UB Physicists First To Demonstrate Critical Prerequisite In Quest For Quark/Gluon Plasma

May 02, 1997

BUFFALO, N.Y. -- Using what they term "a poor man's technique," University at Buffalo particle physicists have successfully demonstrated the most important prerequisite for producing the quark/gluon plasma, the "soup" that existed for an instant following the Big Bang.

They reported recently in Physics Letters B that they are the first to detect a collective flow of nucleons resulting from stopping an incoming high-energy lead beam, the highest energy beam available in the world today.

"Detection of this collective flow is the litmus test for creating the quark/gluon plasma, which will give us our first window on the earlier universe," said Piyare L. Jain, Ph.D., professor of physics at UB and principal investigator. He conducted the work with Gurmakh Singh, Ph.D., research assistant professor of physics at UB.

Their research confirms theoretical predictions that conditions that approach the recreation of the quark/gluon plasma can be achieved by using very-high-energy, heavy- ion beams.

The UB researchers used the world's highest energy accelerator, 7 kilometers in circumference, located at CERN, the European Laboratory for Particle Physics, in Geneva, Switzerland.

In this experiment, a heavy-ion lead beam was accelerated to its total energy of 33 trillion electron volts and directed to hit the researchers' specially prepared thick emulsion, which serves as both target and detector. The interaction of the lead beam with the emulsion target produced more than 20 times the density of ordinary matter.

Jain explained that these results, taken together with those of other groups all working on different aspects of quark/gluon plasma detection, have created increasing optimism in the field.

"Within two to threee years, physicists might be very close to detecting the direct evidence of the quark/gluon plasma," he said.

The achievement by the two UB scientists is particularly noteworthy because in the field of particle physics, teams of 50 or even 100 researchers for a single experimental paper are commonplace.

No other group has been able to show this collective flow with a lead beam.

Once the plasma is directly detected, scientists will be able to study the quark, believed to be the most basic form of matter, because it finally will be freed from 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 become "frozen" into neutrons, protons and other particles of which the universe is comprised.

In heavy-ion accelerator experiments, the interactions of colliding particles create for an instant a high-energy, high-density mass like a fireball that scientists believe is the quark/gluon plasma. The fireball produced in these experiments exists for an extremely short time.

"The bigger the fireball we can create, and the longer it can survive, the more we can see in it," said Jain.

The fireball created in this latest experiment had the highest density that has ever been produced in the laboratory: it resulted in more than 1,000 particles in a single interaction. By contrast, in 1995, Jain conducted experiments with a gold beam at Brookhaven National Laboratory that created a fireball with a total energy of 3 trillion electron volts and produced 300 particles in one interaction, fewer than one-third of the particles produced at the recent CERN experiment.

For a fraction of a second, the temperature in this latest experiment hit a peak of many hundreds of thousands of degrees Centigrade.

The combination of such extremes in energy, density and temperature closely mimics the conditions of the Big Bang.

Unlike with the gold-beam experiment, Jain explained, the plasma produced in a fraction of a second with the lead beam was large enough that he is confident that further analysis will reveal other, important information that will lead scientists closer to detecting the quark/gluon plasma.

"Physicists believe that quarks move freely in the fireball, but once it starts to cool and give birth to new particles, we cannot break the force that binds these particles together," said Jain. "So we need to produce a fireball with even higher volume by achieving interactions with even higher energy and density."

The UB researchers' success was not achieved with multi-million-dollar electronic detectors, the technique of choice among many of the large particle physics research groups working today.

Instead, 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 one hand, register results that allow the scientists, using a high-resolution microscope, to see interactions between particles.

Jain painstakingly customizes the detectors for each experiment. He travels to CERN, the only laboratory in the world with the proper facilities for making a special liquid gel into the 600-micron-thick emulsion he needs for his experiments.

He then conducts the experiment -- in this case it was in the CERN laboratory. Back at UB, he analyzes the emulsions, a process that takes several months to years.

According to Jain, in addition to the achievement of producing collective flow with lead, the results of the most recent experiment also prove that his unique emulsion technique works very well under extreme conditions.

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 hundreds of particles are produced in a single interaction 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 with the highest possible energy and densities available.

To accelerate with higher energy, the UB researchers will have to wait until 2001 when a new accelerator comes on-line at Brookhaven National Laboratory. But Jain noted that experiments with lead also must be conducted with accelerations at lower energies in order to determine exactly when the collective flow of nucleons begins to form.

In a separate paper, published last August in Physics Letters B, the UB researchers also found evidence of a phase transition during the nuclear interactions with the lead beam, a phenomenon that had not been previously observed at such high energies.

Jain explained that the quark/gluon plasma story becomes a bit clearer each time another set of results is published.

"Like archaeologists, each group is helping to add a different piece to reconstruct the main event," he said. "Each new experiment helps physicists to more closely explore the conditions of the early universe."

University at Buffalo

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