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
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, firstname.lastname@example.org) and Gerald A. Miller
(206-543-2995, email@example.com), 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).