July 28, 2003
Berkeley Lab Science Beat Berkeley Lab Science Beat
Jet Quenching at RHIC: the Berkeley Connection
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The recent announcement from Brookhaven National Laboratory that its Relativistic Heavy Ion Collider (RHIC) had created the hottest, densest matter ever observed involved a strong Berkeley Lab connection.

The STAR team is composed of hundreds of scientists and engineers from the U.S. and abroad.

The announced experimental results have convinced scientists 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. Researchers with Berkeley Lab's Nuclear Science Division (NSD) played key roles in both the theoretical and experimental components of those results.

RHIC is the world's largest facility for nuclear physics research. It's designed to recreate the hot, dense conditions of the early universe. At a Brookhaven colloquium on June 18, 2003, it was announced that in comparisons of head-on collisions between either two beams of gold nuclei or a beam of gold nuclei and a beam of deuterons, a phenomenon called jet quenching was recorded at the detector system known as STAR (Solenoidal Tracker at RHIC).

STAR, along with the three other experiments at RHIC -- PHENIX, BRAHMS and PHOBOS -- all detected a suppression of "leading particles," highly energetic individual particles that emerge from nuclear fireballs. Both observations are predicted to be signs of a quark-gluon plasma.

"The observation of jet quenching represents a giant step forward and brings us to the cusp of discovery of the elusive quark-gluon plasma," says Xin-Nian Wang, head of NSD's nuclear theory program. "Much work is still needed to find out the detailed properties of the dense matter, such as its equation of state and whether color deconfinement is achieved. However, taken together with other observed phenomena like collective flow, it should not take too long to conclude that a quark-gluon plasma has indeed been made at RHIC."

The 2,000 particle tracks in this simulated collision of two gold nuclei in the STAR detector are colored according to their ionization values.  

Wang, along with Miklos Gyulassy, then with Berkeley Lab now at Columbia University, developed the theory that links jet quenching to the quark-gluon plasma. Jets are energetic beams of ordinary particles produced when a pair of quarks are knocked out of a proton or neutron during a collision between atomic nuclei. The quarks, moving in opposite directions -- one going in towards the nucleus, the other away from it -- quickly transform into two back-to-back jets that shoot out in opposite directions from the nuclear fireball.

It was the contention of Wang and Gyulassy that if a quark-gluon plasma were to be created, the jet moving toward the nucleus would be drained of energy -- or "quenched" -- so that it could not escape the fireball.

"Analyzing how the jets propagate through the fireball and measuring the amount of quenching that occurs should reveal whether or not a quark-gluon plasma was created," Wang says.

Starting in 2001, researchers at RHIC generated thousands of head-on collisions between the nuclei of gold atoms (which have 79 protons and 118 neutrons) at energies of 100 billion electron volts (100 GeV) per nucleon. The temperature of the nuclear matter in these collisions approached a trillion degrees above absolute zero (about 300 million times hotter than the surface of the sun), which is thought to be hot enough to "melt" the gold nuclei into their constituent quarks and gluons and allow these particles to briefly exist free of one another in the soup-like form of matter called a quark-gluon plasma.

Quarks are one of the basic constituents of matter. Gluons are carriers of the strong force that binds quarks together into protons or neutrons. In the ordinary matter that makes up the world in which we live, quarks are never free of other quarks or gluons.

As different from ordinary matter as water is from ice or steam, the quark-gluon plasma is believed to have been the state of matter that prevailed in the first 10 microseconds after the Big Bang. Though it immediately cooled to the ordinary state of matter, the quark-gluon plasma set the stage for creating the particles that make up our universe today. The ability to produce a quark-gluon plasma at RHIC should yield new insights into how our universe was formed and a better understanding of the behavior of atomic nuclei.

The Time Projection Chamber, built at Berkeley Lab, is the heart of the STAR detector.

To search for the presence of a quark-gluon plasma, the RHIC researchers simultaneously tracked and identified thousands of particles in the debris of their heavy ion collisions. STAR was the only experiment to detect single jets emerging from the collisions between the dual beams of gold nuclei. But in order to draw any conclusions, the researchers needed to compare the gold-gold collision data to collisions in which they would expect to see no jet quenching or suppression of leading particles.

From January to March of this year, researchers at RHIC generated head-on collisions between beams of gold nuclei and beams of deuterons, nuclei consisting of one proton and one neutron. These deuteron-gold experiments, along with experiments using two colliding beams of protons, served as a basis for comparison with the collisions between two beams of gold nuclei. This time the researchers observed back-to-back jets and recorded more leading particles coming from the deuteron-gold collisions.

Hans Georg Ritter, the physicist who heads NSD's relativistic nuclear collisions program, says, "It is the consensus that the quark-gluon plasma has not been found yet. But jet quenching is an exciting new phenomenon that is unique to RHIC."

The findings of the STAR experiment were presented at the Brookhaven colloquium by NSD physicist Peter Jacobs. NSD's David Hardtke developed the method used to analyze the jets in the gold-gold collisions and did the comparison analysis which established that jet quenching was taking place. The analyses were done on the supercomputers at NERSC.

The centerpiece of STAR is the Time Projection Chamber, or TPC, designed and built by researchers, engineers, and technicians in NSD and the Lab's Engineering Division. Construction of the $15 million TPC for STAR was led by NSD physicist Howard Wieman and Engineering's Russ Wells. Construction of the entire $60 million STAR detection system was overseen by NSD physicist Jay Marx and Engineering's Bill Edwards.

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