'Bubbles' of Broken Symmetry in Quark Soup at RHIC
Data suggest symmetry may ‘melt’ along with protons and neutrons
STAR detector
UPTON, NY — Scientists at the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference particle accelerator at the U.S. Department of Energy’s Brookhaven National Laboratory, report the first hints of profound symmetry transformations in the hot soup of quarks, antiquarks, and gluons produced in RHIC’s most energetic collisions. In particular, the new results, reported in the journal Physical Review Letters, suggest that “bubbles” formed within this hot soup may internally disobey the so-called “mirror symmetry” that normally characterizes the interactions of quarks and gluons.
“RHIC’s collisions of heavy nuclei at nearly light speed are designed to re-create, on a tiny scale, the conditions of the early universe. These new results thus suggest that RHIC may have a unique opportunity to test in the laboratory some crucial features of symmetry-altering bubbles speculated to have played important roles in the evolution of the infant universe,” said Steven Vigdor, Brookhaven’s Associate Laboratory Director for Nuclear and Particle Physics, who oversees research at RHIC.
Physicists have predicted an increasing probability of finding such bubbles, or local regions, of “broken” symmetry at extreme temperatures near transitions from one phase of matter to another. According to the predictions, the matter inside these bubbles would exhibit different symmetries — or behavior under certain simple transformations of space, time, and particle types — than the surrounding matter. In addition to the symmetry violations probed at RHIC, scientists have postulated that analogous symmetry-altering bubbles created at an even earlier time in the universe helped to establish the preference for matter over antimatter in our world.
RHIC’s most energetic collisions create the kind of extreme conditions that might be just right for producing such local regions of altered symmetry: A temperature of several trillion degrees Celsius, or about 250,000* times hotter than the center of the Sun, and a transition to a new phase of nuclear matter known as quark-gluon plasma. Furthermore, as the colliding nuclei pass near each other, they produce an ultra-strong magnetic field that facilitates detecting effects of the altered symmetry.
Now, early data from RHIC’s STAR detector hint at a violation in what is known as mirror symmetry, or parity. This rule of symmetry suggests that events should occur in exactly the same way whether seen directly or in a mirror, with no directional dependence. But STAR has observed an asymmetric charge separation in particles emerging from all but the most head-on collisions at RHIC: The observations suggest that positively charged quarks may prefer to emerge parallel to the magnetic field in a given collision event, while negatively charged quarks prefer to emerge in the opposite direction. Because this preference would appear reversed if the situation were reflected through a mirror, it appears to violate mirror symmetry.
“In all previous studies of systems governed by the strong force among quarks and gluons, it has been found to very high precision that events and their mirror reflections occur at exactly the same rate, with no directional dependence,” Vigdor said. “So this observation at STAR is truly intriguing.”
At RHIC, the parity-violating bubbles are formed in a random way, possibly with oppositely oriented charge separation in bubbles at different locations. Averaged over many events there would appear to be no parity violation, even though there were violations locally in each event. Although allowed by quantum chromodynamics (QCD), the underlying theory that describes the strong nuclear force, such local strong parity violation has never been detected directly.
“The key to observing the effect in high-energy nuclear collisions is to study correlations among the particles emerging from the collision,” said Nu Xu of Lawrence Berkeley National Laboratory, the spokesperson for the STAR collaboration.
The theory suggests that particles with the same sign of electric charge should tend to be emitted from such local parity-violating regions in the same direction, either both parallel, or both anti-parallel, to the magnetic field arising in the collision, whereas unlike-sign particles should be emitted in opposite directions.
“We have observed a correlation among emitted charged particles of the predicted type, with the degree of directional preference increasing as the collisions vary from head-on to more grazing,” Xu said.
STAR data also suggest the local breaking of another form of symmetry, known as charge-parity, or CP, invariance. According to this fundamental physics principle, when energy is converted to mass or vice-versa according to Einstein’s famous E=mc2 equation, equal numbers of particles and oppositely charged antiparticles must be created or annihilated. If CP symmetry had not been broken at some very early time in the evolution of our universe, the particles and antiparticles created in equal numbers in the Big Bang would subsequently have annihilated one another in pairs, leaving no matter to form the stars, planets, and people that now populate our world.
While some small violations of CP symmetry have been found in previous laboratory experiments, those violations are far too weak to account for the amount of matter remaining in the universe today. Likewise, the signs of possible local CP violation at STAR cannot explain the global predominance of matter in today’s world, but they may offer insight into how such symmetry violations occur.
“The features observed at STAR are qualitatively consistent with predictions of symmetry-breaking domains in hot quark matter,” said Vigdor. “Confirmation of this effect and understanding how these domains of broken symmetry form at RHIC may help scientists understand some of the most fundamental puzzles of the universe, and will be a subject of intense study in future RHIC experiments.”
“For example,” he said, “we will want to see if the signal disappears, as predicted, at lower collision energies, where the produced matter is no longer hot enough to make the transition to the quark-gluon plasma phase. These future studies will further check the early work, will test more mundane possible explanations for the observed effects, and will explore a wide range of related phenomena.”
Research at RHIC is funded primarily by the U.S. Department of Energy’s Office of Science and by various national and international collaborating institutions.
'Perfect' Liquid Hot Enough to be Quark Soup
Protons, neutrons melt to produce ‘quark-gluon plasma’ at RHIC
PHENIX detector
UPTON, NY — 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 Sun. 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 Review Letters.
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 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.”
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.”
Moving forward
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.
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