RHIC Experimental Findings Explained
While the discovery of quark-gluon plasma remains to be declared, analyses of data from previous RHIC runs have revealed the following:
JET QUENCHING: There is more evidence that sprays of particles, or jets, produced in RHIC’s gold-gold collisions are getting stuck in some sort of “goo” created by the collisions. This phenomenon, known as jet quenching, was first hinted at after RHIC’s first run in 2000 and presented at the Quark Matter 2001 meeting at Stony Brook University and Brookhaven. But findings from more recent runs now confirm that the quenching in gold-gold collisions is a result of the environment created by the collisions and not some preexisting condition.
COLLISION-ZONE SHAPE: Having mapped out RHIC’s collision zone, physicists showed that it is, indeed, almond-shaped, thus giving them yet another way to examine the concept of jet quenching. They found that jets traveling the long axis of the zone are more likely to get stuck than those going the shorter distance across the almond-shaped width.
DIRECT PHOTONS: Because photons coming directly from the collision zone are unaffected by the collision environment, direct photons serve as a thermometer. Just as the color of a hot, glowing object indicates the object’s temperature, the number and wavelength distribution of direct photons are relaying the temperature of the first stage of a collision.
J/PSI SUPPRESSION: The production of J/psi particles (co-discovered at Brookhaven in 1974) is expected to be suppressed by quark-gluon plasma. To see if J/psi suppression is occurring within gold-gold collisions, control experiments during which no plasma can be created were performed by colliding gold ions with deuterons, or hydrogen ions. Deuteron-gold results on J/psi production will be compared to J/psi production during RHIC’s just completed gold-gold run.
QUARK-STRUCTURE SENSITIVITY: At certain momentum, mesons, which are made of a quark and an anti-quark, exhibit less of a collective motion known as elliptic flow than do baryons, made of three quarks. This could be because of their different quark number, or because the most abundant mesons are lighter in mass. Comparing the elliptic flow of phi mesons and protons (a baryon), which are close in mass, the phi meson appears to have reduced flow similar to the other mesons. This indicates that this effect is, indeed, due to the quark structure, not particle mass. This may be crucial evidence that these two- and three-quark particles all came from a hot soup of strongly interacting quarks and gluons within the first moments of the collision.
‘HEAVY FLAVOR’ PRODUCTION: Physicists reported upon the ability to measure particles containing a “heavy” charm or bottom quark, which are the heaviest in the quark family. These quarks are interesting because the extent to which they mimic the behavior of lighter quarks may be an indication of the extreme conditions, such as high temperature, present in the very first moments of collision, when quark-gluon plasma is supposed to formed.
COLOR GLASS CONDENSATE: As gold ions are accelerated to RHIC’s high energies, some postulate, gluons that flit into and out of existence within the gold nucleus would appear to be longer lived. This would result in a dense collection of gluons called color glass condensate, a new type of particle suppression most evident close to the direction of the beam. A small particle colliding with such a gluon “wall” would be less likely to interact with any individual gluon in the particle’s nucleus, resulting in a deficit of jets. Three of RHIC’s experiments have now detected such a jet deficit in deuteron-gold collisions. While some interpret this as evidence for color glass condensate within the gold nuclei, others would like to see more data.
PENTAQUARK PARTICLE: Known particles made up of quarks and/or anti-quarks contain either two or three of them. Indications are, however, that other, multi-quark states exist. CERN in Switzerland and Jefferson National Accelerator Laboratory have uncovered a pentaquark, a new particle composed of five quarks. Now, one RHIC experiment has seen an anti-pentaquark; if confirmed, this will be a new diagnostic tool to analyze quark structure in RHIC experiments.