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correspondent Gary Taubes talks with Dr. Flemming Videbaek about the BRAHMS experiment and its particular contributions to hadron collider research.

 When did you join the BRAHMS experiment?

I've been here at Brookhaven since 1990, when the construction of the Relativistic Heavy Ion Collider (RHIC) was just getting started. I soon got involved in preparing the original proposal for BRAHMS with the heavy ion research group that at the time was led by Ole Hansen, and have been the spokesperson since then.

 What does BRAHMS stand for, and can you tell us about the history of the experiment and what sets BRAHMS apart from the other three RHIC detectors?

BRAHMS stands for Broad Range Hadron Magnetic Spectrometers. When we were first getting ready to propose an experiment for RHIC, we had also talked with the physicists who were forming the STAR experiment—basically a group from Berkeley—and we decided to go our own way. The experiment we had worked on before at the Alternating Gradient Synchrotron (AGS) at Brookhaven was on the order of 50 or 60 people, and we felt this was a comfortable size for an experimental group. The idea, more or less, was to use some of the techniques we used at the AGS but scale them up for RHIC.

Initially we wanted to look mainly at particle production and a broad range of angles. STAR and PHENIX, initially, measured particles around 90 degrees—or perpendicular to the direction of the colliding beams—with a large solid angle, near what is called mid-rapidity.

STAR Detector
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.

The Solenoidal Tracker at RHIC (STAR) is a detector which specializes in tracking the thousands of particles produced by each ion collision at RHIC. Weighing 1,200 tons and as large as a house, STAR is a massive detector. It is used to search for signatures of the form of matter that RHIC was designed to create: the quark-gluon plasma. It is also used to investigate the behavior of matter at high energy densities by making measurements over a large area.

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What we wanted to explore were measurements that would see how this particle production looked in detail when we moved from 90 degrees to as far forward angles as we possibly could. So the experiment was designed to measure down to about 2 degrees from the beamline

In order to do this within the constraints of the funding, we had to build a small solid-angle device. Think of a long telescope consisting of multiple magnets and detectors for tracking particles and determining what kind of species they are—protons, pions, kaons, etc.

 What physics did you hope to uncover looking at these angles so near the beamline?

We wanted to look at what we called the stopping of the incident nucleons. When the heavy ions are colliding, part of the initial kinetic energy they have is transformed to particle production and this mainly shows up in mid-rapidity and also creates this hot dense matter that we've been studying now for many years at RHIC. But this energy comes from the initial energy of the protons and neutrons, and we wanted to see where these initial protons and neutrons end up and to measure how much of the initial energy they lose in the collisions.

In fact, one of the first people to look at this and actually predict it, based on measurements of protons at low energy, was Wit Busza, the spokesperson of RHIC's PHOBOS detector collaboration. Before the time we were proposing BRAHMS, back in 1984, he actually looked at this and made some rough predictions. So one of the important questions in the field was how this manifested itself, but that's still only been a small piece of the totality of measurements that have been done by BRAHMS.

 And this was a different goal than trying to establish whether or not a quark-gluon plasma had been created, and what its characteristics were?

Yes, although in addition to this we would also have detector systems to sit near the center of mass, at the mid-rapidity, and study the hot dense matter as well there, and to see how far forward the effects of the matter can be observed.

 Did the detector change much between the proposal and RHIC turning on?

Not too much. It took a long time before we actually got approved, and there was one addition, to also measure particles near 90 degrees, but the experiment as it was built and completed in 2000 was pretty much how we always envisioned it.

RHIC Tunnel
Courtesy of Brookhaven National Laboratory

Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) is really two accelerators in one — made of crisscrossing rings of superconducting magnets, enclosed in a tunnel 2.4 miles in circumference. In the two rings, beams of heavy ions are accelerated to nearly the speed of light in opposite directions, held in their orbits by powerful magnetic fields. Shown here is an area near the BRAHMS experiment.

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Some of the detector technology was different, in particular the use of small Time Projection Chambers for precision tracking, but the basic idea about the physics goals was in place when we started and when we began taking data 10 years later. Although this does not imply that all the things we measured were actually foreseen when we started—there were a number of surprises there.

 What were the surprises?

In 2003, RHIC had a run in which it collided deuterons on gold nuclei. The main idea was to see what happened when we looked at the production of particles at relatively high-transverse momentum, near the center of mass, and to see whether there was a suppression or not in the deuteron-gold collisions compared to what would be predicted from proton-proton collisions. In the gold-gold collisions, in the two runs before that, a suppression had been observed. This is one of the first surprises that came out of RHIC. So this was, in one sense, a control experiment.

In addition Larry McLerran and Dmitri Kharzeev, theorists at Brookhaven, had also proposed the idea that if we looked at deuteron-gold collisions, as we move away from the central region to the more forward region, we should actually see a suppression, which we can think of the deuterons as probing the nuclear wave function much deeper into the parameter space. This idea is what was called the color glass condensate. So their notion was that we should see this suppression at forward rapidity even though we didn't see it at mid-rapidity.

When we analyzed our data, we did in fact see such a suppression. That's what started this whole discussion about this color glass condensate idea, and it led to some additional experiments that STAR actually carried out later.

 Okay, obvious question: what is a color glass condensate?

It's a way of describing the nuclear wave function. If you look at the nuclear wave function in a framework where it's moving, then you can describe that piece of the wave function as consisting of a lot of gluons, and the heavier the system is, and the faster it moves, the greater the density of gluons in that framework is

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