For a few microseconds after the birth of the universe, quarks and gluons roamed free in a blazing hot jumble of matter known as a quark-gluon plasma. As the plasma cooled, the quarks and gluons condensed into more familiar particles and disappeared. Today, scientists at CERN, the particle physics laboratory near Geneva, announced--gingerly--"compelling evidence" of a new state of matter that might be quark-gluon plasma reborn--unless, that is, it's something else.
The announcement summarizes a 5-year chapter in high-energy physics. Since 1994, CERN physicists have been smashing lead atoms together at enormous speeds, hoping that the colliding nuclei would become so hot and so dense that their protons and neutrons would melt into a soup of quarks and gluons. As early as 1996, CERN scientists saw evidence of a quark-gluon plasma in the unexpectedly low production of an elusive particle known as the J/y (Science, 13 September 1996, p. 1492) as well as other anomalies. The surplus of particles called strange hadrons, for example, is "quite spectacular," says CERN physicist Maurice Jacob.
Yet other physicists are skeptical, and none of the evidence has put the issue to rest. Columbia University's Bill Zajc notes that other mechanisms might account for the destruction of J/y particles, such as collisions with less exotic particles hurtling away from the nuclear smashup.
CERN physicists are careful not to overstate their case. "While all the pieces of the puzzle seem to fit, it needs definite confirmation," says CERN spokesperson and physicist Neil Calder. Upcoming experiments with a bigger collider at Brookhaven National Laboratory in New York might settle the issue, but until then, CERN scientists must be content to celebrate their glimpse of something bizarre. "I don't know whether it's a quark-gluon plasma or not," says Achim Franz, a physicist who worked on two of seven CERN experiments. "But if you take all the experiments together, it's something new and exciting."