First Results from RHIC

Brookhaven's Relativistic Heavy Ion Collider (RHIC) had their first heavy-ion collisions back in June and since then extremely energetic smashups between gold atoms have been lighting up detectors in the four interaction halls, creating fireballs that approximate tiny pieces of the universe as it might been only microseconds after the big bang.

One conspicuous goal at RHIC is to rip apart protons and neutrons inside the colliding nuclei in order to create novel new forms of nuclear matter, such as quark gluon plasma. The beam energies have been as high as 130 GeV per nucleon and the beam density is up to about 10% of its design value.

In this first published RHIC paper, the PHOBOS collaboration (contact Gunther Roland, MIT, gunther.roland@cern.ch) describes the "pseudorapidity" (related to the velocity along the direction of the beams) of the myriad particles emerging from the collisions. The researchers pay special attention to particles emerging at right angles to the incoming beams. These particles emanate from the most violent of collisions, which on average create about 6000-7000 particles per event, more than have ever been seen in accelerator experiments before. The number of particles produced in turn is indicative of the energy density of the fireball produced at the moment of collision; this density, 70% higher than in previous heavy-ion experiments, carries the RHIC researchers into a new portion of the nuclear phase diagram.

The data presented here help to constrain models of this high-density nuclear realm. (Back et al., Physical Review Letters, 9 Oct, Select Articles.) All four RHIC detector groups (STAR, PHENIX, and BRAHMS are the three others) will be presenting their first scientific findings at the American Physical Society Division of Nuclear Physics Meeting in Williamsburg, VA on October 4-7). While no announcement of a quark gluon plasma is expected, researchers plan to describe numerous impressive aspects of RHIC's early operation.