Protons and neutrons melted to produce ‘quark-gluon plasma’ at the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference “atom smasher” at the Brookhaven National Laboratory. This event establishes 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 Sun. This temperature is higher than the temperature needed to melt protons and neutrons into a plasma of quarks and gluons -a truly remarkable new form of matter.
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.
“This research offers significant insight into the fundamental structure of matter and the early universe...,” said Dr. William F. Brinkman, Director of the DOE Office of Science.
These new temperature measurements, combined with other observations analyzed over nine years of operations 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.
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 at RHIC.”
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.
The research program at RHIC will be complemented by studies soon to get underway at the Large Hadron Collider (LHC), a 17-mile-circumference particle accelerator in Europe. The LHC will devote a month each year to colliding heavy nuclei at energies much higher than RHIC’s — extending the exploration of matter one step farther back in time toward the birth of the universe.
Calculations of quantum chromodynamics now predict that as temperatures increase significantly, quark-gluon matter should slowly evolve from RHIC’s perfect liquid to an ideal gas. The LHC will provide the first opportunity to observe this evolution as collision temperatures increase by a factor of 2 to 3 in its own heavy-ion experiments, set to begin in late 2010.
At the same time, RHIC’s upgrades and flexible operations will allow scientists to quantify particle interactions inside the perfect liquid and explore the phase diagram of nuclear matter. “RHIC and the LHC will work together in complementary ways to broaden our understanding of the basic constituents of our universe and the forces that shape them,” Vigdor said.
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.