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April 30, 2001

Near light-speed ion collisions create brief, violent explosions

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Scientists trying to replicate conditions that existed in the first microsecond after the Big Bang have discovered that gold ions ramming each other at nearly the speed of light produce a surprisingly powerful but unexpectedly brief explosion.

“We expected the particles to be released for a much longer time at these high energies,” said University of Washington physicist John Cramer. “Instead, the time is so short that we can’t measure it. The time has grown shorter with increased energy instead of longer.”

The experiments at the Relativistic Heavy Ion Collider, or RHIC, at Brookhaven National Laboratory in Upton, N.Y., are building on previous work at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. Researchers at CERN last year came tantalizingly close to creating a quark-gluon plasma, a stew of subatomic particles (quarks and gluons) that scientists believe existed one-millionth of a second after the Big Bang. Physicists believe such a state of matter hasn’t existed since that first instant after the Big Bang, the explosion that started the universe some 13 billion years ago.

At the CERN Super Proton Synchrotron, the high-energy collisions were arranged by sending one lead nucleus hurtling at nearly the speed of light into another lead nucleus that was held motionless.

At the Brookhaven accelerator, two gold ions are propelled at nearly light speed (a speed physicists call “relativistic”) until they collide with each other. The energy in the collisions is about 10 times greater than at CERN, said Cramer, who has worked at the European laboratory and now leads a group analyzing data from the STAR (Solenoidal Tracker at RHIC) detector facility at Brookhaven.

That group, part of a larger STAR collaboration, first received data in June 2000, and Cramer presented early results during the American Physical Society’s spring meeting in Washington, D.C. A news conference today at which the results were discussed also featured reports from three other detector collaborations at Brookhaven. All agree that it is far too early for anyone working at RHIC to declare the detection of quark-gluon plasma.

Cramer said the collisions so far are producing unexpectedly violent fireballs that expand at about 60 percent of light speed and spray particles for a much shorter time than expected. The velocity of the fireballs is sharply higher than what has been measured previously at CERN. Those fiery collisions disintegrate the ions and create new particles that move outward in a blast wave.

“What we’re seeing is a very violent explosion that was not predicted by any of the theories,” Cramer said.
One goal of those performing experiments is to find results that theorists never considered, he said, adding, “It looks like we did that.”

He said that historically every new high-energy accelerator has produced surprises, and those often have overshadowed the science the facility had been built to accomplish. In this case, the physicists are monitoring the particle collisions in the hope of exploring new territory in physics and understanding better how the universe came to be as it is.

“We are trying to understand how things sorted themselves out just after the Big Bang, on the time scale of about a micro-second,” Cramer said. “We are part of an intricate dance between theory and experiment, prediction, observation and revision of ideas, which always leads to greater knowledge and understanding.”

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For more information, contact Cramer at (206) 616-4635, (206) 543-9194 or cramer@phys.washington.edu