Melting the Nucleus

Cool your jets. Gold-deuterium collisions at RHIC, like this one, are bolstering the case for homemade quark-gluon plasmas.

Things are looking less and less solid at Brookhaven National Laboratory. At a colloquium next week, scientists working on the Relativistic Heavy Ion Collider (RHIC) in Upton, New York, are expected to present the most compelling evidence yet that they have melted gold nuclei into their component quarks, creating for the first time a form of matter known as a quark-gluon plasma. Although nobody is yet making a definitive claim, the new results make it harder and harder to avoid the conclusion that RHIC has created a form of matter not seen since a few microseconds after the big bang.

It is no easy matter to free quarks. Ordinarily, the quarks in a nucleus are bound up in protons and neutrons, tied together by the strong sticky force of gluons. Theorists have long thought that pouring enough energy into a nucleus would "melt" its protons and neutrons, allowing the quarks to roam free, just as water molecules in ice slip their rigid shackles as the crystal melts. RHIC attempts this by slamming together gold nuclei at more than 99% of the speed of light. During the past 3 years, the RHIC team has gathered evidence that a quark-gluon plasma forms in the super-energetic collisions. For example, they witnessed a phenomenon known as "jet quenching," in which particles from a collision tend to disappear at high energies--as if the jets of particles are reabsorbed by a sticky quark-gluon plasma (ScienceNOW, 22 January 2001).

Jet quenching was a strong hint, but scientists wanted to perform another test. This time, instead of smashing gold atoms with gold atoms, they smashed gold atoms with much, much lighter deuterium atoms. The idea was that the collision should look pretty much the same, jets and all, but this less energetic collision shouldn't be enough to melt the nucleus. Without a plasma around, the jets should not be quenched. And they weren't.

The new evidence, spreading by word of mouth, is being greeted with enthusiasm. "It looks convincing," says Karel Safarik, a physicist at CERN in Geneva. Nevertheless, RHIC scientists are still loath to claim they have created a quark-gluon plasma. Late this fall, they'll begin looking for other subtle signatures, such as the destruction of particles known as J/Ys. That may well put the matter to rest.

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