UB Physicists Report First Evidence of Quark/Gluon Plasma

BUFFALO, N.Y. -- In an experiment that has, for the first time, recreated aspects of the Big Bang in the laboratory, University at Buffalo physicists have observed the strongest evidence yet of the most basic form of matter, the quark/gluon plasma.

"With these results, we are entering a new world," said Piyare L. Jain, Ph.D., professor of physics at UB and principal investigator, "a world that only existed during the microsecond of the Big Bang."

The research was published in the Feb. 27 issue of Physical Review Letters, the world's most prestigious physics journal.

The UB experiment, conducted at the Alternating Gradient Synchrotron at Brookhaven National Laboratory on Long Island, is the first to provide unequivocal evidence of the complete stopping of an incoming high-energy gold beam by an emulsion that is both target and detector. This resulted in a collective flow of nucleons, either protons or neutrons, which is a prerequisite for producing the quark/gluon plasma.

Even before it was published, the results were highlighted at the international conference, Quark Matter '95, held in January in Monterey, Calif., and attended by 600 physicists. Jain's work was cited by the eminent physicist Horst Stocker of the Institute of Theoretical Physics, Frankfurt University, who with Walter Greiner originated the theory that in order to see the quark/gluon plasma, it would first be necessary to produce a collective flow of nucleons.

Detection of the quark/gluon plasma, the "soup" that existed for only an instant following the Big Bang, is critical to eventually freeing the quark from its confinement in the nucleon so that it may be studied.

"This is a very exciting time," said Jain. "We are approaching the point where we will be able to recreate the Big Bang in the laboratory. In fact, there are so many different parameters to examine during these experiments, it is possible we have already produced the quark/gluon plasma. But it is quite complex, and our experimental limitations may be such that we have not directly observed it."

Quarks and gluons are subatomic particles, the smallest forms of matter known and which comprise nucleons. Physicists have long wanted to free the quark from gluons, the massless particles that bind quarks together with a very strong force, but they have not been able to do so because quarks are so tightly bound up in the nucleon.

By freeing the quark, scientists will, for the first time, be able to study it and test the current assumption that the quark has no substructure, making it the most basic form of matter.

According to the Big-Bang theory, the quark/gluon plasma was created during the cataclysmic explosion that caused the creation of the universe. During that explosion, temperature and pressure were extremely high and the quark/gluon plasma formed for a microsecond. After the Big Bang, the plasma expanded, temperatures cooled and the quarks became 'frozen' into neutrons, protons and other particles of which the universe is comprised.

By recreating in the laboratory similar conditions that will produce the quark/gluon plasma, physicists and cosmologists will be better able to understand the origins of the universe.

The current research builds on particle-acceleration work with silicon that Jain reported in 1987 and 1988, also in Physical Review Letters. In that experiment, Jain discovered high-energy-density nuclear states, where very high energies are deposited in a small area, in the interacting volume observed in the target material. These high-energy states are essential to observing the nuclear flow.

"In that earlier experiment, we observed 20 times more energy density in the interacting nuclear volume than there is in the real world," said Jain, "an indication that it would be possible to produce the plasma."

But with the silicon beam there was still no evidence of the collective flow of the nucleons.

To produce those interactions, still-higher energy densities were needed, which could only be produced using heavier ions. At CERN, the European Laboratory for Particle Physics in Geneva, Jain tried oxygen, which has 16 nucleons, and silicon, which has 28 nucleons, as well as sulfur, which has 32 nucleons. These experiments were done with high energy beams, but the stopping of the beam was not observed.

"We knew we needed even heavier particles," said Jain. "The problem is that the heavier the particle, the greater the energy that is required to push them around."

Finally, in 1992 and 1993, a gold beam was made available to the UB physicists at Brookhaven National Laboratory. Gold has 197 nucleons.

In the experiment, gold nuclei were accelerated to 10 billion electron volts per nucleon, an extremely high energy.

But it wasn't just the use of the gold beam and the high accelerations that contributed to Jain's recent success.

Unlike similar experiments conducted by other groups that use electronic detectors that are in some cases as large as one-story buildings, Jain has developed his own special, photosensitive detectors made from ordinary photographic film mounted on glass. The detectors, which are small enough to hold in the hand, register results that may be seen with a high-resolution microscope.

"This is a poor man's lab," Jain explained.

For each experiment, he customizes the detectors, a painstaking process that involves complicated logistics. He travels to CERN, the only laboratory in the world with the proper facilities for making a Japanese-manufactured liquid gel into the thick emulsion he needs for his experiments. He then brings the thick film to the laboratory, where the experiment will be conducted -- in this case, Brookhaven. To develop the film, Jain must then go back to CERN. He then returns to UB to analyze the emulsions, a process that may take several years.

"It takes very hard work to produce these results and to compete with other groups that have 40 or 50 researchers on a single paper," said Jain, who conducts all of the measurements and analysis himself with assistance from two colleagues at UB.

What it lacks in sophisticated electronics, Jain's special photo-sensitive emulsion makes up for with extremely accurate space resolution, i.e., the extremely small angles at which the particles are produced in these high-energy nuclear collisions.

It is because of that high resolution that it has achieved what other detectors could not: the first direct evidence of the nuclear collective flow.

"Because the emulsion is so thick, we can see the three-dimensional effect of the reactions of these particles," explained Jain.

"When we are finally able to break the strong force, separating the quark from the gluons and producing the quark/gluon plasma, the energy will remain, so the plasma's energy density confined in the nuclear collision will be very high," Jain said.

In the Brookhaven experiment, during brief head-on collisions between projectiles and the target emulsion, 2 trillion electron volts of energy were produced in space of just 10-15 of a meter.

In their continuing effort to recreate the Big-Bang in the laboratory, Jain recently completed a new experiment at CERN, with the highest possible energy available, using the heaviest element, lead, which has 209 nucleons. The analysis will take between two and three years.

"We hope that with this new beam we will achieve what we have been looking for, direct evidence of the quark/gluon plasma," said Jain.

Jain's co-authors on the recent paper are Gurmukh Singh, Ph.D., research assistant professor at UB and Amitabha Mukhopadhyay, Ph.D., former postdoctoral researcher at UB.

The research was funded by the U.S. Department of Energy and the Research Foundation of SUNY Buffalo.