A closer look at RHIC

A panoramic view of the PHENIX detector's building and counting house. To the left, an entrance to RHIC's particle beam ring is visible. When access is required, the enormous concrete slabs that block the entrance are removed with a crane to expose the route into the tunnel.

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven is a medium-to-high-energy machine that plays a unique role in the study of the early universe. While most particle accelerators collide single particles (like protons and antiprotons in the case Fermilab’s Tevatron), RHIC’s main purpose is to collide gold nuclei, each of which contains 79 protons.

Why the additional mass? The results of a proton-antiproton collision usually look something like this:

A proton-antiproton collision at the Tevatron (courtesy of Rockefeller University/CDF)

Gold ion collisions produce tracks like this:

A gold ion collision at RHIC (courtesy of RHIC, found on Wikipedia)

RHIC uses collisions like that to produce and study quark gluon plasma, the substance that existed ten millionths of a second after the Big Bang. Quarks are the building blocks of protons, neutrons, and most of the other particles that make up normal matter, and these days they are never seen alone. They are held together by the strong force, which is carried by particles called gluons. The strong force acts like a spring: as the quarks get farther away from each other, the force between them increases, which prevents them from ever leaving the confines of the particle. But at the temperatures and pressures that existed right after the Big Bang, quarks and gluons existed free of particles, moving instead in quark gluon plasma.

Scientists at RHIC initially believed that quark gluon plasma would behave, well, like a plasma, or an ionized gas: they expected to see the unconfined quarks and gluons moving freely within the substance. What they saw instead was a substance that behaved more like a liquid. The quarks and gluons, while unconfined, were still limited in how much they could move by the simple fact that there were so many of them.

Not only does the quark gluon plasma observed at RHIC behave more like a liquid than a gas, it has the properties of a perfect fluid, one that flows with no viscosity. (If you stuck your finger in a perfect fluid and started it spinning, it would never stop.) These things were big surprises for the scientists at RHIC, and they are continuing to improve their detectors and explore how quark gluon plasma behaves at different energies. ALICE, the heavy ion experiment at the Large Hadron Collider, will be able to study quark gluon plasma at much higher energies, where there is a chance it will behave like the gas everyone first expected to see.

The two detectors currently operating at RHIC are STAR and PHENIX. Like any experiments using the same machine, they have a healthy rivalry even though they are designed to complement each other. PHENIX is used for precision measurements; it’s able to trace certain kinds of particles back through all their permutations after a collision. STAR looks at the big picture, taking a photograph of all the particle tracks that emerge after a collision.

PHENIX lies behind one wall of this panorama. Though the detector is brought out into this enormous room once a year during the summer for display and maintenance, we visited during an active run. Thus, while we were exposed to the normal workings of the lab, we didn't get a chance to see the detector unveiled. Click on the image to view larger versions of the panorama.

The buildings that house RHIC's PHENIX detector.

(Just in case you hadn't realized how huge these buildings are)

The wall that encases the PHENIX detector

A faulty detector, similar to a portion of the PHENIX detector.

Chris Pinkenburg assured us that they know what all of these cables do.

The buildings that house RHIC's STAR detector.

RHIC operates by accelerating two beams of particles to relativistic speeds, contained in two intersecting rings. Behind this enormous wall lies the STAR detector, placed at one of two points where the beams collide. The other collision point is inside PHENIX.

Both detectors depend on the RHIC accelerator to push the particles close to the speed of light before smashing them into each other. But RHIC can’t do it alone — it needs several booster accelerators to get the particles up to top speed. The most notable of these is the Alternating Gradient Synchrotron (AGS), which was one of the most powerful accelerators in the world in the 1960s and 1970s. Work done with the AGS produced three Nobel Prizes in phyiscs (1976, 1980, and 1988) and set the stage for all current and future high energy physics research.

The AGS now serves as the injector for RHIC — a good example of the responsible, innovative recycling of lab resources and a poetic statement about how today’s scientific research depends on work that was done in the past. As Anne Sickles, a shift leader at PHENIX, eloquently put it, “Today’s Nobel Prizes are tomorrow’s calibrations.”

Making sure RHIC and its booster accelerators are running smoothly is the job of Main Control, which monitors the workings of the accelerator 24/7 during runs:

The machine went down just before we visited Main Control, and everyone was working to analyze the problem and estimate the time until it would be fixed. Eventually the problem was traced to the Radio Frequency department.

Main Control sorting out the problem.

Another view of Main Control

At Fermilab, we would learn that these keys correspond to portions of the beam rings, which can be flooded with radiation when the machine is on. By requiring every key to be in place before operation, Main Control can ensure that no one is exposed to unsafe conditions during a run.

More Main Control

Brookhaven is a relatively small lab and RHIC is a relatively small project, but that made the experiments seem particularly personal. Working 24/7 to take data can certainly be exasperating, but it also seemed exhilarating in a way unique to projects dependent on teamwork and collaboration. Each experiment at the LHC will have thousands of scientists working on it, and I wonder if they will all feel the sense of ownership, pride, and camaraderie evident in the much smaller halls of RHIC.

Chris Pinkenburg and Anne Sickles

-text by Lizzie, photos by Nick