What did we learn at Quark Matter 2009?
Edited by Peter Steinberg, BNL
The main auditorium at Quark Matter 2009
Quark Matter 2009, the 21st international conference on high energy heavy ion collisions, was held in Knoxville, Tennessee and ended just two weeks ago (March 30 - April 4). Although the conference covers just one subfield of nuclear physics (itself a subfield!), those attending often found themselves overwhelmed by the sheer variety of results and all of the questions raised about the nature of the collisions we make at CERN and BNL. For the benefit of RHIC News readers, the editors asked a group of RHIC-related scientists (primarily from the US, but not exclusively) the question:
"What was the most interesting thing you learned at Quark Matter 2009?"
RHIC News received about twenty replies, which was considered to be a sufficiently-large sample. The open-ended nature of the question, and the inevitable differences in people's free time and mental space, led to replies ranging from a few words to several pages, which made combining the results non-trivial. In the end, it was decided to simply tabulate the responses and count the number of times various topics were raised by the respondents, either in a supportive or critical way. Interest is interest, right?
Two topics in particular came up the most frequently in the replies we received:
- The application of jet reconstruction algorithms developed in high energy physics experiments is naturally difficult, due to the large fluctuations in the background multiplicity, and the expectation that jets should be modified by the creation of a strongly-interacting medium. And yet, the two large RHIC experiments each found a few brave souls undaunted by the data analysis and its related uncertainties, who showed the first results from their work at Quark Matter. The PHENIX group showed progress using their technique of "Gaussian filtering" for p+p and Cu+Cu collisions, with jets reaching out to 35 GeV. STAR used their large acceptance to applied two well-known jet algorithms (kT and anti-kT), unfolded the background fluctuations in Au+Au, and reported possible changes in jet shape in heavy ion collisions as a function of the jet energy. While all of those involved, and those replying to the the RHIC news survey, seem to feel that it is still too soon to make any strong physics conclusions from these first attempts, there is palpable excitement about this pioneering development in the field.
- Parity violation (or, as some call it, "topological fluctuations") are manifest as a different response of positive and negative charged particles to the shape of the collision zone in each event. The story is a long one, stemming from work done in the very early days in the RHIC program, but only now coming to fruition with the large data sets from a variety of collision systems and energies, particularly by STAR, as shown in a talk by Sergei Voloshin from Wayne State University. Rather than looking directly for parity (P) or charge-parity (CP) violation in small regions in each event, the focus now is looking for evidence that strong magnetic fields are generated in the passage of the two nuclei, pushing different charges in opposite directions. While no quantitative theoretical calculations exist for direct comparison to data, many were excited about the prospect of the measurements as a spur to future work in both theory and experiment.
While jets and parity violation seem to be the most sensational results, there was widespread enthusiasm about the continuing development of the theoretical tools needed to interpret the most striking data from RHIC: jet quenching and viscous hydrodynamics. The sustained work of the TECHQM collaboration on both of these topics was noted by many respondents, in particular the consistent application of various jet quenching codes with similar assumptions about the flow of the underlying medium. Others noted the progress shown in new measurements, like the correlation of high momentum photons with hadrons (from PHENIX & STAR), and the studies of jet suppression relative to the reaction plane (from PHENIX). These measurements should provide direct access to modified fragmentation in the presence of a strongly-coupled medium, and will be a stringent test for the theoretical calculations.
Finally, there was a small group who independently noted the recent analysis of J/Psi suppression by the NA60 experiment at CERN, which is directly relevant to similar measurements at RHIC by both PHENIX and STAR. The new measurements of proton-nucleus collisions by NA60 have now been incorporated into the models used to calculated "normal suppression", and find essentially no additional "anomalous" suppression in the Indium-Indium data. This is a striking result to some, which suggests that further developments may eventually lead to a disappearance of J/Psi suppression at CERN energies -- itself puzzling considering that the overall suppression of the J/Psi is similar at RHIC and the SPS.
Quite a few other issues were raised by the respondents, and
this selection is by no means complete. However, it is hoped
that gives some sense of what issues caught the imagination of
the RHIC community at Quark Matter, and perhaps a hint of what
awaits us in Annecy, France in 2011.
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
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from the smallest subatomic particles, to the largest stars.