 Research
Nuclear & Elementary Particle Physics
UNM Researchers:
Dr. Timothy L. Thomas, Senior Research Scientist, Dept. of Physics
and Astronomy, University of New Mexico
Prof. Bernd Bassalleck, Chair, Dept. of Physics and Astronomy, University
of New Mexico
Funding Sources:
HPC@UNM computational resources: U.S. National Science Foundation
PHENIX experiment at Brookhaven: U.S. Department of Energy
The Early Universe and The Relativistic Heavy-Ion Collider
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. [#1] 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. 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.
[#2]
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. [#3]
The PHENIX Detector
The results of these high-energy ion-ion collision are "events",
each containing up to ten thousand produced particles that must
be tracked, separated, and studied to yield information about QGP
formation and decay characteristics. This is accomplished through
the use of large, high-tech particle detectors. To vastly oversimplify,
these basically consist of thousands of very high-precision Geiger
and scintillation counters. [#4]
[#5] 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 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
the University of New Mexico (UNM). Our group, within UNM's Center
for Particle Physics, is responsible for a significant component
of the Muon Tracking System hardware and for related analysis software.
Simulations
[#6] 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, the "PHENIX
Integrated Simulation Application" (PISA) is based on the GEANT
code, a detector description and simulation tool that has been under
continuous development for more than 25 years at CERN, the European
Organization for Nuclear Research in Geneva, Switzerland. GEANT
contains an enormous amount of information about the physics of
particle propagation and interaction in materials.
A direct, or "central," gold-gold collision event contains
up to ten thousand charged and neutral "primary" particles-the
original particles produced by the collision or by the decays of
very short-lived produced particles. Each of these primaries may
produce dozens or even hundreds of "secondaries" via interaction
and/or decay during propagation through the detector materials.
A single event can therefore contain close to 100,000 total objects.
[#7] Such an event takes about 20
minutes of CPU time per node to simulate on the HPC@UNM's LosLobos
supercluster. During the summer of 2001, over 150,000 CPU hours
of LosLobos time was used to simulate a total of 5.5 million ion-ion
collisions, using the PISA model. A variety of species and "centrality"
conditions were simulated, including gold-on-gold, deuterium-on-gold,
silicon-on-silicon, and copper-on-copper. This computation represents
the highest throughput simulation effort in the PHENIX collaboration's
history.
Future Work
[#8] There are many analyses that can
be performed on data sets collected with a large multi-purpose particle
detector like PHENIX. Each analysis typically makes use of at least
one and often several types of simulated data sets. By mixing the
events produced at the HPC@UNM with other, more specialized simulated-signal
data sets that already exist, one can obtain an understanding of
signal and background features, estimate signal-to-noise levels,
and study the acceptance and efficiency for various signals.
These analyses can be performed both at the High Performance Computing,
Education & Research Center (HPC@UNM) and on computers located
at Brookhaven, where the real data are now being collected. The
events simulated at the HPC@UNM-over a terabyte of data-have been
transferred to BNL and will be shared among the over 400 PHENIX
collaborators, who will use it to analyze and interpret in various
ways the real data sample being collected by the experiment this
year and next.
Web Site
http://thomas.phys.unm.edu/LLsims/
Tim's Graphics:
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