Physicists investigating heavy-particle collisions believe they are on the track of a universal form of matter, one common to very high energy particles ranging from protons to heavy nuclei such as uranium. Some think that this matter, called a color glass condensate, may explain new nuclear properties and the process of particle formation during collisions. Experimentalists have recently reported intriguing data that suggest a color glass condensate has actually formed in past work.
Particles such as protons and neutrons consist of smaller particles called quarks and gluons. Just as electrons have an electrical charge and transmit their force via photons, quarks have a "color" charge and transmit their force via gluons. But one major difference is that gluons, unlike photons, interact strongly with one another. As protons or heavy nuclei, such as gold, are accelerated to nearly the speed of light, the quarks and gluons inside flatten into a pancakelike structure, a relativistic effect called Lorentz contraction. The energy of acceleration also produces more gluons. The flattened multitude of gluons then begins to overlap, falling into the same quantum state, similar to the way atoms in a low-temperature Bose-Einstein condensate overlap and behave collectively as one gigantic atom.
Besides being similar to Bose condensates, the squashed matter "bears some resemblance to ordinary glasses," says Larry McLerran, a theorist at Brook-haven National Laboratory who first formulated the concept of a color glass condensate. For instance, the color fields produced by the gluons point in random directions, like the small, diffuse electrical fields generated by the orientation of atoms in glass. Just as regular glass is an amorphous solid for short periods (years) but flows over long intervals (centuries), these high-energy gluons are in a glassy planar state that changes very slowly compared with timescales typical of nuclear systems. This state is common to all extremely high energy particles and should enable physicists to describe the distributions and scattering probabilities of particles produced during collisions.
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The color glass condensate can "shatter" in a collision. The shattering can produce a quark-gluon plasma, a bulk form of quarks and gluons. Although a discovery has not yet been announced, many physicists believe that a quark-gluon plasma, which would provide clues about the early universe, has been created in heavy-ion collisions in the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven lab.
As a precursor to the quark-gluon plasma, the color glass condensate should have been created if the plasma formed, as McLerran and some experimentalists believe it has. Electron- proton scattering in the HERA accelerator in Hamburg, Germany, provided indications of a color glass condensate. But perhaps the clearest signals have taken place in collisions in the RHIC: both in gold-gold and in deuteron-gold collisions. (Deuterons consist of one proton and one neutron.)
To detect a quark-gluon plasma, physicists examine the spray of particles emitted perpendicularly to the beam axis. But to tease out signs of a color glass condensate, detectors look at very small angles (about four degrees) relative to the beam axis. There the effects of a large number of very low momentum gluons dominate. Both deuteron-gold and gold-gold collisions produce fewer particles (relative to other proton-proton collisions) at these small forward angles, a sign that the gold nuclei was in a color glass condensate state. The effect was first seen by the multi-institutional group referred to as the BRAHMS collaboration (for Broad RAnge Hadron Magnetic Spectrometer); two other collaborations--the PHOBOS and PHENIX--confirmed the BRAHMS data.
"I think this is a very interesting hint that something is happening here," comments Gunther Roland of PHOBOS. "But I think there¿s still a lot of work on the theoretical side that is needed to confirm the color glass condensate as the reason for the effect we¿re seeing experimentally."
Theorist Miklos Gyulassy of Columbia University believes, however, that the experimental evidence for a color glass condensate is too indirect: "What has been presented so far is not enough, for me." He says that the condensate should in fact appear for gluons moving with even lower momenta than have been measured. Direct evidence for the condensate might not happen until the more energetic proton collisions occur in the Large Hadron Collider at CERN near Geneva in about three years or until there is an upgrade at Brookhaven, probably a decade away.
David Appell is based in Lee, N.H.
