Quark-Gluon Plasma: Has It Been Observed?

Barbara Jacak of Stony Brook University in Long Island, N.Y., is a member of the PHENIX team, a large detector collaboration studying the high-energy smashup of gold nuclei at Brookhaven National Lab's Relativistic Heavy Ion Collider (RHIC). Delivering a plenary talk at this week's American Physical Society April Meeting in Dallas, Jacak argued that new experimental data provide evidence that in collisions the gold nucleus, including its complement of neutrons and protons, and all their quark constituents, are being melted into a true plasma of quarks and gluons. This plasma possesses the highest energy density of any substance made in a lab -- up to 15 gigaelectronvolt per cubic femtometer.

At last year's APS April meeting all the RHIC teams unanimously agreed that a peculiar liquid of quarks had been created in the collisions. Peculiar and unexpected: instead of a gas of weakly interacting quarks, the collision fireball ensuing from a head-on interaction of the two nuclei resulted in a liquid of strongly-interacting quarks (see PNU 728). But this wasn't quite the same thing as claiming that this fluid was a true plasma.

To form a plasma, the quarks must reside outside their customary groupings of two or three; two quarks (a quark-antiquark couplet) together are called a meson while three-quark groupings are called baryons. Mesons and baryons in turn are collectively referred to as hadrons. One of the observed properties of hadrons is that they are "color-neutral" (just as ordinary atoms are charge neutral), "color" being the fanciful name for the strong-nuclear-force equivalent of electrical charge. For example, a proton would normally consist of a red, blue, and green quark which (in a color sense) adds up to zero.

And just as an electrical plasma is one in which the particles are charged so a nuclear plasma would be one in which the particles possess color. At last year's April meeting the observation that the matter is liquid was presented. According to Jacak, further studies over the past year now provide, at least for her and a growing number of RHIC scientists, the necessary proof for a plasma state.

One notable fact supporting the plasma contention is the fact, apparent in recent data analysis, that charm-quark jets are being suppressed. In the fireball, charm quarks are being produced, albeit at much lower rates than the light quarks (up, down, strange).

Because of their heft the charm quarks (or, to be precise, the jets of hadrons they engender) ought to be able to punch their way out of the plasma to be observed in outside detectors, but they’re not. What seems to be happening is this: the plasma of mostly light quarks are taking up or engulfing the heavy quarks through frequent and intense interactions.

As Jacak says, it's as if a strongly rushing river were picking up stones off the riverbed and pulling them along with the stream. A river of hadrons (quarks bundled up into color-neutral clumps) wouldn't be able to do this as readily as a river of mostly unattached quarks.