Nu Xu Talks About the STAR Collaboration at Brookhaven National Lab

Special Topic of Hadron Colliders Interview, May 2010

Nu Xu

Photo: Lawrence Berkeley Nat'l Lab - Roy Kaltschmidt, photographer.

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

BNL also ranks at #20 among the 704 institutions comprising the top 1% in the field of Physics in Essential Science IndicatorsSM from Thomson Reuters. Currently, their record in this field includes 4,175 papers cited 100,493 times between January 1, 2000 and December 31, 2010.

ScienceWatch.com 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 STAR Collaboration. Among BNL's papers in our Special Topic, 369 papers with 4,449 cites dealt with STAR in some way; four of these papers rank among the top 20 over the past decade and over the past two years.

ScienceWatch.com correspondent Gary Taubes talks with Dr. Nu Xu about the STAR Collaboration and its role in hadron collider research.


SW: When did you first start working with the STAR Collaboration?

I joined STAR in 1997 and in 2008 was elected spokesperson of the experiment.

SW: Can you describe for us the original physics goal of STAR and RHIC?

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.
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  >

In high-energy collisions, we understand the fundamental interactions involved with quarks and gluons pretty well. We know each element, but we don't know how hadrons are formed. We don't understand, for example, the properties of a proton, the underlying mechanism known as confinement. So we are trying to study it using nuclear collisions—gold on gold, for example.

Imagine 197 nucleons smashing into another 197 nucleons (protons and neutrons). The idea was to heat up the system such that all these particles—protons and neutrons—will melt away, leaving only the quarks and gluons. This is what we call the quark gluon plasma, and it's speculated that this existed some 10 microseconds after the Big Bang.

The hope with RHIC has been to recreate this post-big-bang situation in the laboratory, study its properties, understand what's happening with confinement and how the universe evolved since then. That was the ultimate goal.

SW: As a detector/experiment, what would STAR do that the other three RHIC experiments wouldn't?

STAR does what's called large acceptance very well. Compared with the other detectors, it is literally larger by a factor of 100 in terms of the range of angles over which we can simultaneously measure emitted particles. Per event we can detect up to 4,000 particles per collision, for example. Boom, there's the collision, and we take it. PHENIX, on the other hand, has smaller acceptance and specializes in identifying electrons and photons. Early on, the STAR apparatus was not best suited and optimized for measuring electrons.

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.
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.

The interview series with Brookhaven National Labs  >

Since 2005, PHENIX has upgraded to improve its acceptance and STAR is involved in an upgrade now to do a better job of identifying particles. We can now do identification of electrons very well, and we have very fast data acquisition and a sophisticated trigger system.

We can now take up to 1,000 heavy ion collisions per second. It's really very good for measuring particles whose production rates are small—rare particles. STAR has discovered the heaviest anti-matter.

SW: Of STAR's two most-cited papers, let's start with the second, "Elliptic flow in Au plus Au collisions at root s(NN)=130 GeV," (Ackermann KH, et al., Phys. Rev. Lett. 86: 402-7, 2001). What was that paper reporting and why is it so important?

As I said, what we're doing with heavy ion collisions is studying the quark gluon plasma. In the jargon of physics, we want to study the degrees of freedom of the matter—quarks and gluons.

All matter has well-defined properties—for example, how hot it is, how strong is the collective motion—and what we call the elliptic flow is a measure of those quantities. This is measured in non-central collisions.

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