STONY BROOK, NEW YORK--Ever since the Relativistic Heavy Ion Collider (RHIC) was turned on last June, physicists have been eagerly awaiting the results from this newest, biggest particle accelerator on the block. Now, the wait is over. The first data, presented here at an international particle physics conference last week, suggest that physicists have created a state of matter not seen since the big bang.
Inside the tunnels of RHIC at Brookhaven National Laboratory in New York, gold nuclei are accelerated to more than 99.99% of the speed of light and slammed into each other. By analyzing the resulting shower of particles, physicists are trying to figure out how matter behaves when so much energy is poured into so small a space, hoping to understand the laws of physics that reigned during the very first seconds of the universe.
Last year, scientists at CERN in Geneva implied that their collider had slammed nuclei together so hard that the protons and neutrons that make up nuclei "melted," creating a liquid melange of the particles' components, though the scientific community is still unsure whether the CERN results will hold up (ScienceNOW, 10 February 2000). Thus, when RHIC started up in June, physicists hoped that the new data would show evidence of this soup of components, known as a quark-gluon plasma.
And they appear to. At low energies, gold nuclei behave something like a clump of hard wax pellets; slam two into each other, and particles shoot off in all directions. Scientists detect jets of such particles spraying from the sides of these collisions. But when the nuclei collided at RHIC's higher energies, fewer high-energy particles zoomed sideways out of the collisions than expected, says Yale physicist John Harris, spokesperson for STAR, one of RHIC's four experiments. It's as if some of the wax pellets had melted, forming a sticky fluid that slows down particles shooting out the sides.
"It's a very exciting observation," says Tim Hallman, a physicist at Brookhaven working on STAR. "It's early enough that people are guarded, but it matches predictions pretty well of when you make a transition to the quark-gluon plasma." Still, some are suspicious. CERN physicist Carlos Lourenco warns that the RHIC measurements could not show a sharp, well-defined transition between ordinary matter and a quark-gluon plasma, which scientists would like to see before declaring victory. This might happen during RHIC's next run, scheduled to begin in May, which will be longer and use higher energies and more sophisticated detectors.