P.O. Box 5000
Upton, NY 11973-5000
phone 631 344-2345
fax 631 344-3368
managed for the U.S. Department of Energy
by Brookhaven Science Associates, a company
founded by Stony Brook University and Battelle
Interesting findings from RHIC so far:
Early evidence for jet quenching
Jet quenching is a suppression of “jets,” or the stream of particles emerging from gold-gold collisions. Scientists believe that gold-gold collisions create a very hot, dense soup of quarks and gluons. When an energetic quark tries to shoot out through this thick soup, it loses a lot of energy. In simple collisions, such as those of protons on protons, the energetic quarks are measured. The energetic quarks usually come in pairs that fly out back-to-back in both proton-proton collisions, where there is no soup of quarks and gluons, and in gold-gold collisions, where they are truly “in the soup.”
However, preliminary data from the gold-gold collisions at RHIC indicate that sometimes, when one of the pair of quarks gets out to form a jet, its partner does not; the second jet is “quenched.” The reason, scientists suspect, is that the “lost” quark has to traverse a longer distance through the thick gluon soup it encounters in its path. The quark’s energy loss can be related to the density of gluons, and, thus, may allow scientists to measure how thick the soup is.
Collective flow of particles
Although the scientists are fairly certain that they have created a hot, dense soup of quarks and gluons in a tiny volume the size of an individual gold nucleus, to describe this soup as a plasma, they need evidence that the quarks and gluons “thermalize,” or interact as components of a single system. In RHIC, the degree of interaction can be measured in terms of the collective “flow” of particles, acting much like a fluid, that are emitted from the collision region. In asymmetric collisions (versus those that occur head-on between gold nuclei), the degree of flow changes with the angle from which the particles exit the collision zone. The asymmetry, referred to as “elliptic flow,” has been observed at RHIC and indicates a large number of interactions.
However, the same “flow” calculations disagree with measurements of the spatial size of the hot fireball. This disagreement is one of the real puzzles in the field and represents a conundrum that the scientists are working to understand.
Matter, anti-matter density
The early universe was made up of an almost exactly equal number of particles and anti-particles. Most of these quickly paired up and annihilated each other. However, a very tiny excess of particles over anti-particles in the early universe has persisted until the present time, allowing stars, planets and, eventually, people to exist. In the collisions at RHIC, the scientists start with gold nuclei made of particles, not anti-particles.
However, among the particles produced in RHIC collisions, equal amounts of matter and anti-matter are observed, in a ratio that is approaching that of the primordial QGP in the early universe.
First observation at RHIC of J/Psi Particle
The J/psi, a particle discovered simultaneously at Brookhaven’s Alternating Gradient Synchrotron and the Stanford Linear Accelerator Center for which the 1976 Nobel Prize in physics was awarded, is made up of a charm quark and anti-charm quark. The two do a dance that eventually ends when the quark and anti-quark annihilate each other to form a new set of particles. It is theorized that, in a quark-gluon plasma, there would be so many other quarks and gluons banging into the charm partners that the dance could not take place, lowering the number of J/psi particles observed. The first observation of J/psi particles at RHIC has been reported, but the statistics are too small to draw any definitive conclusions about whether the numbers are suppressed. This early measurement is just the tip of the iceberg, and measurements with definitive statistics must be pursued at higher RHIC beam intensities over the next few years to really understand how the charm dance is changed.
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