Wit Busza Interivew - Special Topic of Hadron Colliders
Special Topic of Hadron Colliders Interview, April 2010
Is this what you're discussing in your 2001 Physical Review Letters article on charged-particle pseudorapidity density distributions, which is the second most highly-cited article from PHOBOS (Back BB, et al., Phys. Rev. Lett. 87: 102303)?
Yes, one of the things we were able to do with PHOBOS, and what we focused on, was to get the big picture. How much energy density is created in these collisions? After the explosion occurs, you look at the fragments, and we had the most complete coverage for seeing all the fragments. As the accelerator went to higher and higher energies, we always made those measurements because our detector was most suited to this kind of work.
We have a number of papers that basically state, "In the collisions of gold and gold, this is what all the fragments from the collisions are doing, in the collisions of copper and copper, this is what they're doing, etc." And we found certain regularities that to the present day are not understood.
From that first era, the first five years, our biggest contribution was the study of the total energy density produced and the regularities seen in the overall production of fragments. And also we answered the question, "When one fragment does one thing, what do the other fragments do?" Many of our other measurements were done by all four experiments. You probably heard of jet quenching, for instance. We also observed and studied that but we were not the first, and that wasn't our greatest strength.
What would you consider the most surprising or interesting results to come out of RHIC, in total, not just PHOBOS?
PHOBOS Detector
Courtesy of Brookhaven National
Laboratory.
Working on the paddle trigger counters of the PHOBOS detector.
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Now if I speak about all experiments together, then the most important result has to be the so-called flow. You'll hear this from all of the experiments, because everyone soon realized that the matter created in these collisions is flowing like a liquid.
Could you explain to us what you mean by the flow of matter?
Let's look at two extreme possibilities. One is you can imagine these nuclei colliding and all that happens is the individual protons and neutrons hit each other, as if nothing else existed around them. In these collisions, you would see nothing other than the same physics we see in the collisions of two nucleons, except that there would be many such collisions occurring at the same time. That would be boring. It would be as if we were studying nucleon-nucleon collisions many times over.
Fortunately for everybody, that did not happen. And the simplest way we could see that it didn't happen is that we saw a collective behavior of the particles produced in these collisions. The particles don't behave independently of each other, but seem to form some kind of a system, which behaves like an expanding gas or a liquid drop—some kind of expanding fluid. If you distort it, for instance, you would see the same kind of pattern that you would see if you distort a liquid. That is what I mean by "flow of matter."
To change gears, maybe the following would help give a big picture of what we were studying. Let me give you the analogy that I always give to undergraduates. There are two classes of interactions that, together with gravity, describe the world around us. One is quantum electrodynamics, and the other is the strong interaction, quantum chromodynamics.
Now in quantum electrodynamics, there are very few ingredients. There are the positive charges on the nuclei and the negative charges, which are on the electrons, and they interact with each other. Yet the result of this is all the things you might see, for instance, if you look around your room. It's amazing. You have atoms, molecules, your brain, the walls, the gasses, etc. All that follows essentially from quantum electrodynamics, all this tremendous variety!
Now if you look at the consequences of the strong interaction, it seems to be boring. There appears to be a lack of any variety. There is a proton and a neutron, which then combine to form nuclei. End of story. Yet quantum chromodynamics should be much more interesting. The ingredients are much richer. There are these so-called color charges, which come in three forms, but which can make up combinations all the way up to eight colors and white. And the interactions between the quarks are mediated by gluons which themselves are colored. And there are many quarks. There's a lot of variety. So you can ask, "Why there is not more variety in the room due to the strong interactions?"
PHOBOS Detector
Courtesy of Brookhaven National
Laboratory.
The PHOBOS detector is designed to examine and analyze a very large number
of unselected gold ion collisions. For each collision, the detector gives a
global picture of the consequences of the collision and detailed
information about a small subset of the nuclear fragments ejected from the
high energy-density region.
The interview series
with Brookhaven National Labs >
And the answer is very simple. Here's an analogy: imagine that you were in outer space and there was one electron and one proton every cubic meter. What a boring universe that would be. There would be no solids, essentially nothing you have in your room. Just one electron and proton per cubic meter. The reason why it is so interesting in your room is that you have just the right density of electrons and nuclei to be able to make all these different materials.
For quantum chromodynamics, your room is like outer space. It is boring. So the question is this: if we manage to produce a higher density of this stuff, do we start getting interesting things? And the answer we now know is yes, we do. And we are lucky that with the accelerators available, like RHIC and now the LHC, we can produce a density of this stuff that is interesting.
So what interesting thing did you see?
One of the first interesting things we saw was that if you produce this higher energy density in these collisions, it is high enough to start producing a system that has collective properties. That's the most exciting thing that has come out of this work. Nuclei colliding at high energy are not a bust. In fact, yes, in these collisions we are beginning to reach a density of energy that has very interesting properties, for example fluid like, that flows.
What are you hoping to learn from the upcoming heavy ion run at the LHC?
Another way to ask the question is what will be the difference between the collisions at RHIC and at the LHC? Well, at the LHC we will have much higher energy in the collisions, so presumably we will create a hotter system that will last for a longer time and produce bigger volumes of this hot matter. At the same time, because of the higher energy, many of the particles that emerge, that are particularly good for studying the properties of this material, will be produced in larger numbers. For example the J/Psi, Upsilon, and Z particles will be produced in large numbers.
So we will have a bigger volume, hotter material, and very powerful probes for studying the properties of this material. That's one very important aspect of that program.