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Columbia Scientists Help Discover Hottest Most Dense Matter

By Michael Larkin

William Zajc

Columbia scientists joined with more than 1,000 other scientists from around the world in generating the hottest and most dense matter ever observed. In addition to providing a window into how the universe was formed, the discovery promises to offer a greater understanding of the qualities that hold atomic matter together.

"The discovery of this state of matter is relevant because it's probably the same sort of stuff from which everything we see around us originated," said William A. Zajc, a Columbia physics professor who has been involved with the project since 1986. "That is, it's the very dense matter that was present in the first few microseconds after the big bang."

Using the Relativisitic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in Upton, New York, scientists have been colliding sub-atomic particles and recording the results for several years as part of a research project called Pioneering High Energy Nuclear Interaction Experiment (PHENIX).

Columbia professors have a longstanding working relationship with Brookhaven, and have been involved in a leadership role with RHIC and PHENIX since their inception. Zajc has managed the experiment as its spokesperson since 1997.

Beginning in the summer of 2000, a series of experiments were performed where the nuclei of gold atoms were collided into each other. The recorded results produced an anomaly when compared with previously recorded collisions. A collision between two nuclei, which reaches temperatures of more than a trillion degrees, would typically exhibit two back-to-back jets signatures resulting from a pair of quarks being knocked out of a proton or neutron. In this particular collision, however, only one jet was visible. A second series of experiments illustrated similar results.

Physicists theorized that the collisions had melted the nuclei down to their most basic components, or to what is referred to as quark-gluon plasma. The second jet was not visible because the super dense material may have absorbed it. This state of matter has never before been observed and is believed to be how the universe was composed a few millionths of a second after its birth.

To bolster their assertions, the scientists ran a controlled collision of gold nuclei and smaller, lighter deuterium nuclei. These deuterium-gold collisions produced only two-jet signatures. A one-jet signature would have indicated that the reaction observed between the two heavy gold nuclei was not unique, and hence not of significance. But all the collisions between the heavy gold nuclei and the lighter deuterium resulted in two jets. This controlled measurement points to the creation of quark-gluon plasma.

"The particular effect we were studying is an unusual suppression of the high momentum particles that are expected to emerge from such collisions if the matter were transparent," said Zajc. "The very fact that it is not transparent, but instead results in nearly complete suppression of high momentum particles, indicates that we have formed the densest matter ever created."

Columbia's faculty participation in PHENIX is much like a two-sided coin, as theorists and researchers work in tandem in search of discovery. Zajc and Brian Cole, associate professor of physics, are intricately involved with the research itself. Working with Columbia's Senior Engineer Bill Sippach and Associate Research Scientist Cheng-Yi Chi, Cole oversaw much of the electronics and real-time computing that were built specifically for RHIC at Columbia's Nevis Laboratories in Irvington.

Professors Miklos Gyulassy, Norman Christ, Alfred Mueller and Robert Mawhinney are theoretical physicists. They had long theorized that a simpler state of matter could be created and was attainable; but it has been the researchers' job to create the matter and measure its existence through their work at PHENIX.

Zajc was quick to point out that the evidence is less than conclusive, and that much more testing is required to confirm that they had in fact created quark-gluon plasma. "I would call it [evidence] persuasive, but not conclusive. It's crucial to note that persuasive doesn't cut it in the scientific literature, and attempting to sell persuasive as conclusive is not conducive to good science."

Zajc will lead further testing at PHENIX over the next several years to confirm their creation and to further explore the anatomy of the atom.

Gyulassy -- the theorist -- believes the findings were more than convincing and will open a new realm of research. "The new data confirmed our theoretical predictions. The data, together with a new type of elliptic motion of the thousands of produced particles discovered previously, convince me that quark gluon plasma has been produced. Now the exploration of its properties can begin."

Published: Jul 17, 2003
Last modified: Jul 16, 2003

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