Nu Xu Talks About the STAR Collaboration at Brookhaven National Lab
Special Topic of Hadron Colliders Interview, May 2010
Physical Review Letters will only publish at most six-page articles, including the author list, which in our case takes one and a half pages by itself. This review paper was more than 80 pages long so the Brookhaven management made a deal with the editorial office at Nuclear Physics A and all four papers were published there.
STAR is currently upgrading the detector. What's being done and what do you hope to learn with the improvements?
At the moment, STAR is doing two upgrades so it can do two major measurements. What I think is crucial to pin down the quark-gluon plasma formation at RHIC, first of all, is to measure heavy quark formation—the charmed, bottom, and top quarks. RHIC only has enough energy to produce the charm and bottom quarks—the top quark has an energy of about 200 GeV—and we want to measure the collective motion of these heavy quarks. That will serve as a gauge of the thermalization of the collisions.
The second thing is that we can now do a good job of measuring electrons from the collision and, particularly, we can measure pairs of electrons, known as di-electrons. From these di-electron distributions we can get a direct measurement of the temperature at which the quark-gluon plasma is formed. Once we have those two measurements, we can say, "Okay, the quark-gluon plasma is there, with this temperature, this density, and this collective flow."
What do electrons tell you that other particles don't?
STAR Detector
Courtesy of Brookhaven National
Laboratory.
The STAR detector at Brookhaven's Relativistic Heavy Ion Collider (RHIC).
As big as a house, STAR searches for signatures of the form of matter that
RHIC aims to create: the quark-gluon plasma.
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Electrons interact only via electro-magnetic interactions. They do not feel the strong interactions. If you take other particles on the way out of the collisions, they interact so many times that any initial medium information they may have carried has been lost. But the electron doesn't feel these interactions, so we say it's more penetrating. It gives clearer information about the early stage of the collisions and that's why we measure them.
When is the upgrade scheduled for completion?
The detector upgrade should be completed by the summer of 2013 and so ready for the run in 2014. As I see it, we should have a definitive answer by 2018.
How would you compare your expectations back in the late 1990s with what you've done and learned over the past decade at RHIC?
I don't really know how to answer this question. I was relatively young in the field when RHIC was being planned, so I didn't know what to expect. This field, nuclear physics, is experimentally driven. Our theoretical ability to calculate the properties of fundamental particles is still in the developmental stage. It's not because the theorists aren't very smart, but because it's very, very difficult to do, and so we don't get much guidance from theory. This is why, for example, before RHIC we thought the interactions were not going to be as strong as what we've observed.
So you're satisfied with what the RHIC experiments have accomplished so far?
Yes, very much. RHIC really made a huge advance in nuclear physics and our understanding of the universe. It has contributed to the advancement of accelerator technology, detector, and human resource development. It's been tremendous and I hope the next decade will continue along the same lines. Our experiment, STAR, has been publishing 15 to 20 papers each year on what we've been learning and we have graduated more 130 graduate students. It's very satisfying and I feel great about it.
Nu Xu
Nuclear Science Division
Lawrence Berkeley National Laboratory
Berkeley, CA, USA
and
Spokesperson, the STAR Collaboration
Brookhaven National Laboratory
Upton, NY, USA
BROOKHAVEN NATIONAL LAB'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
- Yao WM, et al., "Review of particle physics," J. Phys. G-Nucl. Particle Phys. 33(1): 1-+ Sp. Iss. SI July 2006 with 2,980 cites.
- Adams J, et al., "Experimental and theoretical challenges in the search for the quark-gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions," Nucl. Phys. A 757 (1-2): 102-83, 8 August 2005 with 757 cites.
Source: Essential Science Indicators from Clarivate.
ADDITIONAL INFORMATION:
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Menu for the institutional interview series with Brookhaven National Lab from the Special Topic of Hadron Colliders
KEYWORDS: BROOKHAVEN NATIONAL LAB, NUCLEAR PHYSICS, RHIC, RELATIVISTIC HEAVY ION COLLIDER, STAR COLLABORATION, HIGH-ENERGY COLLISIONS, HADRONS, PROTON FORMATION, CONFINEMENT, NUCLEAR COLLISIONS, GOLD-GOLD, QUARK-GLUON PLASMA, BIG BANG, LARGE ACCEPTANCE, ELLIPTIC FLOW, THERMALIZATION, JET QUENCHING, HEAVY QUARK FORMATION, CHARMED QUARK, BOTTOM, TOP, COLLECTIVE MOTION, DI-ELECTRONS.