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Using TeraGrid resources, a group of scientists has calculated key
properties of quark-gluon plasma at a resolution previously unattainable
The birth of protons
and neutrons
T e r a G r i d
S c i e n c e
H i g h l i g h t s
3902c1
TG
22
Artistic conception of a heavy-ion collision producing
quark-gluon plasma. Modified image reproduced with
permission from the Press Office of CERN (European
Center for Nuclear Research), Geneva, Switzerland.
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oments after the Big Bang, the early
universe did not resemble the cosmic
cluster of stars, planets and galaxies visible today.
Instead, it began as a hot plasma mixture of quarks
and gluons that eventually cooled and condensed into
the protons and neutrons that formed the heavenly
bodies.
To apply these results to a rapid nuclear collision,
scientists use hydrodynamics, which describes the
plasma as a fluid characterized by many properties,
including temperature, energy density, pressure,
entropy and nuclear-matter density. In particular,
from the QCD simulation, they need the equationof-state that describes how energy-density and
pressure change as the plasma transforms between
a quark-gluon plasma and ordinary matter.
Using TeraGrid computing cycles from TACC’s Ranger,
LONI’s Queen Bee and NCSA’s Abe and Tungsten, a
team of physicists from the MILC (MImd Lattice
Computation) collaboration led by Ludmila Levkova
and Carleton DeTar from University of Utah, has
calculated the plasma equation-of-state at nonzero
nuclear-matter density at a resolution never previously
attained, which will help physicists understand better
how the early universe formed.
Nuclear-matter density in the early universe was
very low, so many groups study the plasma in zero
nuclear-density conditions, assuming equal amounts
of matter and antimatter. But in a collision of heavy
nuclei, nuclear-matter density is greater than zero.
Calculating at nonzero density is difficult, so MILC
collaboration scientists used TeraGrid resources.
Their simulation approximates space and time with a
grid of points called a "lattice.” The smaller the
spacing of lattice points, the closer it simulates real
conditions. Researchers aim to make lattice spacing
much smaller than the size of a proton—smaller than
one femtometer—to increase accuracy.
To tell when the lattice resolution is small enough to
be considered accurate, researchers calculate at
smaller lattice spacings and compare results. If the
results agree, the calculation is converging. Previously,
the smallest achievable spacing at the transition
temperature was about 0.24 femtometers. Thanks to
TeraGrid resources, the MILC collaboration has cut
this resolution to 0.16 femtometers, the best to date.
The group presented their initial findings from this
high-resolution lattice at a conference in July 2008.
Isentropic energy-density and pressure (p) versus temperature (T) for several
fixed values of entropy per baryon number (S/NB). The values of S/NB = 30, 45,
300 correspond to various plausible experimental conditions. The zero baryon
number density case of infinite S/NB is also shown below.
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The MILC Collaboration
The MILC Collaboration
Alexei Bazavov and Doug Toussaint
University of Arizona
Subhasish Basak and Steven Gottlieb Indiana University
Claude Bernard and Jack Laiho
Washington University
Ludmila Levkova, Carleton DeTar,
University of Utah
and Tommy Burch
NSF grants: PHY04-56556, PHY05-55235, PHY04-56691, PHY05-55243,
PHY06-09852, PHY05-55234, PHY05-55397
TeraGrid grant: MCA93S002S
Urs Heller
American Physical Society
James Hetrick
University of the Pacific
More information:
Bob Sugar
University of California-
http://physics.indiana.edu/~sg/milc.html
Te r a G r i d 2 0 0 8
Physicists are conducting experiments involving
collisions of heavy nuclei to temporarily recreate that
primordial quark-gluon plasma and see how it may
have cooled and condensed. Quantum chromodynamics (QCD), the well-established theory of interacting
quarks and gluons, offers a series of equations for
plasma formation. Solving them to simulate the
plasma requires high-performance computing.
Such simulations describe the plasma only as it
would exist in an equilibrium environment, which
does not account for the rapidly changing conditions
that occur in a nuclear collision.
The birth of protons and neutrons
M
Santa Barbara