The underlying assumption has always been that in the very first moments of the universe, when the universe was at extremely high temperatures, matter didn’t exist as it does in our world. The conjecture is that at sufficiently high temperatures, matter will break down into its fundamental constituents: quarks and gluons. This is the state known as a quark-gluon plasma. Around 25 years ago, visionary people like T.D. Lee at Columbia University began to postulate that one way to access, or create in the laboratory, conditions similar to those that existed in the early universe was to collide heavy nuclei together at the speed of light. That idea grew, through a number of meetings, into the proposal for the Relativistic Heavy Ion Collider facility. RHIC started running in 2000 and since then there have been several very striking discoveries that suggest we have indeed produced a new state of matter, but this in turn has recently led to a debate about what it is we’ve actually discovered.

  What’s the basis of the debate?

Back in the day when all this was being formed as a direction for nuclear science in the U.S., people discussed the possible discovery of a QGP, perhaps without defining it completely or worrying about the definition too much. It was envisioned to be a state of matter in which quarks and gluons would deconfine from the hadrons in which they are normally constrained. So, for some people, and this is certainly true for some of the people in the STAR collaboration, the real thrust of the program, as it was articulated back in this formative stage, was to discover the QGP. Other people would say that the real goal was to discover a new state of matter without specifying the detailed properties. So this debate heated up about a year ago, and it continues at a somewhat lower pitch today.

  Is this why you still haven’t actually, officially announced the discovery of a quark-gluon plasma?

Highly Cited Papers by the STAR Collaboration, including T.J. Hallman, Published Since 1995 (Ranked by total citations) Rank Paper Citations 1 K.H. Ackermann, et al., "Elliptic flow in Au + Au collisions at root sNN=130 GeV," Phys. Rev. Lett., 86(3): 402-7, 2001. 239 2 C. Adler, et al., "Disappearance of back-to-back high-pT hadron correlations in central Au + Au collisions at root sNN=200 GeV," Phys. Rev. Lett., 90(8): 082302, 2003. 164 3 C. Adler, et al., "Centrality dependence of high-pT hadron suppression in Au + Au collisions at root sNN=130 GeV," Phys. Rev. Lett., 89(20): 202301, 2002. 161 4 J. Adams, et al., "Transverse-momentum and collision-energy dependence of high-pT hadron suppression in Au + Au collisions at ultrarelativistic energies," Phys. Rev. Lett., 91(17): 172302, 2003. 137 5 C. Adler, et al., "Pion interferometry of root sNN=130 GeV Au + Au collisions at RHIC," Phys. Rev. Lett., 87(8): 082301, 2001. 129 SOURCE: Thomson Scientific Web of Science

Yes. The question is whether we should make "the big announcement" of the discovery of the QGP or "the big announcement" that we’ve discovered a new state of matter and say that it represents the full success of what we originally set out to do. Within STAR, our view, as embodied in a white paper, was that we’re really seeking the QGP and we have a specific definition of that. And there are definitely some specific questions that need to be answered before we can say with any certainty that that’s what we’ve discovered. We’ve been a little bit conservative in this sense, certainly to the frustration of some people who want us to go beyond that.

  But you have discovered something new?

There is no question that we’ve indeed produced a new state of matter, and that’s extremely exciting.

  So can you tell us what you’re seeing and why it doesn’t satisfy the full requirement of a QGP?

There are certain key parts of our definition, and we’re trying to be precise. Our official definition, as phrased in technical jargon in our bible here, is that the QGP is a (locally) thermally equilibrated state of matter in which quarks and gluons are deconfined from hadrons so that the colored degrees of freedom become manifest over nuclear rather than merely nucleonic volumes. In other words, instead of protons, which we know have quarks and gluons in them, we’re trying to demonstrate that we have a volume of matter in which quarks and gluons exist—deconfined—over the size of a gold nucleus, rather than merely in the protons and neutrons themselves. There are a few key things you need to show to prove that this is what you’ve got, and now we have some major pieces of experimental evidence. One is the measurement of something called elliptic flow. The other is called jet quenching.

  Okay. Can you describe these in a way that we can understand?

The first is a measure of the asymmetry in how particles are produced in the collision. And we see what we expect to see there. That’s strong evidence that the matter in the particles colliding is what we call locally thermally equilibrated, which means the pieces interact with each other strongly and collectively share the energy from the collision.

Jet quenching is another very striking observation. Because quarks can’t exist in nature "undressed," which means without being bound to other quarks or anti-quarks, when you collide these particles and two quarks hit each other, they go off in different directions and immediately decay into a spray of particles called a jet. These jets result from the hard scattering of two quarks, or a quark and a gluon, or two gluons. And we know from lots of work at other facilities that when we collide two protons, we should expect to see jets of matter going sideways some known percentage of the time. It’s predicted, though, that in a quark-gluon plasma, the interaction of the particles when they exit the plasma might influence what we see. If one of the scattered quarks or gluons had to go through this QGP goop, it might be so strongly impacted that we wouldn’t see a jet of particles coming out on that side. So that’s what we call jet quenching, and that was also observed. When we collide two nuclei of gold together, they have 197 protons and neutrons in each one, but inside each of these particles are quarks and gluons, so we’re colliding swarms of quarks and gluons, and we look for these jets of particles going out sideways; we expect to see one on one side and a balancing jet on the other. But what we observe, with a much higher probability than we’d expect, is a jet coming out one side of the collision but no recognizable jet of particles balancing it out on the other side. We’ve shown that there is still energy coming out of the other side—the energy has to be conserved. But because of the interaction with this new state of matter, there’s much greater dissipation, and so it’s not readily recognizable as a jet of particles.

  So what does that tell you, and why isn’t that enough to establish that what you’ve got is this long-sought QGP?

If we use the theory to interpret this suppression of jets, or jet quenching, what we come to understand is that matter is produced that has an energy density 50 to 100 times that of normal nuclear matter. That’s indirect proof that this matter is almost certainly not composed of protons and neutrons, and that the "degrees of freedom" are those of quarks and gluons. It’s indirect evidence of deconfinement. And so these two measurements almost get us there, but there is a further refinement. In effect, we’ve proven that the system is thermally equilibrated at an early time and we’ve proven, at least indirectly, that there is deconfinement and that the degrees of freedom are those of quarks and gluons, but we haven’t proven that we have all of these simultaneously. There’s one little further push we have to make, and that’s what we’re working on now. We’re studying it by measuring particles that contain heavy quarks, known as charm and bottom quarks.

  In the papers that have come out, you’ve made much of this notion of a perfect liquid, as opposed to a QGP. What does that mean?

It’s clear to everyone that something very exciting has been accomplished at RHIC—that we’ve discovered this new state of matter and it has some very intriguing properties. One such property is that, unlike the original expectations, the constituents of this matter are not weakly coupled, as they would be in gas, but seem to be very strongly coupled and very strongly interacting, as would be the case in a liquid. A plasma can be a liquid or a gas, so it doesn’t mean it’s not a QGP. We just have to find out where it falls on the continuum between the two, and that depends on the ratio of the potential energy of the constituents to their kinetic energy. Our plasma at RHIC seems to be closer to a liquid than a gas. But there’s another facility that will do similar studies when it comes online in a year or so, the Large Hadron Collider at CERN, which will have significantly higher energies, a factor that some people think will make the matter the LHC produces somewhat closer to being a gas than a liquid.

  Do you feel the pressure to make a definitive announcement of a discovery one way or the other, before the LHC can possibly beat you to it?

Our business is competitive by its nature. Certainly we think we’re very close at Brookhaven, and we want to let people know about our big successes as soon as we feel the case is strong enough. We think that will be before the LHC turns on. We think it’s close.

  In the heat of all this controversy, RHIC and Brookhaven have had funding problems and have gotten help from an unlikely source. Can you tell us about that?

Well, our country has a lot of challenges right now, and how we fund all the priorities is a very challenging task. We feel that nuclear science and RHIC are a high priority, but RHIC recently got hit by a couple of realities. One was that because of the need to balance priorities, funding for RHIC went down in the budget this year. That, by itself, we could probably deal with, although not without some really deep cuts. But the other critical factor is our power costs. We had a long-standing agreement on what power would cost us, but that agreement expired. Now we’re renegotiating, and the price of electricity has jumped by close to a factor of two. That and the reduced budget made it so that we weren’t going to have enough weeks of running time to make a run worthwhile. Then, toward the end of last year, we had an up-turn in the situation. I don’t really know the details, but the gist of it is that several members of the board that helps manage Brookhaven, led by board member Jim Simons of Renaissance Technologies, an investment company, organized a $13 million contribution. Simons used to be a mathematician here at SUNY Stony Brook. This contribution, combined with the DOE funding that was already there, will allow us to have a very significant run this year. At the moment, things are looking reasonably positive.