long island

Physicists at the Brookhaven National Laboratory on Long Island last week sent gold ions around the first ring of the Relativistic Heavy Ion Collider (RHIC), a $600 million nuclear-physics facility built to create and study quark-gluon plasma.

Next, the RHIC team will send ions in the opposite direction around a second, 3.8-km circumference, ring, opening the way for the facility's experimental programme to start this November.

The programme is the culmination of a two-decade quest by US nuclear physicists for an experiment powerful enough to explore the behaviour of quarks and gluons when nuclei collide at energies high enough to wrest these particles from their usual status as the constituents of protons and neutrons.

The tunnels that carry RHIC's rings, as well as some of the equipment which will feed ions into them, were constructed 20 years ago as part of a particle physics experiment, called Isabelle, to study proton collisions. But in 1983, high-energy physicists abandoned Isabelle for the more ambitious Superconducting Super Collider — itself abandoned by Congress ten years later.

The demise of Isabelle led Nick Samios, who had become director of Brookhaven in 1982, to work with nuclear physicists to create a heavy-ion collider. Samios, who was ousted as director of Brookhaven two years ago, can now watch his dream reach fruition as an ordinary physicist working at the laboratory.

RHIC will serve as the main focal point for the Brookhaven laboratory, which has been in turmoil following a leak of radioactive tritium from the fuel storage tank of its research reactor. The reactor has been closed since the leak was discovered in 1997 (see box).

Almost 1,000 investigators will use the four detectors at RHIC — two large and two small — to study the character of the quark-gluon plasma. This plasma is expected to be created when gold ions collide at combined energies of 40 TeV, or 200 MeV for each proton or neutron involved in the collision.

The detectors will use differing methods to detect the byproducts of the collision, reflecting uncertainty about what clues will betray the existence or the nature of the plasma. “We're talking about the interaction of hundreds of quarks and gluons,” says Samios. “There's lots of speculation about what will happen. The problem is that you don't know what you are looking for.”

Broadly speaking, one of the large detectors, called Phenix, will identify rare by-products of the plasma explosion, such as the J/ψ meson, while the other, Star, finds the tracks of the more common ones. Phenix has cost about $90 million and Star $70 million; each involves about 400 investigators.

Two more specialized detectors, Brahms and Phobos, each involve 50 investigators and cost about $8 million.

Japan and Russia have been heavily involved in the design and construction of the RHIC detectors — their combined contribution is worth about $40 million, according to project director Satoshi Ozaki. But there has been little European involvement. European physicists are planning their own experiment, called Alice, using the Large Hadron Collider when it opens in six years' time at the European Laboratory for Particle Physics (CERN).

Collision course: rings have been placed inside a tunnel intended for ill-fated Isabelle accelerator. Credit: BROOKHAVEN NATIONAL LABORATORY

The 200 MeV energy for each nucleon in a collision at RHIC is ten times the energy of the most powerful such experiment so far conducted at CERN, and four times the minimum energy that should smash the colliding nuclei into a plasma of quarks and gluons. Physicists are confident that plasma will be created at RHIC, but unsure of the characteristics of the transition to plasma. “The phase transition is a subtle thing to observe,” says Tom Ludlam, associate director of the project.

The rewards of finding the plasma, however, are great. RHIC could recreate the conditions thought to characterize the Universe for the first fraction of a second of its existence, before quarks and gluons coalesced into protons and neutrons. The discovery of the plasma would confirm an important prediction of quantum chromodynamics, and its characterization could give a new insight into the nature of matter.