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Researchers seek answers to universal questions.
More than 13 billion years ago, some scientists theorize, the cosmos underwent a super-fast expansion, in effect growing from the size of an atom to that of a grapefruit in a fraction of a second. Initially, according to the Big Bang model, our universe was a hot soup of quarks, electrons and other particles. But, before the split-second expansion ended, the cosmos cooled enough for quarks to clump into protons and neutrons. According to this theory, some 300,000 years later electrons combined with protons and neutrons to form atoms, mostly the hydrogen and helium that formed stars. Most of these "first-generation" stars subsequently exploded, giving rise to all the chemical elements that formed planets and made possible life on Earth. Today the cosmos—which includes ordinary matter (4.6%), dark matter (23.3%) and dark energy (72.1%)—is expanding at an ever-increasing rate. Theorists postulated that the matter existing in the first ten-millionth of a second after the Big Bang consisted of free quarks, anti-quarks and gluons that collectively behaved like a gas, a "quark-gluon plasma," or QGP. A large international team of physicists set out to recreate this state of matter, under laboratory conditions, at the Relativistic Heavy Ion Collider, or RHIC, at the Department of Energy's Brookhaven National Laboratory. At RHIC nuclei collide at nearly the speed of light, resulting in extremes of temperature and density that have not existed since shortly after the Big Bang. Strong evidence suggests that the extreme conditions created in RHIC collisions briefly free quarks and gluons from their prisons inside protons and neutrons. The results also suggest, however, that the QGP behaves more like a liquid than a gas. Oak Ridge National Laboratory researchers are part of the international team that has been conducting QGP experiments in the 1980s and 1990s at CERN in Switzerland and, since June 2000, at Brookhaven National Laboratory's RHIC facility in New York. Vince Cianciolo, an ORNL physicist who helped develop one of RHIC's particle detector arrays called PHENIX, recalls that the measurements at the collider showed that the recreated QGP was almost a "perfect liquid" because its viscosity was measured to be close to zero. "The QGP flowed better than water," he notes. Recreating an extreme form of matter, like that existing in the first ten-millionth of a second after the Big Bang, requires a collider, extreme in both size and capabilities. RHIC uses 15 megawatts of electricity and 1600 miles of superconducting wires to create magnetic fields that steer two beams of gold nuclei in opposite directions in a giant ring more than two miles in length. RHIC's accelerators boost the energies of these beams to 100 billion electron volts per proton or neutron in the beam, bringing them to 99.995% of the speed of light. Some constituent particles in the gold nuclei collide head-on, causing protons and neutrons to melt together to form a quark-gluon plasma similar to that formed at the beginning of time.
For an average collision of two gold nuclei, 6,000 particles are emitted. The particles are detected and analyzed by PHENIX using nearly 500,000 individual particle detector channels. ORNL researchers designed and built the PHENIX muon identifiers and lead-glass electromagnetic calorimeter. They also developed electronic components for other PHENIX particle detectors. Some findings at RHIC did not agree with theoretical predictions. Two such findings involve a heavy quark known as the charm quark, a particle that has a mass about 2,900 times that of an electron. The charm quark is one of six elementary particles having electric charges of one-third or two-thirds that of the electron. This particular quark is not found in everyday matter. Measurements in gold-on-gold collisions show that particles made of light quarks lose a substantial amount of energy as they pass through the QGP. This result was expected. However, according to Cianciolo, "When we looked at charm quarks, we found that they lose just as much energy as light quarks. That result was surprising because of the difference in mass. Theorists are developing models for the energy loss to explain that finding." "The charm quark was also observed to participate in the collective flow of the QGP medium. Using an analogy, if we throw a short stick (representing a light quark) on top of a flowing stream, we would expect the stick to be carried along with the water's motion. But if we push a boulder (representing a heavy quark) into the stream, we would be amazed to see the boulder flow as readily as the stick. Theorists must try to explain this surprising result." Other theorists pose questions about the universe that may be answered by studying the neutron using two instruments at DOE's Spallation Neutron Source at ORNL. Most SNS instruments use neutrons to decipher the structure and molecular motions of various physical and biological materials. In contrast, experimenters from 30 collaborating institutions at the SNS Fundamental Neutron Physics Beam Line will focus on the neutron itself. "For us the neutron is a rich complex object," says Geoffrey Greene, professor of physics at the University of Tennessee and leader of ORNL's fundamental neutron physics project. "We will study neutron properties. We know the neutron has a mass, a magnetic moment and a lifetime, which we hope to measure. It is unstable and decays by emitting an electron. "Although the neutron is electrically neutral, theorists conjecture that it may have an electric dipole moment in which the positive and negative charges at the neutron's poles are slightly displaced. We hope to measure this little asymmetry in the neutron's charge distribution." If the researchers can determine whether the electric dipole moment is zero or another value, the measurement could shed light on "spontaneous symmetry breaking," which in physics describes a phenomenon in which tiny fluctuations act on a system at a critical point, determining the system's fate. "For example, no law of nature states that every person driving in America must drive in the right lane," Greene says. "In most countries people drive in the right lane because of tradition, practicality and police officers. However, in a few nations, people must drive on the left, breaking the symmetry." Theorists believe the universe once had equal amounts of matter and antimatter, reflecting symmetry between matter and antimatter. Early in the Big Bang, matter and antimatter annihilated each other. Astronomical observations indicate that the universe is, for all practical purposes, made entirely of matter. The matter we see is the result of some interaction that led to an extremely small excess of matter. Remarkably, the amount of matter left in the universe is only one part in 10 billion parts of the original amount. "But why such a tiny amount of matter?" Greene asks. "We suspect that initial symmetry between matter and antimatter were broken some way and that the same interaction that broke this symmetry could also create a neutron electric dipole moment." Scientists have been searching unsuccessfully for the electric dipole moment for more than 50 years. Interestingly, the first electric dipole moment search occurred in 1950 at ORNL. Greene says that the effect is so small that, if the neutron was blown up to the size of the Earth, the electric dipole moment would correspond to one electron being displaced a few microns from the North Pole. "If we see this tiny effect at the SNS, we would have very strong support for the notion that the matter-antimatter asymmetry is the result of a spontaneous symmetry breaking," Greene explains. "If we do not see the effect, that would imply that theorists' version of matter-antimatter asymmetry is wrong or unlikely." In 2009 the electron dipole moment and the neutron lifetime experiment will be performed on separate instruments at the Fundamental Neutron Physics Beam Line. Because the SNS will provide 10 times more neutrons per pulse than any previous machine, Greene and his colleagues will more precisely measure a neutron's half-life by detecting the energetic electron emitted by a decaying neutron. Nuclear radioactivity—also called beta emission or the "weak force"—is the emission of electrons by decaying neutrons. A free neutron has a half-life of about 10 minutes, according to previous experiments elsewhere by the international team. Measurements of neutron lifetime and details of decay will shed light on basic scientific questions: Why does the universe at a particle level show a preference toward left-handedness, an effect known as parity violation? Does a typical neutron decay in a preferred direction, ejecting a lone electron from the south pole rather than the north pole? Or, is a little right-handedness left over? Experimenters at the SNS hope to make extremely precise measurements, providing tiny but important clues to help answer large questions that are, quite literally, universal.—Carolyn Krause
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