Barbara Jacak Talks About Brookhaven National Lab's PHENIX Experiment
Special Topic of Hadron Colliders Interview, March 2010
With a hadron calorimeter, we can do better selection and measurement of the jets, and that's related to one of the surprises at RHIC, which is how opaque this matter is to other quarks and gluons. The plasma is very opaque to all particles that have color, that feel the strong interaction, and it is completely transparent to particles that only feel the electromagnetic interaction. This means that photons get out, but hadrons that come from the fragmentation of quarks or gluons get stuck.
And why was this a surprise?
Well, we expected some opacity, but we did not expect the plasma to be as opaque as it turns out to be. One of the biggest surprises is that even the heavy charm quarks lose a lot of energy to the plasma and ultimately flow along with everything else. Nobody predicted that. This is closely related to the jet-quenching issue. There were predictions that some jet quenching would occur, but the fact that the plasma would look almost black to fast quarks and gluons was a surprise. And the charm energy loss was an even bigger surprise, because heavy quarks should radiate fewer gluons and so make it through easier.
You're the first of the RHIC physicists I've interviewed to use the term "opacity" to describe this property. Isn't this a plasma physics term more than a nuclear physics one?
Well, a number of theoretical papers do refer to the opacity of the quark gluon plasma. But, I have spent some time talking to regular plasma physicists and have adopted some of their language. In fact the behavior of the strongly coupled plasmas in the regime that we study and some observations in the electromagnetic regime, which is what normal plasma physicists are scratching their heads about, turn out to have some of the same kinds of surprising flows. In part, I think they do for the same reason—the strong coupling.
They don't just have pair-wise interactions, but multiple interactions that you have to worry about. You think of a plasma usually as a hot and ionized gas, with pairs of particles colliding. But when the coupling is strong, it's more complicated than that, with multiple particles interacting at once. This is really, really cool.
PHENIX Detector
Courtesy of Brookhaven National
Laboratory.
PHENIX Detector at Brookhaven Lab's Relativistic Heavy Ion Collider.
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What's interesting is that I've also been talking to my cosmologist friends and asking them how it affects their understanding of the current universe that the plasma has these properties just before it hadronizes into the normal matter we see today, and their answer is always some variant of, "Hmmm, we'll have to think about that and get back to you." The early universe, after all, did go through the same transition from the quark-gluon plasma to normal matter as the stuff at RHIC does.
You mentioned J/Psi particle suppression as another interesting result, yes? What is that and what are the implications?
I wanted to tell you about that, because that was one of the predicted, A1, great signatures of quark-gluon plasma, and we do indeed see J/Psi suppression. The J/Psi is a bound state of a charm and anti-charm quark, and those beasts are heavy. They're made in the first moments of the gold-gold collisions, when the gold ions just hit each other. The prediction was that when you send a pair like that through a quark-gluon plasma, they should just fall apart, since the color is screened by the plasma. What we see is that some of them fall apart but not all.
The big question is whether some J/psi's stay together because the plasma is strongly coupled—so even though there is some screening of the color, they may not all completely fall apart—or whether they all fall apart, but then some of them find each other at the end and reform as bound pairs. That's a mystery that's making a very simple signal—less than we'd expect to see—more interesting.
Okay, so in total, how many surprises have there been and what were they?
I would say three. One is the high opacity, one is the fact that the matter flows so freely with nearly zero viscosity per particle, and the third is that the J/Psi suppression is somewhat mysterious. Still, we'll figure it out. And, if you'll humor me, I can offer a fourth surprise that's theoretical rather than experimental. In fact, in some ways it's an even bigger surprise than the actual science itself. It turns out that the strongly coupled quark-gluon plasma is describable with ideas that grow out of string theory. And I can say definitely that nobody expected that.
Isn't one of the criticisms of string theory that it can, effectively, encompass or describe any phenomena in any possible universe?
PHENIX Staff
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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with Brookhaven National Labs >
Perhaps. But the fact that we see such strong interactions within the quark-gluon plasma means the usual techniques to study colored matter don't work very well. They all rely on weak coupling between particles. Yes, people are working to make them better. But you can try the exact opposite assumption—that the coupling is infinitely strong. Surprisingly, it turns out you can easily make calculations about infinitely strongly coupled matter by making the same calculations people do about the behavior of stuff in the vicinity of a black hole.
So this is inspired by string theory, particularly the ideas of Juan Maldacena of the Institute for Advanced Study at Princeton, who sugested a duality between a theory a lot like quantum chromodynamics and what happens near a black hole. Now string theorists are applying this not only to our quark-gluon plasma but also to very cold atoms with strong interactions and to condensed matter as well. It's just astounding that the hottest and coldest matter on the planet is so similar. And some of our string theory friends predicted that the charmed quarks should stop in the quark-gluon plasma, or at least lose a lot of energy.
Which string theorists are involved in this work?
It was Steve Gubser and his group at Princeton who made the heavy quark prediction. They also said that if we do see a response of the plasma to the quarks that dump in a lot of energy, we should see a diffusion wake, so we're off looking for that.
Are you looking for it in the old RHIC data or in the coming runs as well?
Both. This needs lots of data, so we'll probably have to combine several years' worth to get any evidence.
Would this constitute evidence that string theory is true, that it's the appropriate description of the universe, or just that the mathematics of string theory is useful?
It could be.
It could be which?
It could just be that the duality is useful. But I do find it interesting that the discovery of this strongly coupled, quark-gluon plasma has inspired a number of string theorists to actually try to calculate things that can be tested experimentally, even in our own universe.