Barbara Jacak Talks About Brookhaven National Lab's PHENIX Experiment

Special Topic of Hadron Colliders Interview, March 2010

Barbara Jacak. Image Courtesy of Brookhaven National Laboratory.

Image Courtesy of Brookhaven National Laboratory.

According to our Special Topics analysis of hadron colliders research over the past decade, the work coming out of Brookhaven National Laboratory (BNL) ranks at #1 by total cites, #2 by number of papers, and #20 by cites per paper, based on 1,529 papers cited a total of 27,320 times.

Among the 681 institutions comprising the top 1% in the field of Physics in Essential Science IndicatorsSM from Thomson Reuters, BNL ranks at #20. Their current record in this field includes 4,104 papers cited 98,631 times between January 1, 2000 and October 31, 2010. has conducted a series of interviews with representatives of the various projects at BNL that have contributed to its citation record, particularly with regard to hadron collider research. This interview relates to the PHENIX experiment. Among BNL's papers in our Special Topic, 323 papers with 5,317 cites dealt with PHENIX in some way; one of these papers is ranked at #5 on the 10-year paper list.

In this interview, correspondent Gary Taubes talks with Dr. Barbara Jacak about the PHENIX experiment and its particular contributions to hadron collider research over the years.

SW: When did you first get involved with the PHENIX experiment?

I've been involved in the experimental program at RHIC since the very beginning when it was just a gleam in all our eyes. I've been in it for long, long time.

SW: What set PHENIX apart from the other three detectors? What were the physics goals and what made the detector itself special?

PHENIX had an interesting history, which is why it's called PHENIX. In the initial round of proposals for experiments at RHIC, a lot of proposals were submitted. Three shared some key characteristics: all were looking at rare probes of the quark-gluon plasma.

In particular, these were probes that were either produced early in the collisions, at calculable rates, so we could see what the plasma does to them, or they were electromagnetic probes, meaning they wouldn't feel the strong interaction and would make their way out of the plasma undisturbed. As a result, they would carry initial-state information about the plasma at its hottest and densest. We wanted to look for photons and electron pairs and muons.

Because these three proposals all had variants of those kinds of measurements, the program committee sent us all home and told us to have a shotgun marriage and get together and come back with one proposal. Thus, PHENIX rose out of the ashes.

SW: Why is it an acronym then, all caps?

The PHENIX detector at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) records many different particles emerging from RHIC collisions, including photons, electrons, muons, and quark-containing particles called hadrons.
PHENIX Detector
Courtesy of Brookhaven National Laboratory.

The PHENIX detector at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) records many different particles emerging from RHIC collisions, including photons, electrons, muons, and quark-containing particles called hadrons.

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In this case the acronym came after the name. It stands for Pioneering Hadron Electron Nuclear Interaction Experiment.

SW: And what did PHENIX do to allow it to look for these rare probes?

Photons, electrons, and muons are all rare in these collisions and they all have very large backgrounds from decays of the original common hadrons. So you need very good particle identification to tell which particles are electrons and photons, in particular. For the muons, you need a big absorber to get rid of the hadrons that decay into muons.

Basically, you need lots of particle identification and you need a good selective trigger and a high-rate capability. Since all the big experiments were given a budget envelope, we had to trade off some coverage for the particle identification and high rate.

If you look at a picture of PHENIX, you'll see that it's really a collection of four large spectrometers: two at central rapidity, where the action is highest—the highest temperatures and density—and one forward and one backward. The goal of these is to measure the muons that also come out of the plasma unperturbed. They also allow us to measure bound states, such as J/Psi and upsilon, which are bound states of a quark and an anti-quark. The central spectrometers measure photons in a highly granular electromagnetic calorimeter. And then we also measure electrons and hadrons in the central spectrometers with tracking detectors and a suite of detectors to identify the few electrons out of the many everything else.

If you look at pictures, one other thing you'll notice about all the RHIC detectors compared to other collider detectors, is that none of us have a hadronic calorimeter. Most collider detectors do. That's driven in part by the high particle densities at RHIC. Gold–gold collisions can produce up to 10,000 particles if the nuclei hit head on. We are making plans to remedy that lack, so detectors in the next round will look lot more like other collider detectors.

SW: What do you learn by studying the hadrons themselves?

That's how we see the collective flow patterns. One of the big surprises of the quark-gluon plasma is that it flows very freely. The way we measure that is by looking at flows of many different particles, including clusters of hadrons arising from a single energetic quark or gluon—jets, in the lingo. The way particles are correlated with one another tells us about collective motions in the matter giving rise to them.

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