Intriguing Oddities in High-Energy Nuclear Collisions

Missing debris in the smashup between gold nuclei going at close to the speed of light suggests the creation of a highly unusual plasma environment, researchers have announced at Brookhaven National Laboratory. By smashing together gold ions at Brookhaven's Relativistic Heavy Ion Collider (RHIC), scientists are attempting to make and study a state of matter that existed only millionths of a second after the big bang. Called a quark-gluon plasma (QGP), it is a hot, dense soup of individual quarks and gluons. In today's universe, by contrast, quarks come in groups of twos and threes, held together by gluons. This spring, Brookhaven researchers performed a "control" experiment, in which they collided a gold nucleus with a deuteron, a light nucleus consisting of just a proton and neutron. In these and other kinds of nuclear collisions, a pair of quarks from a proton or neutron occasionally gets ejected. In turn each ejected quark produces a stream or "jet" of particles in its wake. In some of the gold-deuteron collisions, the researchers indeed observed pairs of jets flying in opposite directions. But in head-to-head collisions between two gold nuclei, researchers observed only one, rather than two, jets. This property, called jet quenching, suggests that the particle jet traveling in the direction of the collision region is getting absorbed by a hot, dense state of matter. Jet quenching is predicted to occur in the correspondingly hot, dense environment of a quark-gluon plasma, but RHIC experimentalists are not ready to claim the QGP prize quite yet. To verify its presence and rule out rival scenarios, they are planning numerous other experiments for finding other signatures of a QGP. However, the new data has convinced Columbia theorist Miklos Gyulassy that the RHIC team is already seeing a QGP. The gold-gold collisions, he and his colleagues calculate, produce an environment 100 times denser than ordinary nuclear matter and display properties predicted in QGP models based on quantum chromodynamics (QCD), the theory of the strong force which holds nuclei together. On June 18, three of the four RHIC experimental groups have submitted papers on the new results to Physical Review Letters and researchers discussed these new results at a special Brookhaven colloquium today. (Brookhaven press release, June 11)