Heavy-Ion Collisions & Other High-Energy Physics News from Brookhaven National Lab
Institutional Feature, January 2011
Why do string theorists find it so interesting?
One reason is because of a prediction that comes out of string theory. This matter produced here behaves as though it's almost frictionless. If you think about what it takes for a liquid to flow without friction, you'll think it's not possible to get down to really zero friction in a liquid that is governed by quantum mechanics. And that's because in quantum mechanics, every particle acts like a wave also and has some finite wavelength, and that means that to get to zero friction you have to have point-like particles interacting with zero interaction length. You have to have what we call zero mean free path. This means that the average distance particles travel between successive collisions goes to zero. You can't do that in quantum mechanics.
String theories have predicted a quantum lower limit on the viscosity of a relativistic fluid, and what we observe at RHIC is matter that is at least very close to that lower limit. We don't know exactly how close, just because the experimental and theoretical uncertainties are still large enough that all we can say is that it's probably no more than a factor of three above this quantum limit. But that's already closer than any other liquid that's ever been studied. So string theorists are very excited, because they finally have the option of maybe making a connection to experimental measurements from something they know how to predict.
Is it possible to say, without getting too technical, how this prediction works?
They actually predicted a mathematical kinship between the behavior in a strongly interacting system like in this RHIC matter and a black hole. It's a connection with black hole physics that was completely unexpected. This was done by a bunch of theorists working primarily with Dam Thanh Son at the University of Washington.
What are the biggest uncertainties remaining about the quark-gluon matter controversy and whether or not that is indeed what you've created at RHIC?
Relativistic Heavy Ion Collider
(RHIC)
Courtesy of Brookhaven National
Laboratory.
A view of the superconducting magnets at Brookhaven's Relativistic Heavy
Ion Collider. As gold particles zip along the collider's 2.4 mile long
tunnel at nearly the speed of light, 1,740 of these magnets guide and focus
the particle beams.
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The biggest uncertainty right now is in what physicists would call the degrees of freedom. What we mean by that is what is actually interacting inside this liquid. The reason we ask the question is that we know fundamentally that quark and gluon interactions are covered by QCD, and QCD does predict that the interactions should be relatively weak. But to explain this nearly frictionless liquid behavior, we need strong interactions.
It leads to the expectation that we're dealing with some sort of quasi-particles, perhaps clusters of quarks and gluons. We don't really know exactly what those quasi-particles are, what the effective degrees of freedom in the matter are.
A related question to that is the fact that we get this very strongly correlated liquid at RHIC. Is that just an accident? The LHC at CERN is just now starting its first run for heavy-ion collisions.
So it's an open question whether the matter created at the LHC will also be an essentially frictionless liquid or will it change a lot. There are QCD predictions that it will move toward the ideal gas limit.
So the question is how does matter behave when we make it still hotter than occurs at RHIC? Then what I mentioned before about these symmetry-violating bubbles is still an open question. The data are consistent with predictions, but we have to rule out alternative, more mundane explanations.
Another question we still have is about something we originally thought we'd see at RHIC—matter that has gone through what we call a first-order phase transition, an abrupt discontinuous transition. This is what happens, for instance, when water makes the transition to steam. It's actually discontinuous in the sense that when you dump more energy in, the temperature stays constant—at the boiling temperature—until all of the water is converted to steam, and only then does it start to increase.
We now know that what we observe is not this first-order phase transition. What we observe is rapid but not discontinuous. There are theoretical reasons to believe that at higher net densities of quarks minus antiquarks, and at lower temperatures, we'd see an abrupt transition. We're searching for that right now. Whether a "critical" point that separates continuous from discontinuous transitions exists or not is up for grabs.
Brookhaven From The Air
Courtesy of Brookhaven National
Laboratory.
Aerial view of Brookhaven National Laboratory taken in August 2007. The
Relativistic Heavy Ion Collider (top, center) is 2.4 miles in
circumference, and dominates Brookhaven's 5,265-acre campus.
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with Brookhaven National Labs >
We've seen another very interesting phenomenon here. In high-energy physics, if you scatter two quarks you tend to see back-to-back jets of particles coming out. We don't see that at RHIC. If we see one jet, the oppositely directed jet seems to be suppressed. But what we do see, if we look in detail, is what looks reminiscent of a water wake, what a boat makes when it goes through water. This is very similar to a sonic boom. And we don't know if that's the right analogy for what we're observing, but, if it is, it means we can effectively measure the speed of sound in this QCD matter, and that will help us pin down what we call the equation of state—how the pressure of matter varies with energy density.
And a last remaining uncertainty is that, everything we observe is consistent with the matter reaching thermal equilibrium in an incredibly short time—in less time than it would take light to travel across a nucleus. The initial energy of motion of the two light-speed nuclei gets converted to heat on that time scale. We don't understand the mechanism by which that happens so rapidly, and there are speculations that it is related to a phenomenon we call gluon saturation.
What that represents is the idea that as we look with higher and higher resolution microscopes inside a proton we see more and more gluons, and as far as we've looked so far, the density of gluons is growing continuously. But it can't grow forever. That would violate the fundamental laws of physics. At some point, it must stop growing. It has to saturate.
So the speculation is that when these two ions collide at near light speed, we're seeing this saturated density of gluons in one nucleus collide with a saturated density of gluons in the other. This would be a really unique manifestation of QCD in matter. It would be unique and also universal. It's predicted to be there in all matter that contains quarks and gluons. It may be related to how the masses of protons are actually determined by nature. Those are all things that are associated with these collisions involving heavy nuclei.
RHIC has also collided polarized protons together. What is the goal of that research and what have you learned?
There the main unanswered question is where the spin of the proton comes from. That's been a mystery for the last 20 years and after some relevant measurements at RHIC, it's still a mystery. We know where it's not coming from, but we can't put together a model that allows us to say where it does come from.