Barbara Jacak Talks About Brookhaven National Lab's PHENIX Experiment
Special Topic of Hadron Colliders Interview, March 2010
One of things I found so interesting about what we're discovering at RHIC is the connection to other fields—the connection of high-energy nuclear physics and particle physics is not surprising. But the connection to string theory is quite surprising to us, and the connection to atomic physics and ultimately maybe condensed matter physics was really something I did not expect.
Other than your review paper in Nuclear Physics A in 2005, the most-cited of the PHENIX publications is a 2002 article in Physical Review Letters on the suppression of hadrons with large transverse momentum (Adcox K, et al., 88: 2002). Is this the opacity issue, you're reporting?
Yes, that was our very first observation of this incredible opacity of the plasma to fast quarks and gluons. We measured pions—neutral ones in the calorimeter and charged ones in our tracking system—and we did the measurement in gold-gold collisions and in proton-proton collisions. When we took the ratio of what we saw in gold-gold over what we expected from proton-proton, we discovered we had a fifth as many high-momentum pions as we should have.
Were all four experiments seeing the same thing and publishing simultaneously?
That one was PHENIX publishing first. At that time, only PHENIX and STAR could observe what we were seeing. The two smaller experiments didn't have the capabilities to look at high-momentum in the right way. They eventually confirmed this, but at the time they didn't have the statistics to do the right measurement.
When you realized what you had seen, were you then trying to beat the STAR collaboration into print?
PHENIX Detector
Courtesy of Brookhaven National
Laboratory.
A computer image generated from data collected at the PHENIX detector
during RHIC's second run cycle. Reconstructed tracks (in green) point
towards the location of the collisions. The beam path is shown in
red.
The interview series
with Brookhaven National Labs >
You know how this goes. It's a competition of course, but a friendly one. So when one experiment finds something weird, the first thing you do is find out if the other guys see it too. So in the first quark matter conference after RHIC started up, we presented these results, but we had already talked in the back hallways with the STAR people, saying, "We see something weird," and they said, "Gee, so do we." So we presented the preliminary results at the conference, and then we published and then STAR published not that long after. STAR published the results on the hydrodynamic flow first, and PHENIX published not long after.
How do the RHIC results inform and relate to what is likely to be found in the heavy ion collisions at the LHC?
Well, we found this strongly coupled quark-gluon plasma here at RHIC. Now the obvious question is what's going to happen at the LHC. As best as we can tell, the LHC should reach temperatures maybe twice what we have at RHIC. If you look at the predictions from quantum chromodynamics, you find out that maybe the quark-gluon plasma at the LHC should be less strongly coupled.
But, even if it starts out less strongly coupled it has to cool also, and this will take the system back down through the critical temperature and the phase transition back to hadrons where the quarks and gluons are no longer free. So the LHC should have a phase that's not as strongly coupled and, of course, everybody is dying to find out whether this is going to be the same thing we see or is it going to be different.
One of the things we've been doing at RHIC is starting to get quantitative. We know that matter flows very freely, but we would like to actually put a number from experiment to the viscosity-to-entropy ratio. We'd like to pin down the mechanism of interaction of the fast quarks and gluons with the plasma. How does the energy loss proceed? We thought it would be all gluon radiation, very much as charged particles going through matter radiate photons as bremsstrahlung radiation. However, that doesn't work for charmed quarks. That mechanism doesn't seem to be enough to explain what we see.
It turns out, though, that RHIC is a very good place to measure what's going on by picking out the jets, using the planned detector upgrades to really quantify the energy loss of different quarks. We want to look at the energy loss of heavy quarks in two ways—one is with our new silicon detectors that can separate charm and bottom quarks. Since the bottom quark is really heavy, it has to lose less energy. And we have some results that suggest that the bottom does indeed lose less energy.
PHENIX Detector
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.
The interview series
with Brookhaven National Labs >
We'll do the definitive measurement in the next year if everything works out right. For more details of the energy loss mechanism, we actually need to measure jets carefully, to reconstruct them, and we also need to look at jets from charmed quarks. So we have a plan for upgrading PHENIX to be able to do that.
What are you doing in the upgrade and what's the schedule?
The silicon upgrade to separate charm and bottom quarks is being installed, literally, now, as we speak. The bigger upgrade for picking apart jets and reconstructing jets from charmed quarks is something we'd like to start building in the next few years and install five years from now.
How many of the questions you're hoping to answer now with the new equipment are questions you might have asked 10 years ago before RHIC got going?
None of the questions we're asking now are questions we thought about before RHIC started! For example, we do know from comparing hydrodynamics to RHIC data that the plasma comes into equilibrium very quickly, but we really don't know how. And nobody predicted that the equilibration would be that fast. We know it happens quickly, otherwise we wouldn't see the large asymmetries we observe in the collective flows.
Another question that would never have occurred to us 10 years ago is what exactly is in the plasma? If it's strongly coupled, does it have quasi-particles? The string theory guys would say no, it's just a field. But those of us who think of quarks and gluons as actually existing as individual particles, we would say well, it's probably composite stuff in there, like multiple particle clumps.
So we're trying to figure out how we can answer those questions experimentally, and we think it can be done. Now that we have the combination of experiments at much higher energy at the LHC together with the experiments at RHIC, where we can change the beam energy, change the size of the colliding system, and look very carefully at particles that come from jets. That combination will allow us to answer the exciting questions.
Dr. Barbara Jacak
Distinguished Professor, Physics & Astronomy
Stony Brook University
Stony Brook, NY USA
and
Spokesperson, RHIC PHENIX collaboration
Brookhaven National Laboratory
Upton, NY, USA
BROOKHAVEN NATIONAL LAB'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
- Yao WM, et al., "Review of particle physics," J. Phys. G-Nucl. Particle Phys. 33(1): 1-+ Sp. Iss. SI July 2006 with 2,980 cites.
- Adcox K, et al., "Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX Collaboration," Nucl. Phys. A 757 (1-2): 184-283, 8 August 2005 with 696 cites.
Source: Essential Science Indicators from Clarivate.
ADDITIONAL INFORMATION:
-
Menu for the institutional interview series with Brookhaven National Lab from the Special Topic of Hadron Colliders.
KEYWORDS: BROOKHAVEN NATIONAL LAB, NUCLEAR PHYSICS, PARTICLE PHYSICS, RHIC, RELATIVISTIC HEAVY ION COLLIDER, PHENIX, QUANTUM-GLUON PLASMA, PROBES, INITIAL-STATE INFORMATION, PHOTONS, ELECTRON PAIRS, MUONS, PIONEERING HADRON ELECTRON NUCLEAR INTERACTION EXPERIMENT, QUARK-GLUON PLASMA, BACKGROUNDS, DECAYS, PARTICLE IDENTIFICATION, SELECTIVE TRIGGER, HIGH RATE CAPABILITY, SPECTROMETERS, COLLECTIVE FLOW PATTERNS, OPACITY, JET QUENCHING, STRONGLY COUPLED PLASMAS, J/Psi SUPPRESSION, STRING THEORY, CHARMED QUARKS, ATOMIC PHYSICS, CONDENSED MATTER PHYSICS, PIONS, GOLD-GOLD COLLISIONS, PROTON-PROTON COLLISIONS, SILICON UPGRADE, JETS.