A Puzzling Signal in RHIC Experiments

A puzzling signal in RHIC experiments has now been explained by two researchers as evidence for a primordial state of nuclear matter believed to have accompanied a quark-gluon plasma or similarly exotic matter in the early universe. Colliding two beams of gold nuclei at Brookhaven's Relativistic Heavy Ion Collider (RHIC) in New York, physicists have been striving to make the quark-gluon plasma, a primordial soup of matter in which quarks and gluons circulate freely.

However, the collision fireball has been smaller and shorter-lived than expected, according to two RHIC collaborations (STAR and PHENIX) of pions (the lightest form of quark-antiquark pairs) coming out of the fireball. The collaborations employ the Hanbury-Brown-Twiss method, originally used in astronomy to measure the size of stars. In the subatomic equivalent, spatially separated detectors record pairs of pions emerging from the collision to estimate the size of the fireball.

Now an experimentalist and a theorist, both from the University of Washington, John G. Cramer (206-543-9194, cramer@phys.washington.edu) and Gerald A. Miller (206-543-2995, miller@phys.washington.edu), have teamed up for the first time to propose a solution to this puzzle. Reporting independently of the RHIC collaborations, they take into account the fact that the low-energy pions produced inside the fireball act more like waves than classical, billiard-ball-like particles; the pions' relatively long wavelengths tend to overlap with other particles in the crowded fireball environment.

This new quantum-mechanical analysis leads the researchers to conclude that a primordial phenomenon has taken place inside the hot, dense RHIC fireballs. According to Miller and Cramer, the strong force is so powerful that the pions are overcome by the attractive forces exerted by neighboring quarks and anti-quarks. As a result, the pions act as nearly massless particles inside the medium.

Such a situation is believed to have existed shortly after the big bang, when the universe was extremely hot and dense. As the pions work against the attraction to escape RHIC's primordial fireball, they must convert some of their kinetic energy into mass, restoring their lost weight. But the pions' experience in the hot, dense environment leaves its mark: the strong attractive force (and the absorption of some of the pions in the collision) would make the fireball appear reduced in size to the detectors that record the pions. According to Miller, looking at the fireball using pions is like looking through a distorted lens: the pions see the radius as about 7 fermi (fm), about the radius of an ordinary gold nucleus, while the researchers deduce the true radius of the fireball to be about 11.5 fm (Cramer, Miller, Wu and Yoon, Phys Rev Lett, tent. 18 March 2005).