Several ORNL physicists have played key roles in the search for one of the early universe's signature ingredients. Internal "seed-money" funding, the forerunner of today's Laboratory Directed Research and Development Program, allowed this ORNL physics program to flourish.
Researchers conducting experiments at the Relativistic Heavy Ion Collider (RHIC) at the Department of Energy's Brookhaven National Laboratory (BNL) may have come close to simulating the Big Bang. Whether they created a quark-gluon plasma, the signature of the first 10 microseconds of the universe's birth, ORNL physicists involved in these experiments are not ready to say.
"The good news is that it looks like Brookhaven has found an odd form of matter," says Glenn Young, director of ORNL's Physics Division. "What it is, we don't know. Science only disproves the existence of phenomena predicted by theory; it doesn't prove a theory correct. The hypothesis that a quark-gluon plasma could be produced at RHIC has survived this set of experiments."
At a Brookhaven colloquium on June 18, 2003, researchers announced that experimental results at RHIC, the world's largest facility for nuclear physics research, provided evidence they are on the right path to discovering a quark-gluon plasma-an elusive form of matter believed to have existed in the first microseconds after the universe was born. Quarks and gluons are thought to have roamed freely back then. As the universe cooled down, after 10 microseconds, the quarks and gluons "froze" into protons and neutrons, which make up the nuclei of atoms. Each neutron or proton is composed of three quarks held together by gluons.
Three types of experiments-gold-on-gold, deuterons-on-gold, and protons-on-protons-have been performed at RHIC. In each case nuclei in two beams are stripped of all their electrons, accelerated to almost the speed of light, and sent in opposite directions around a circular track. At several points along this track the beams are steered into each other, and detectors at those locations record the resulting collisions.
Occasionally quarks inside the nuclei of the two counter-circulating beams will collide head-on and create jets of particles that are emitted in back-to-back directions. "Quarks can't show up in the real world," Young says. "But if we visualize them as colliding invisible marbles, they shatter into fragments we can see." In gold-on-gold collisions many fewer jets were observed than expected, suggesting that some jets had to plow through dense matter that might have been a quark-gluon plasma. This phenomenon, called jet quenching, was one of the predicted signatures of the production of a quark-gluon plasma. Jet quenching was recorded by each of the four detectors at RHIC, including PHENIX, in which ORNL researchers played key roles.
Proton-on-proton collisions were then measured, verifying the baseline expectations. Theoretical studies suggested that an effect other than quark-gluon-plasma formation could be responsible for the observed jet quenching. If so, a similar but smaller effect was predicted for deuteron-on-gold collisions. Significantly, this effect was not observed.
Nevertheless, Young says, "Many measurements are needed to see if RHIC has actually produced a quark-gluon plasma. We must look for muons, heavy electrons that come from the decay of heavy particles, like the J/psi particle, which is a charmed quark bound to its antiquark. We are using the muon identifier at the PHENIX detector at RHIC to find them. If quark-gluon plasmas are present, they would modify the production of J/psi particles, and the muon identifier should detect this change."
Young played a major role in getting ORNL involved in the RHIC experiment. In fact, it was Young's idea to build a muon identifier at PHENIX. Young's colleague Paul Stankus suggested that PHENIX could detect jets and jet quenching by focusing on rare, high-energy particles. Vince Cianciolo and Ken Read co-led the detailed design and 60-person crash construction of PHENIX's muon identifier. Cianciolo also spearheaded the development of its readout electronics to find an occasional muon among thousands of other particles. Partly because of this work, he received a Presidential Early Career Award for Scientists and Engineers in 2002. Read is the PHENIX Detector Council representative responsible for the muon identifier, on which he has worked since 1993.
A photograph of PHENIX graced the October 2003 cover of Physics Today. The PHENIX collaboration has 400 physicists from 12 nations and 40 institutions.
"It all started with a seed money project back in 1989," Young says. "We got in the game of developing gamma-ray detectors called calorimeters when Brookhaven was starting its research program and accepting proposals for detector experiments on RHIC. I had learned about calorimeters from colleagues at BNL and ORNL. Before then, Frank Plasil, a member of our group, had a connection with a European group doing experiments in search of quark-gluon plasmas at the European Organisation for Nuclear Research (CERN) in Switzerland. So we offered to build calorimeters for that experiment. Our offer was accepted.
"We collaborated from 1986 through 1996 with Russian and German scientists in developing calorimeters using the Russians' lead-oxide bricks for experiments at CERN in Switzerland. These bricks stopped gamma rays, causing the emission of a 'shower' of electrons and positrons and changing their last bit of energy into visible light that can be detected by a photomultiplier. We developed electronics to read out the data, so we could figure out what was formed when nuclei collided in the CERN synchrotron. ORNL's Terry Awes became spokesman for the last CERN experiment."
ORNL wrote a set of winning proposals to work with the Russians and Germans to move the lead-glass calorimeter to PHENIX, with ORNL focusing on developing the electronics to read out and sort through the data to find meaningful signals. To develop the electronics for many subsystems at PHENIX, Young worked with electrical engineers in ORNL's Engineering Science and Technology Division; the project leads were Gary Alley, Chuck Britton, Bill Bryan, Miljko Bobrek, and Alan Wintenberg.
Stankus, who had worked on the last experiment at CERN in which ORNL was involved, pondered the problem of identifying jets at PHENIX. "Paul told the PHENIX collaborators that when quarks collide and shatter into lots of particles, the leading particle in the jet carries off most of the energy," Young says. "He proposed that some of the fastest leading particles could be pions, which could be detected by our calorimeter, because a pion, which is a quark-antiquark pair, decays into two gamma rays. He was right. Lo and behold they are easy to see and they came out clean as a whistle at PHENIX."
The collaborators detected jet quenching. "We saw only 20 to 30% of what we should see in pion decays," Young says. "Maybe some of the fragments had a real hard time pushing their way out when two quarks collided, because the matter they encountered looked like thick molasses. It was more opaque than a normal nucleus by a factor of 40 to 50. A quark-gluon plasma will be the densest form of matter ever seen, if it's actually found."
Young first reported on the data analysis of the results of the gold-gold collision experiment at a physics meeting in 2000.
Stankus oversaw the analysis and led the writing of the paper featured on the cover of the January 14, 2002, issue of Physical Review Letters.
More measurements using the muon identifier and the analysis of data already obtained will be needed to help verify whether the dense matter detected in the RHIC experiments actually is a quark-gluon plasma. The muon identifier consists of panels of steel that stop most particles—except muons—and detectors that produce current pulses when the gas inside them is ionized by particles passing through them.
The identifier's trigger electronics that Cianciolo helped develop will be key to finding the needles in the haystack-the muons that indicate the presence of a quark-gluon plasma. "It's like finding short scenes in a movie that could interest us the most and reviewing them many times to see if what we're looking for is there," Cianciolo says. If muons are seen, they will be the stars.
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