Physicists have found new signs that fiery particle collisions within a giant accelerator two years ago created a state of matter identical to what might have been the stuff of the newborn universe.
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MATTER SPLATTER. A near-light-speed impact of a gold nucleus and a deuteron–the nucleus of the hydrogen isotope deuterium–spews subatomic particles whose trajectories appear as colored streaks on this diagram of a huge detector called STAR. |
Stunning results announced this week are prompting a growing chorus of physicists to say that it’s time to declare success in a decades-long quest to make quark-gluon plasma–an extremely hot, dense soup of matter that contains loose fundamental particles known as quarks and gluons (SN: 8/26/00, p. 136: Seeking the Mother of All Matter).
“This really is a decisive moment,” says theorist Miklos Gyulassy of Columbia University. “I feel, at this stage, we’ve actually seen it.”
While mainly theorists take this stand, experimental physicists largely remain cautious.
Theorists have predicted that smashing together heavy atomic nuclei accelerated to nearly the speed of light can create a quark-gluon plasma. The resulting fireballs, which can reach temperatures measured in trillions of degrees, are expected to melt the protons and neutrons that compose ordinary nuclear matter.
That process would briefly liberate the quarks and gluons that make up protons and neutrons.
Producing a quark-gluon plasma would replay in miniature the critical scene early in the cosmos when the plasma gave birth to ordinary matter, researchers say. Studying the plasma, scientists could expose the fundamental nature of matter and the vacuum that permeates the cosmos.
In experiments conducted earlier this year, physicists at Brookhaven National Laboratory (BNL) in Upton, N.Y., smashed heavy gold nuclei with much lighter nuclei called deuterons–the nuclei of deuterium, a hydrogen isotope. The researchers produced those collisions in the lab’s Relativistic Heavy Ion Collider, or RHIC–a giant device that is now the main tool in the hunt for the quark-gluon plasma.
Creating a quark-gluon plasma wasn’t the aim of these experiments. The goal was to determine whether observations from earlier RHIC experiments could be explained with theories that don’t summon the quark-gluon plasma.
The new gold-deuteron tests focused on jets of particles emanating from the collisions. Jets are produced when two highly energetic quarks bounce off each other. If one or both escape the fireball, they break up into sprays of other particles. Such jets may show up as single bursts or oppositely directed pairs.
However, during the previous RHIC experiments, scientists had observed that collisions between pairs of gold nuclei yielded fewer jets than would be expected at that collision energy.
Physicists had come up with two explanations for the discrepancy. In one scenario, if energetic quarks collide at a fireball’s edge, the quark heading away from the collision’s center might get away while its partner bogs down in the soup of still agitated, colliding particles–the quark-gluon plasma.
Alternatively, some physicists suggested, a subtle property of nuclei known as gluon saturation could have caused the jet suppression.
To sort out what was happening, the RHIC teams turned to the less energetic collisions between gold ions and deuterons. If gluon saturation were at work, jet suppression would persist at these lower energies. If not, jets would emerge in undiminished numbers because the gold-deuteron impacts generate too little energy to create a jet-absorbing quark-gluon plasma.
Jets weren’t suppressed, three independent RHIC teams announced Wednesday at a BNL colloquium. The stunning implication is that a quark-gluon plasma had been present in the gold-gold experiments.
Nonetheless, most of the hundreds of physicists who have been conducting the RHIC experiments don’t consider the case closed, say members of those teams. Despite the new findings and other, previous hints of quark-gluon plasma at RHIC, a stronger case will require observing several additional features of collision debris to rule out some other form of hot, dense matter.
The experimentalists’ reticence stems in part from fallout from an announcement 3 years ago by the European Laboratory for Particle Physics, or CERN, near Geneva.
Researchers stated that something akin to the quark-gluon plasma, if not the plasma itself, had been produced in an accelerator there (SN: 2/19/00, p. 117: Melting nuclei re-create Big Bang broth). To many other scientists, the announcement gave the impression that CERN researchers were prematurely claiming discovery of the quark-gluon plasma when rigorous proof of such a claim was still lacking.
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