Nu Xu Talks About the STAR Collaboration at Brookhaven National Lab
Special Topic of Hadron Colliders Interview, May 2010
Imagine two pancakes hitting each other, two parallel plates. If you let them displace a little, you see the overlap region and this is the elliptic flow. The strength of that flow depends on the pressure gradient driven by interactions of the participating particles, and to what degree the thermalization is achieved, and now I'll have to define thermalization. This is the equal partitioning of energy.
For example, our body temperature is more or less synchronized with our surroundings. The outside temperature may be cold, but you keep warm by putting a jacket on. Between the jacket and your body, you are equilibrated. You don't feel the temperature gradient; you don't feel the cold. That's what we mean by thermalization. Or imagine putting your hand in a bucket of warm water. At first you can feel how warm it is. After a while you don't because your hand and water readjusts to a common temperature, and that process is thermalization.
So what do you learn from the thermalization process in the collisions?
STAR Detector
Courtesy of Brookhaven National
Laboratory.
The Solenoidal Tracker at RHIC (STAR) is a detector which specializes in
tracking the thousands of particles produced by each ion collision at RHIC.
Weighing 1,200 tons and as large as a house, STAR is a massive detector. It
is used to search for signatures of the form of matter that RHIC was
designed to create: the quark-gluon plasma. It is also used to investigate
the behavior of matter at high energy densities by making measurements over
a large area.
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Before RHIC, we did not expect to see such a strong elliptic flow in the collisions. Most models predicted a factor of two less. The fact that there is such a strong collective flow, or elliptic flow, is very clear evidence that we have really produced a state of matter that we never had produced before. It has a lot to do with the degrees of freedom being dominated by gluons and a lot to do with the high degree of thermalization in the collisions. So that's the very first indication of a new state of matter formed at RHIC and why we're all excited about this and why the paper is so heavily cited.
So what's been created at RHIC is, indeed, the long-sought quark-gluon plasma?
We can't say that from this evidence alone. But since then we've collected more and more evidence pointing us toward that conclusion. Although I still can't say that what we see in these collisions has all the properties of a quark-gluon plasma as we originally envisioned it. We're still working on it.
The STAR paper with the most citations is the 2005 Nuclear Physics A article, "Experimental and theoretical challenges in the search for the quark-gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions," (Adams J, et al., 757: 102-83). Tell us about the genesis of that paper and what it reported.
We call that the RHIC white paper. All four experiments wrote one—PHENIX, STAR, BRAHMS, and PHOBOS—and we all summarized what we have learned experimentally since the beginning of the collisions in 2000. This was the STAR summary paper. The challenge to writing the paper was how to put all this information together.
In addition to this collective flow paper, we had another key paper about jet quenching. This was another important discovery at RHIC and it was STAR that made it. In this case, imagine a balloon full of water and a water gun that you're shooting at it. If this jet of water has enough energy it will go through the balloon. If it has less, it might be absorbed by the balloon. It will not pass through the other side. The denser the material in the balloon, the more energy is needed in the jet to go through it. If it doesn't make it through to the other side, we say the jet has been quenched.
STAR Detector
Courtesy of Brookhaven National
Laboratory.
The STAR Detector at Brookhaven's Relativistic Heavy Ion Collider tracks
and analyzes thousands of particles, such as protons, neutrons, and pions,
that may be produced inside the detector.
The interview series
with Brookhaven National Labs >
Now imagine what happens in these high-energy gold-on-gold collisions. They generate hot and dense matter that you can think of as the balloon. One thing we see in these collisions is a jet produced but these jets always come in pairs—back to back. So one jet goes into the balloon, into the hot, dense volume, and the other comes to you. If the density of the material in the balloon is relatively low, we can see and measure both jets.
In these gold-on-gold collisions, we tag the jet coming to our detector, but we don't see the back-to-back jet. That one seems to have disappeared and that's clear evidence that it's been quenched. And this can only happen when we have a hot and dense medium—a quark-gluon plasma-like medium. And this is another piece of evidence that we have formed something like a quark-gluon plasma. This is the kind of evidence we summarized in that 2005 paper and the challenge, theoretically, was how to understand the fundamental properties of this medium.
How was the decision made to publish these articles in Nuclear Physics A?
Well, if we have a very good result—a good measurement—we will always try to publish it in Physical Review Letters. The best paper I ever published personally—my best paper—was in Physical Review Letters. These four papers went into Nuclear Physics A because we wanted to publish them simultaneously and they weren't a first-discovery or a first-measurement paper; they were summary review papers.