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One of the hypothesized phase transitions in the early universe is the
Confinement Transition. Prior to this transition, quarks and gluons
were free to move over extended regions of space-time, and the quarks
had virtually no mass. This Quark-Gluon Plasma (QGP) state existed at
a temperature above 12 thousand billion degrees Kelvin.

After about a microsecond, when the universe's expansion had cooled it
below this temperature, the QGP condensed into a more "familiar" state
of hot hadronic matter, which continued to cool. Baryons coalesced
into nuclei, and these eventually formed atoms, stars and galaxies,
planets... and us.

Experimental proof of the existence of the QGP phase, and a study of
its characteristics, are among the most important research activities
in high-energy nuclear and elementary particle physics.

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Experiments at the Relativistic Heavy Ion Collider (RHIC), a
high-energy particle accelerator located at Brookhaven National
Laboratory on Long Island, are presently searching for this exotic
state of nuclear matter.

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RHIC is housed in a circular tunnel 2.4 miles in circumference. Two
vacuum pipes carry counter-circulating beams of heavy ions, such as
gold, that have been accelerated to nearly the speed of light. These
beams are made to collide head-on at various locations around the ring
where the experiments are located.

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The result of each high-energy ion-ion collision is an "event"
containing tens of thousands of produced particles.  These must be
tracked, separated, and studied to yield information about QGP
formation and decay characteristics. To give a sense of the density of
information which must be handled, this figure displays the long-lived
charged tracks in a single real event, as seen by one of the
experiments, the TPC-based STAR detector.

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PHENIX is the most complex of the experiments at RHIC. It contains
eleven detector systems comprising more than 300,000 detection
channels. Weighing over 500 tons, the detector and associated read-out
electronics and data-acquisition and analysis computers occupy an
industrial-sized complex. It collects data, such as the real event
partially displayed above left, at rates well over 30 megabytes per
second. The detector was designed and built by a collaboration of over
400 physicists and engineers from 43 laboratories and universities
worldwide, including UNM.

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Highly detailed computer models of the known physics taking place
within the PHENIX detector have been assembled by the collaboration as
essential ingredients in the analysis and interpretation of the actual
signals produced by the detector. The most CPU-intensive of these
models, called PISA, is based on CERN's GEANT code, which contains
enormous amounts of information about the physics of particle
propagation and interaction in materials. Shown at far left are only
10% (for clarity) of the tracks present in the final state of a
simulated "central" (direct-hit) gold-gold collision event.

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A single event of this kind can contain close to 100,000 total
objects. Such an event takes about 20 minutes of CPU time per node to
simulate on the AHPCC's Los Lobos supercluster. During the summer of
2001, over 100,000 CPU hours of Los Lobos time was used to simulate a
total of 5.5 million ion-ion collisions of various species, using the
PISA model. This is the highest throughput simulation of this sort in
the PHENIX collaboration's history.  Future work includes reconstruction
and analysis of these simulated data sets, followed by their use in
real-data analysis.

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