > 0 for ingoing flow and v0f < 0 for
outgoing flow.
We fix the total energy E = Efl + Eth, and vary the fraction
n of the flow energy, n = Efl/E, where Efl=mV02/2R2Sumi=1A
ri-2and Eth are the
flow energy and the thermal energy, respectively.
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Because of the way in which
the system is built up, these energies will fluctuate from event to event with
a relative uncertainty of the order of A-0.5.In our simulations we have chosen nuclear-scale parameters: m =
940 MeV, rhc = 0.5 fm, R0 = 1.2 fm, 0 <= v0f <= 0.5 (in units of the ve-
locity of light, c = 1), but since the behavior of the model only depends on
the two combined parameters mv0f2
and rhc/R, the choice of nuclear scale is
not crucial. We choose A = 50, so with these parameters the initial radius of
the system is 4.2 fm.We focus on four different types of event: 'th20': The particles are
started in 100% thermal motion inside a spherical container of radius 4.2 fm,
at t = 20 fm/c the container walls are removed. 'in': 100% ingoing
flow. After interacting, the particles will move out again. This implosion-
explosion process is intended to simulate some features of a heavy-ion col-
lision. '50/50': 50% thermal motion + 50% outgoing flow, simulating an
explosion from a non-thermalized state. . '100out7': 100% outgoing flow in-
side a spherical container of radius 7 fm.
The results are averaged over an ensemble of 20 events of each kind.
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