RHIC Resumes Operation With First Full-Energy Collisions

Higher energy, detector upgrades to yield more data in quest for quark-gluon plasma

Ion collision

UPTON, NY - Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have taken their search for an elusive form of matter to a new level by bringing the Relativistic Heavy Ion Collider (RHIC) up to full collision energy. All four detectors at the 2.4-mile-circumference, two-ringed particle accelerator are now recording these full-energy collisions, which are expected to produce 100 times more data than collisions during RHIC's first run last year. The result will be a clearer picture of what happens when gold ions slam together at nearly the speed of light.

The goal of this research is to recreate (on a microscopic scale) the hot, dense conditions that are thought to have existed when our universe first formed, so scientists can study the basic components of matter as they existed in its earliest form. Those components, the quarks and gluons normally bound together inside protons and neutrons, should exist freely for a fleeting instant in the hot, dense "quark-gluon plasma" RHIC aims to create.

An event display showing particles emerging from collisions enlarge

An event display showing particles emerging from collisions and striking the pad chamber detectors (green area) and Time-of-Flight detectors (gray area) in the two "central" arms of PHENIX, one of RHIC's large experiments. Several hundred particle tracks (in cyan) are seen which indicate the collision vertex at the center of the image.

RHIC has already produced some very intriguing results, indicating that we are on the right track toward producing quark-gluon plasma," said Thomas Kirk, Brookhaven's Associate Laboratory Director for High-Energy and Nuclear Physics. But, he added, "those results were based on a very small amount of data. This next run will be much more substantial."

In its first run, RHIC collided ions at a maximum energy of 65 billion electron volts (GeV) per nucleon (proton or neutron). The new collisions, at 100 GeV per nucleon, will deposit more energy into the collisions, raising the temperature higher. These higher temperatures increase the chance that the colliding particles will reach the "boiling point," the temperature at which quarks and gluons can escape from their confined existence inside protons and neutrons - similar to the way water molecules escape as steam from a pot of boiling water as more energy is added.

gold-gold collision at the maximum RHIC energy enlarge

A spectacular gold-gold collision at the maximum RHIC energy as seen by the Phobos detector. Phobos consists of a cylindrical array of silicon detectors and two spectrometer arms surrounding the interaction region where the gold nuclei collide. Colored dots show the locations where silicon was struck by the thousands of produced particles. The red lines are reconstructed trajectories of some of those particles.

If they are successful, one result would be a dramatic increase in the number of particles produced in the collisions, as well as changes in the distribution of particle types. For example, scientists expect to see more particles that contain strange quarks and antiquarks, as well as distinct changes in the pattern of particles produced relative to the directions of the colliding beams. As the statistics improve with additional running time, more subtle effects connected with the kinds of particles produced should emerge. All of these new collision properties are characteristic only of relativistic heavy ion collisions and, all taken together, will allow scientists to reliably infer the production of quark-gluon plasma.

Operating at top energy is only the first step in enhancing the physics output of RHIC. Very significant gains will also be obtained by increasing the luminosity, the collision rate at each of RHIC's four detectors - the "eyes" the physicists use to observe the collisions. Since last year, each of the detectors has also been upgraded significantly. This will increase their ability to see and track particles as they emerge from the collisions.

An end-on view of one of the first full-energy collisions between gold ions at  the Solenoidal Track enlarge

An end-on view of one of the first full-energy collisions between gold ions at the Solenoidal Tracker At RHIC (STAR) detector. The tracks indicate the paths taken by thousands of subatomic particles produced in the collisions as they pass through the STAR Time Projection Chamber, a large, 3-D digital camera.

In addition, the computers that analyze the enormous volumes of data collected by RHIC's detectors have been upgraded as well. "The raw computing power at RHIC has more than doubled, while storage capacity has quadrupled," said Bruce Gibbard, head of the RHIC Computing Facility. RHIC's computers can now store 1.2 petabytes, or 1.2 million gigabytes, of data. The average desktop computer, by comparison, has a capacity of only 20 gigabytes.

This run is expected to last about four times as long as the initial data run in 2000. In addition to the heavy ion collisions at full energy, this run will also explore collisions of polarized proton beams for the first time in RHIC.

A graph from RHIC's BRAHMS detector

A graph from RHIC's BRAHMS detector representing how close together in time particles emerging from each collision strike two Zero Degree Calorimeters (ZDC) situated opposite each other along the beam path, 20 meters from the collision point. The central peak at zero nanoseconds (ns) indicates the number of collisions happening at a point equidistant from the two ZDCs.

"The achievement of the full-energy RHIC physics program represents the culmination of a decade-long effort to create new knowledge about the birth of the universe," said Kirk. "It is difficult to know how the resulting insights will change and influence our technology, or even our views about nature, but history suggests there will be changes, and some may be profound. Brookhaven is proud to be at the center of this advance."

This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields.

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