### Quark Matter 2009 Highlights

The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, colloquially referred to as Quark Matter 2009, took place in Knoxville, Tennessee from March 30th to April 4th, 2009. The Quark Matter conference is the largest and most important conference in the field of Relativistic Heavy-Ion Collisions and traditionally the experimental collaborations reserve it as the forum to present their latest and most important data. Results shown at this particular meeting were clearly indicative of the field making a transition from the discovery phase to the exploratory phase. This conference was not so much about big surprises and novel phenomena, but about qualitative progress, both in data and theoretical understanding. This theme was emphasized by a series of excellent overview talks:

- William Zajc gave a noteworthy overview on the current status of the field and a preview on what to expect in the near and intermediate term future.
- Carla Vale in her PHENIX overview talk nicely managed to tie the latest lineup of PHENIX results to the currently most prevalent physics questions and challenges.
- Jörn Putschke in his STAR overview talk showed how the new success of full jet reconstruction extends the reach of hard probes accessible to the RHIC experiments.
- Scott Pratt provided a detailed account of years of painstaking work by many theorists (including his own) which contributed to a comprehensive understanding of the
*RHIC HBT Puzzle*, which for all practical purposes can now be considered resolved. - Peter Petreczky gave a comprehensive review on the current state of the art regarding Lattice QCD at finite temperature. This talk provides for one-stop shopping for anyone interested in getting up-to-date on the current status of Lattice EoS calculations.
- Thorsten Renk presented an insightful discussion on jet-tomography and jet-medium correlations.
- Paul Chesler explained nicely to the general audience how to calculate jet properties in AdS/CFT models of strongly coupled gauge theories.
- Paul Romatschke gave a great introduction into viscous relaticistic hydrodynamics.
- Agnes Mocsy impressed with her highly didactic presentation on quarkonium spectral functions.
- Chihiro Sasaki gave a comprehensive survey of the complexity of, and the present limits of our definite knowledge about, the complete QCD phase diagram.
- Abhijit Majumder provided an oustanding summary of the status and progress in the field of jet-medium interactions in his rapporteuer talk.
- Likewise impressive was Bedanga Mohanty‘s rapporteur talk on the QCD phase diagram, phase transition and fluctuations.

Among the noteworthy new developments of the conference was that of quantitative and comprehensive theory-data comparisons: models need to confront the data (which is now available in impressive detail and precision) and must be verifiable or falsifiable. A noteworthy example for this was Jamie Nagle‘s “Grim Reaper” talk in which he addressed (and debunked) a variety of models on the formation of the “ridge”. The TECHQM initiative can be generally considered as a leader on that front.

Probably the most exciting “new” topic of the conference was the the progress in full jet reconstruction analysis (see plenary talk by Sevil Salur and parallel talk by Mateusz Ploskon) and full jet energy loss MC simulations. Several MC codes are currently being developed for the description of full jet energy-loss in the medium:

- JEWEL (Jet Evolution With Energy Loss) by K. Zapp et al.
- Q-PYTHIA / Q-HERWIG by N. Armesto et al.
- YaJEM (Yet another Jet Energy-loss Model) by T. Renk
- MARTINI (Modular Algorithm for Realistic Treatment of heavy IoN Interactions) by the McGill group (S. Jeon et al.)
- PYQUEN/HYDJET by Lokhtin et al.

While the full jet evolution MC field is still in its infancy, this is definitely a development to watch out for, and given the recent advances on the experimental side, the timing is perfect. Particularly noteworthy was Zapp’s talk on the implementation of the LPM effect – an issue which has bugged the theory community for quite a while.

In the transport sector, viscous hydrodynamics has certainly come of age and there is impressive agreement among the different codes. The current effort is now shifting from understanding shear viscous effects to bulk viscous effects (see e.g. talks by H. Song and A. Monnai). Another important item on the vRFD to-do list is the incorporation of a temperature-dependent shear-viscosity to entropy-density ratio (see talk by N. Demir). On the side of microscopic transport, it is now possible to verify microscopic transport algorithms by comparing them to analytical hydrodynamic solutions (see talks by D. Molnar and I. Bouras), which should greatly improve the confidence in these microscopic descriptions and their application to off-equilibrium phenomena outside the reach of (viscous) hydrodynamics.

By far the most frequently shown figure / most cited result of the conference was figure #5 of “Systematic Comparison of Jet Energy-Loss Schemes in a realistic hydrodynamic medium” followed by figure #8 of “Conformal Relativistic Viscous Hydrodynamics: Applications to RHIC results at s(NN)**(1/2) = 200-GeV”

Some of the most heated discussions of the meeting were generated by the Knudsen number *K*. The Knudsen number is the ratio of the mean free path to the system size and is a useful tool to characterize the medium in transport calculations, e.g. in viscous hydrodynamics. Examples for its application in exploring the scaling behavior of model calculations could be found in talks by C. Gombeaud and R. Snellings. It was pointed out, however, that there is a risk of pushing the envelope too far, in particular with phenomenological Knudsen-Number fits to data, e.g. on elliptic flow, which then relate an extracted Knudsen Number to the shear viscosity-over-entropy density ratio. In general, both quantities in this ratio vary strongly over the course of a heavy-ion reaction. Assuming that a Knudsen number fit to final-state data may be indicative of its value at a well-defined instant in the evolution (e.g. at the formation time of the QGP), is most likely strongly model-dependent and not a reliable method to extract physics from the data. Both, STAR and PHENIX, have made uncontrolled use of the Knudsen number fit method – the intense discussions generated on this topic will be beneficial to putting such kinds of analysis into a realistic perspective and hopefully lead to improvements in our understanding of how to extract transport coefficients from data.