Wit Busza Interivew - Special Topic of Hadron Colliders

Special Topic of Hadron Colliders Interview, April 2010

Photo Courtesy of MIT

Photo courtesy of MIT.

According to our Special Topics analysis of hadron colliders research over the past decade, the work coming out of Brookhaven National Laboratory (BNL) ranks at #1 by total cites, #2 by number of papers, and #20 by cites per paper, based on 1,529 papers cited a total of 27,320 times.

Among the 704 institutions comprising the top 1% in the field of Physics in Essential Science IndicatorsSM from Thomson Reuters, BNL ranks at #20. Their current record in this field includes 4,175 papers cited 100,493 times between January 1, 2000 and December 31, 2010.

ScienceWatch.com has conducted a series of interviews with representatives of the various projects at BNL that have contributed to its citation record, particularly with regard to hadron collider research. This interview relates to the PHOBOS experiment. Among BNL's papers in our Special Topic, 62 papers with 1,052 cites dealt with PHOBOS in some way; one of these papers is ranked at #4 on the 10-year paper list.

In this interview, correspondent Gary Taubes talks with Dr. Wit Busza about the PHOBOS experiment and its role in hadron collider research.


SW:  What was the initial conception behind PHOBOS and how did you hope to carry it out?

When I heard that RHIC was going to be constructed in the 1980s, I did some semi-analytical work trying to see what one could learn from it, and convinced myself that interesting systems could be formed in these collisions. So I dove in and proposed an experiment at RHIC, even though I didn't really have a good idea of what might be the most interesting things to actually measure. It was a relatively cheap experiment—under 10 million dollars compared to close to 100 million for the big detectors.

The idea was to have a quick first look and, based on that look, we could then decide what to do next. And this has worked out magnificently. We worked at Brookhaven for five years with PHOBOS, learned a fair amount, and now my group has moved to CERN, to the LHC, where we will shortly be looking at collisions of lead on lead at unprecedented energy. We're part of the CMS (Compact Muon Solenoid) detector. So that's the brief history of how I got involved and how PHOBOS came to be.

SW:  How did you come about calling it PHOBOS?

Well, we first proposed a slightly more expensive experiment called the Modular Array for RHIC Spectra, or MARS. That was considered too expensive, so we came up with a reduced version and one of my colleagues at MIT said that, since Mars was too expensive, why not build the moon of Mars, which is Phobos. And if that was still considered too expensive, we figured we'd come up with Deimos, a still smaller moon of Mars. So that's the origin of the name. We were interested, like everybody else, in what kind of matter can exist at extremely high energy or matter density, comparable to that which existed in the very early stages of the universe.

SW:  How did PHOBOS differ from the other three RHIC experiments?

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.
PHOBOS Detector
Courtesy of Brookhaven National Laboratory.

The PHOBOS experiment at Brookhaven National Laboratory's Relativistic Heavy Ion Collider is based on the premise that interesting collisions are rare, but when they occur, new physics is readily identified.

The interview series with Brookhaven National Labs  >

It was orthogonal. Very crudely, the philosophy of PHOBOS was to look at all the charged particles (or fragments)—over the full four-pi solid angle—and then over a very narrow region of phase space have a high-quality detector like a telescope looking directly into the hottest part of the explosion. Where we differed from the others is that we were the only experiment looking at everything, although we were the smallest and the cheapest. It was complementary to the other ones.

One other thing about this detector—by the uncertainty principle, if one produces a new form of matter over a big volume, then one might produce lots of slow particles, particles with a big wavelength comparable to the source. So one of our specialties was to look at the very slow particles. We hoped we would see a large excess of these, which would give us a hint of how big this fireball is and how it blows up. But we didn't. Nature worked against us. What we found was that there was no evidence of an increase of slow particles. I find it surprising still, but that is the case. The experiment worked, but the study of slow particles didn't lead to anything interesting.

SW:  Would you consider that the most surprising finding to come out of PHOBOS?

No, but it was the most disappointing for me. If it had turned out to be true, PHOBOS would have been the only one of the four experiments designed to study it. As for the most interesting or surprising results from PHOBOS, there were a few of them. The first was that the overall number of particles produced was smaller than everybody expected.

Here I have to brag a little bit. We got the first results from RHIC and PHOBOS was first to publish. Within a few days of the first collisions, we had an interesting result and six weeks later we published it. This was sort of unprecedented in the field. That result showed that the number of particles—related to the energy density—was smaller than people had expected.

Following that, the most important results, as far as I'm concerned, are all sorts of measurements which show how that system as a whole evolves. Because we covered the full solid angle, we had insight into the production of particles from all regions of that initial explosion.

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