Latest RHIC Results Make News Headlines at Quark Matter 2004
Gold, Deuterons, Protons
The RHIC data presented at Quark Matter are from January through March of 2003, when beams of heavy gold nuclei were collided with beams of deuterons, which are much smaller and lighter nuclei, each consisting of one proton plus one neutron. These deuteron-gold runs, along with other data from colliding two beams of protons, serve as a basis for comparison for the gold-gold collisions at RHIC
When two gold nuclei collide head-on, the temperatures reached are so extreme — more than 300 million times the surface temperature of the sun — that the protons and neutrons inside the merged gold nuclei are expected to melt, releasing their quarks and gluons to form quark-gluon plasma. In contrast, when a much smaller deuteron strikes a large gold nucleus, only a small part of the gold nucleus is heated up, and so the matter within the gold nucleus is thought to remain close to its normal state, with distinct protons and neutrons.
In either type of collision, a pair of quarks can be knocked loose from a proton or neutron, with each of these loose quarks producing a spray, or “jet,” of ordinary particles. The two jets will emerge back to back from the collision region — unless they are stopped within a dense medium. In the 2003 deuteron-gold experiments, back-to-back jets were seen; but, in head-on collisions during the earlier gold-gold runs, one of the two jets was missing. In addition, fewer highly energetic individual particles were observed coming from gold-gold than from deuteron-gold collisions.
One explanation of the missing jets is that a quark traveling through this environment would interact strongly, losing most of its energy. Thus, if a quark pair is produced near the surface of the nuclear fireball resulting from a head-on gold-gold collision, then the outward-bound quark is likely to escape, while the inward-bound quark is absorbed. As a result, only one jet is detected. This phenomenon, called “jet quenching,” is predicted to occur within quark-gluon plasma. The same calculations also predicted the observed suppression of individual high-energy particles.
RHIC physicists are intrigued by these distinctions, which clearly show that head-on gold-gold collisions are producing a nuclear environment quite different from that of deuteron-gold collisions. These phenomena are new at RHIC, as they have not been observed in previous experiments at lower energies.