Recent analyses from the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference
"atom smasher" at the U.S. Department
of Energy's (DOE) Brookhaven National Laboratory, establish that collisions
of gold ions traveling at nearly the speed of light have created matter at a
temperature of about 4 trillion degrees Celsius - the hottest temperature ever
reached in a laboratory, about 250,000* times hotter than the center of the
This temperature, based upon measurements by the PHENIX collaboration at RHIC,
is higher than the temperature needed to melt protons and neutrons into a plasma
of quarks and gluons. Details of the findings will be published in Physical
These new temperature measurements, combined with other observations analyzed
over nine years of operations by RHIC's four experimental collaborations
- BRAHMS, PHENIX, PHOBOS, and STAR - indicate that RHIC's
gold-gold collisions produce a freely flowing liquid composed of quarks and
gluons. Such a substance, often referred to as quark-gluon plasma, or QGP, filled
the universe a few microseconds after it came into existence 13.7 billion years
ago. At RHIC, this liquid appears, and the quoted temperature is reached, in
less time than it takes light to travel across a single proton.
"This research offers significant insight into the fundamental structure
of matter and the early universe, highlighting the merits of long-term investment
in large-scale, basic research programs at our national laboratories,"
said Dr. William F. Brinkman, Director of the DOE Office of Science. "I
commend the careful approach RHIC scientists have used to gather detailed evidence
for their claim of creating a truly remarkable new form of matter."
According to Steven Vigdor, Brookhaven's Associate Laboratory Director
for Nuclear and Particle Physics, who oversees the RHIC research program, "These
data provide the first measurement of the temperature of the quark-gluon plasma
Scientists measure the temperature of hot matter by looking at the color, or
energy distribution, of light emitted from it - similar to the way one
can tell that an iron rod is hot by looking at its glow. Because light interacts
very little with the hot liquid produced at RHIC, it bears accurate witness
to the early cauldron-like conditions created within.
Said Vigdor, "The temperature inferred from these new measurements at
RHIC is considerably higher than the long-established maximum possible temperature
attainable without the liberation of quarks and gluons from their normal confinement
inside individual protons and neutrons.
"However," he added, "the quarks and gluons in the matter
we see at RHIC behave much more cooperatively than the independent particles
initially predicted for QGP."
Hot gas vs. hot liquid
Scientists believe that a plasma of quarks and gluons existed a few
microseconds after the birth of the universe, before cooling and condensing
to form the protons and neutrons that make up all the matter around us -
from individual atoms to stars, planets, and people. Although the matter produced
at RHIC survives for much less than a billionth of a trillionth of a second,
its properties can be determined using RHIC's highly sophisticated detectors
to look at the thousands of particles emitted during its brief lifetime. The
measurements provide new insights into Nature's strongest force -
in essence, what holds all the protons and neutrons of the universe together.
Predictions made prior to RHIC's initial operations in 2000 expected
that the quark-gluon plasma would exist as a gas. But surprising and definitive
data from RHIC's first three years of operation, presented by RHIC scientists
in April 2005, showed that the matter produced at RHIC behaves as a liquid,
whose constituent particles interact very strongly among themselves. This liquid
matter has been described as nearly "perfect" in the sense that
it flows with almost no frictional resistance, or viscosity. Such a "perfect"
liquid doesn't fit with the picture of "free" quarks and gluons
physicists had previously used to describe QGP.
In the papers published in 2005, RHIC physicists laid out a plan of crucial
measurements to clarify the nature and constituents of the "perfect"
liquid. Measuring the temperature early in the collisions was one of those goals.
Models of the evolution of the matter produced in RHIC collisions had suggested
that the initial temperature might be high enough to melt protons, but a more
direct measurement of the temperature required detecting photons - particles
of light - emitted near the beginning of the collision, which travel outward
undisturbed by their surroundings.
"This was an extraordinarily challenging measurement," explained
Barbara Jacak, a professor of physics at Stony Brook University and spokesperson
for the PHENIX collaboration. "There are many ways that photons can be
produced in these violent collisions. We were able to 'eliminate'
the contribution from these other sources by exploiting RHIC's flexibility
to measure them directly and to make the same measurement in collisions of protons,
rather than of gold nuclei. Thus we could pin down excess production in the
gold-gold collisions, and determine the temperature of the matter that radiated
the excess photons. By matching theoretical models of the expanding plasma to
the data, we can determine that the initial temperature of the 'perfect'
liquid has reached about four trillion degrees Celsius."
The discoveries at RHIC have led to compelling new questions in the
field of quantum chromodynamics (QCD), the theory that describes the interactions
of the smallest known components of the atomic nucleus. To probe these and other
questions and conduct detailed studies of the plasma, Brookhaven physicists
are planning to upgrade RHIC over the next few years to increase its collision
rate and detector capabilities.
"These technical improvements will facilitate studies of rare signals
providing measurements of even better precision on temperature, viscosity, and
other basic properties of the nearly perfect liquid quark-gluon plasma created
at RHIC," Vigdor said.