Color Glass Condensate

Color glass condensate (CGC) is the name for an extreme form of nuclear matter that may have been created in recent experiments at Brookhaven's Relativistic Heavy Ion Collider (RHIC).

At this week's Quark Matter 2004 conference in Oakland, California, experimentalists presented possible preliminary evidence for this novel state of matter. While nuclear physicists are debating the evidence for a CGC, the concept itself is an accepted, if evolving, theoretical idea that may describe a universal form of matter at high energies.

In RHIC experiments, researchers ordinarily collide a beam of gold ions with another beam of gold ions. But during the first quarter of 2003, they studied the collision of gold ions with deuterons, nuclei which each consist of a proton and neutron. They used a deuteron beam precisely to avoid making the coveted quark-gluon plasma (QGP), the hypothetical soup of individual quarks and gluons that the RHIC researchers hope to recreate in their future experiments. They did this in order to better observe the CGC state, which many believe would be a precursor to QGP.

So what is a color glass condensate? According to Einstein's special theory of relativity, when a nucleus travels at near-light (relativistic) speed, it flattens like a pancake in its direction of motion. Also, the high energy of an accelerated nucleus may cause it to spawn a large number of gluons, the particles that hold together its quarks. These factors--relativistic effects and the proliferation of gluons--may transform a spherelike nucleus into a flattened "wall" made mostly of gluons. This wall, 50-1000 times more dense than ordinary nuclei, is the CGC (see Brookhaven page for a letter-by-letter explanation of the CGC's name). How does the gluon glass relate to the much sought quark-gluon plasma? The QGP might get formed when two CGC's collide.

Reporting their gold-deuteron data at the Quark Matter conference, researchers in the BRAHMS collaboration (Jens Jørgen Gaardhoje, gardhoje@nbi.dk) observed fewer-than-usual high-momentum particles emitted transverse (sideways) to the direction of the collision. According to Gaardhoje, the data, which includes BRAHMS's ability to detect particles at small angles to the beam, provided evidence that the deuteron nucleus formed a CGC. Meanwhile, the PHOBOS collaboration (Gunther Roland, MIT, gunter.roland@cern.ch) confirms the experimental effect seen by BRAHMS, though Roland cautions that direct calculations that confront the CGC theory with the observed effect need to be performed.

According to Brookhaven theorist Larry McLerran (mclerran@quark.phy.bnl.gov), the BRAHMS and PHOBOS
observations provide evidence for this new state of matter.
However, Columbia theorist Miklos Gyulassy
(gyulassy@mail-cunuke.phys.columbia.edu), disagrees.

BRAHMS spokesperson Gaardhoje points out there are conflicting theoretical views, but considers the suppressed production of high-momentum particles to be "a necessary feature" of a CGC. Whether it is sufficient evidence is another story, he says, and the next RHIC runs should provide further insights.

Nonetheless, Gyulassy believes that CGC is a valid concept and that the RHIC researchers should actively search for signs of it, just as they continue to try to create and study the QGP (which, incidentally, he believes RHIC has already produced--see Update 642). (Gaardhoje adds that evidence for the existence of a CGC state has already appeared in electron-proton collisions at HERA in Germany.)

According to McLerran, the CGC has the potential to explain many things in high-energy nuclear physics such as the mechanisms by which particles are produced in nuclear collisions as well as the distribution of gluons inside nuclei. (For more information, see Brookhaven news release; for more background on RHIC, see October 2003 Physics Today article)