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 Quantifying the Properties of Hot QCD Matter
 (INT Program 10-2a: May 24 - July 16, 2010)

  Reported by Brian Cole, Ulrich Heinz, Peter Jacobs, Yuri Kovchegov, Berndt Mueller, and Jamie Nagle
  Date posted September 16, 2010

The program on Quantifying the Properties of Hot QCD Matter was held at the INT from May 24 to July 16, 2010. The goal of the program was to bring theorists and experimentalists together to assess the results of the experimental heavy ion physics program at RHIC in the light of state-of-the-art models of relativistic heavy ion collisions, the dynamics of hot QCD matter, and of the interactions of hard probes with the hot medium. The timing of the program also permitted a first discussion of the results of measurements made in proton-proton collisions at the LHC. The program succeeded well in achieving this goal bringing together more than 107 experimentalists and theorists to discuss and work together during the eight weeks of the program.

Following an initial one-week workshop with special emphasis on the first results from the LHC, the program was organized into six categories: initial conditons, relativistic hydrodynamics and bulk dynamics of hot QCD matter, the physics of jet quenching, theories of strongly coupled gauge plasmas, heavy flavor physics and quarkonium in hot QCD matter, and electromagnetic probes.

  • Start-up workshop: The experimental speakers provided overviews of the latest results from the RHIC heavy ion program, presented first results from the p+p run at the LHC, and reviewed the upgrade plans of the RHIC detectors. The theory talks covered current research highlights from hard probes of hot QCD matter to event-by-event fluctuations.

  • Initial conditions: The two major exciting developments were progress in the theory of saturated gluon matter, where running coupling and next-to-leading order calculations are becoming available, and the rapidly expanding insight into the role of initial state density fluctuations and long-range rapidity correlations in novel final-state effects such as the "ridge".

  • Bulk dynamics: Viscous relativistic hydrodynamics coupled to final state hadronic Boltzmann transport are rapidly becoming the state of the art for bulk matter dynamics modeling. The combination of high-quality modeling and high-statistics data is expected to soon permit reliable extraction of bulk matter properties (equation of state, shear viscosity, etc.). Lattice simulations of bulk properties of hot QCD matter have made remarkable progress over the past few years; the news here was that the results from the major groups are finally converging.

  • Jet quenching: The great excitement in this area revolves around the efforts aimed at full jet reconstruction in heavy-ion collisions at RHIC. The week allowed for a healthy discussions among members of different experimental collaborations about the strengths and weaknesses of the currently employed algorithms. On the theory side, progress is being made on Monte-Carlo jet quenching schemes as well as on calculations of coincidence observables.

  • Strongly coupled plasmas: Gauge-gravity duality based methods have revolutionized the theory of strongly coupled gauge theory plasmas. New approaches to thermalization, jet quenching, and hydrodynamic expansion of strongly coupled quark-gluon plasmas are being explored and much insight beyond the perturbative picture is being gained. The search for a realistic quantitative connection to QCD continues.

  • Heavy flavor probes: The reason for the unexpectedly large suppression of the yield of mesons with open heavy flavor at medium to high momentum at RHIC is still not well understood. The complex phenomenology of charmonium suppression remains a challenge for theorists; a comprehensive explanation of the data from SPS, RHIC, and Fermilab is still elusive. Impressive progress is being made on formulating an effecting theory of charmonium at finite temperature.

  • Electromagnetic probes: The thermal photon measurement, by way of low-mass electron pairs, of PHENIX garnered considerable attention, because it provides the first direct glimpse of the hot QCD matter created in Au+Au collisions at RHIC. State-of-the-art calculations confirm that a fraction of the observed photons are emitted from the expanding quark-gluon plasma. While the NA60 data provide clear evidence for broadening of the rho-meson, the PHENIX dilepton data in the mass range below the rho-meson remain unexplained. The analysis of the data from the hadron-blind detector, which were taken recently, is eagerly awaited.