Virtual Journal on QCD Matter

The 20th International Conference on Ultra-Relativistic Nucleus Nucleus Collisions, Quark Matter 2008, took place February 4-10 in Jaipur, India. The Quark Matter conference is the biggest 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 results. However, due to the recent high frequency of Quark Matter conferences and restrictions on available beam time at the experimental facilities, the amount of new data presented at the last two QM conferences was somewhat less than at the preceding ones.

Among the hottest topics of the conference was The Ridge: Measurements by the STAR and PHENIX collaborations have shown that high pT hadrons are accompanied on the near side by a wide ridge (or pedestal) of secondary hadrons, which extends along the beam direction over significantly more than one unit of rapidity in both directions. The usual cone-shaped pattern of secondary hadrons, which closely resembles the jet cone around a hard scattered parton in nucleon-nucleon collisions, sits on top of the ridge.

The ridge made its first preliminary appearance at Quark Matter 2005 in Budapest (discussed by J. Putschke), and by now has been observed by STAR, PHENIX and PHOBOS. It is currently being studied as a function of centrality, reaction-plane, transverse momentum and hadro-chemistry, was featured in all plenary overview talks and had an entire parallel session (Session# VIII) devoted to it. A variety of theoretical models have been suggested to explain the ridge and its features (see e.g. our post relating it to the presence of turbulent color fields) – the high quality data presented at Quark Matter will hopefully allow for verification or falsification of most of these.

Other noteworthy talks and sessions at QM 2008 were:

• Bedanga Mohanty‘s overview talk on high pT physics in STAR highlighted the richness of phenomena associated with jet-medium interactions.
• Krishna Rajagopal gave a very insightful plenary talk on the application of AdS/CFT to strongly coupled QCD matter. The range of applications of AdS/CFT to QCD matter has increased tremendously over the past year and had an entire parallel session devoted to it at this conference (session IV). Note that N=4 SYM is not QCD and a lot of open questions the applicability of the AdS/CFT correspondence to QCD remain.
• Sanja Damjanovic presented very impressive results of the NA60 Collaboration on the rho spectral function and thermal dileptons at the SPS. In particular noteworthy is the measurement of radial flow of late-stage rho mesons (measured via their di-muon decay) in concert with indications of higher mass di-muons exhibiting far less radial flow, which can be attributed to a partonic origin (via quark-antiquark annihilation).
• Dinesh Srivastava gave a very comprehensive overview on Direct Photon production in Relativistic Heavy-Ion Collisions, describing the tremendous progress the field has made over the past decade.
• The session on viscous fluid dynamics (parallel session XVI) revealed some promising progress among the various developers of viscous hydro models: while the discussion on which dissipative terms need to be included in the viscous hydro equations is still ongoing, it was revealed that the Romatschke and Heinz/Song implementations yield the same results for elliptic flow if the same terms are used, and that Molnar’s microscopic simulation of viscous hydro matches well on Romatschke’s results. The apparent convergence of these different implementations is encouraging and points towards significant progress in the field. Note, however, that the best viscous hydro implementations are still 2+1D.
• Brian Cole gave highly noteworthy (and slightly controversial) summary talk on Hard Probes. He highlighted the need for a comprehensive and quantitative theory analysis which treats all aspects of high pT and bulk physics consistently: i.e. the same sophisticated evolution model used to describe hadron spectra and yields (e.g. viscous hydro or hybrid hydro+micro approaches) needs to be utilized as medium for state-of-the-art jet energy-loss calculations, high-pT multi-particle correlations as well as the medium response to hard probe (e.g. Ridge and Mach Cone) and heavy-quark diffusion. This theory analysis should go hand in hand with the experimental effort to yield quantitative results.

If you missed out on Jaipur, you may click here for some photos.

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