The beautiful Asilomar resort, on the Pacific coast of the Monterey Peninsula in northern California, attracted 130 participants to the Second International Conference on Hard Probes of High Energy Nuclear Collisions, on 11–19 June 2006. The Hard Probes series brings together experimentalists and theorists to discuss perturbative quantum chromodynamics (pQCD) in the context of relativistic heavy-ion physics. Penetrating, hard probes provide essential tools for understanding the properties of the hot and dense QCD matter that is produced in nuclear collisions at the Super Proton Synchrotron (SPS), the Relativistic Heavy Ion Collider (RHIC) and, in the near future, at the Large Hadron Collider (LHC). The programme was divided into three areas: jets and high transverse-momentum (high pT) hadrons, heavy flavour and quarkonia, and photons and dileptons.
Jet quenching
Producing jets by the hard scattering of quarks and gluons from incoming projectile particles is the hard probe par excellence. One of the most striking early results at RHIC was the discovery that jets are quenched in hot QCD matter, providing a direct measurement of the parton number density and transport properties of the system that is produced. Since that initial discovery, RHIC experiments have extended their studies of jet quenching in many directions. The Pioneering High Energy Nuclear Interaction eXperiment (PHENIX) now measures strong suppression of pion production up to a pτ of 20 GeV, while observing that direct photons (which do not carry colour charge, in contrast to the jets generating the pions) are not suppressed, as Gabor David of Brookhaven National Laboratory (BNL) explained.
One of the highlights of the conference was the discussion of the unexpectedly large suppression of high-pτ D and B mesons, measured by PHENIX and by the Solenoidal Tracker At RHIC (STAR). These results challenge the robust QCD prediction that heavy quarks experience smaller radiative energy loss in matter than light quarks or gluons, as Carlos Salgado from the University of Rome “La Sapienza”, Magdalena Djordevic of Ohio State University, Che-Ming Ko of Texas A&M University and others described. Matteo Cacciari from the Université Pierre et Marie Curie reviewed the pQCD calculations of charm and bottom production at colliders and the implications for RHIC. In addition, in two exciting ad hoc night sessions, theorists debated vigorously the merits of various approaches to calculating radiative energy loss in QCD, while the experimentalists kept score.
New insights into jet quenching featured in the talk by Krishna Rajagopal of Massachusetts Institute of Technology (MIT). He presented a recent calculation of the jet quenching parameter q^ in string theory using the intriguing anti-de Sitter space/conformal field theory correspondence between strongly coupled QCD and weakly coupled gravity. Other non-static parameters of QCD-like hot matter can also be calculated in this approach, in particular the viscosity and heavy-quark diffusion coefficient, as discussed by Ed Shuryak of Stony Brook University (SUNY), Pavel Kovtun of the Kavli Institute for Theoretical Physics, and Urs Wiedemann from CERN. These new theoretical developments provide insight into dynamical properties of non-perturbative QCD that cannot be directly treated by either perturbative or lattice methods.
Another important focus of discussion was the modification of dijet azimuthal correlations in the medium. Thomas Peitzmann, of Utrecht University/NIKHEF, showed how STAR has put the back-to-back nature of dijets to good use, most recently reporting the measurement of the high-momentum “punch through” products of the recoiling jet. Given the large jet-energy loss, it is natural to ask where the lost energy goes and how the medium responds to it. Theorists have proposed that Mach cones or Cherenkov radiation might be produced in the process, as Abhijit Majumder of Duke University and Thorsten Renk of Jyvaskyla University discussed. Two-particle correlation measurements have shown previously that the recoiling jet is both softened and broadened in matter, but insight into the specific mechanisms at play requires higher-order correlations. Marco van Leeuwen of Lawrence Berkeley National Laboratory (LBNL) reviewed three-particle correlation techniques and their subtleties, and Jason Ulery of Purdue University and Nuggehalli Ajitanand of SUNY presented new, high-statistics three-particle correlation measurements from STAR and PHENIX, respectively. The data suggest the formation of a cone structure from shock waves or Cherenkov radiation. With improved statistical and systematic uncertainties in the near future, such a measurement could provide important information on the speed of sound or the dielectric constant in the strongly interacting quark–gluon plasma.
The STAR collaboration also reported “near side” correlations in which the jet structure is elongated owing to coupling with the longitudinally flowing medium, a theoretical prediction that Nestor Armesto of Santiago de Compostela reviewed. The jet-quenching results from RHIC have stimulated the reanalysis of high pτ heavy-ion data from the SPS, described by Christoph Blume and Mateusz Ploskon of Frankfurt University, which show surprisingly similar (albeit less spectacular) effects. Jet measurements will undoubtedly play an important role in the heavy-ion programme at the LHC, as CERN’s Andreas Morsch, MIT’s Gunther Roland, and BNL’s Helio Takai from the ALICE, cmS and ATLAS experiments, respectively, explained.
Heavy quarkonium and dimuouns
In 1986, Helmut Satz of Bielefeld University, together with Tetsuo Matsui, suggested that deconfinement would be signalled by the melting of heavy quarkonium states, and quarkonium suppression was well represented at the conference. Masayuki Asakawa of Osaka University, Takashi Umeda from BNL and Agnes Mocsy from RIKEN-BNL presented the latest lattice gauge calculations on heavy quarkonium at finite temperature which show that, in contrast to early calculations, the ground states (J/ψ, Y) survive at least up to twice the critical QCD temperature, whereas excited states such as the ψ’ and χc melt around Tcrit. At the conference Satz interpreted the similar J/ψ suppression pattern at RHIC and SPS, reported by Abigail Bickley of Colorado University/PHENIX and Roberta Arnaldi of INFN Torino/NA60, respectively, as resulting from the dissociation of ψ’ and χc, which contribute via feed-down decay to 40% of the J/Ψ yield. Robert Thews of Arizona University argued alternatively that direct J/ψ suppression is partially counterbalanced by heavy-quark recombination in the dense medium.
The venerable heavy-ion programme at the SPS continues to provide surprising and interesting results. Sanja Damjanovic from CERN presented the NA60 experiment’s new, high-statistics low-mass dimuon measurements, which address the important question of the restoration of chiral symmetry. The spectral shape of the ? meson in hot matter broadens but is not shifted in mass, in contrast to a long-standing prediction by Gerry Brown of SUNY and Mannque Rho of Saclay. Theorists were excited by these new data, which may provide a new window into the mechanisms underlying the breaking of chiral symmetry in the strong interaction.
All in all, the conference showed once again that hard processes are excellent probes of matter under extreme conditions of temperature and density. The large attendance, lively discussions, and marked experimental and theoretical progresses reported during the conference guarantee a strong future for the Hard Probes conference series. To maintain the now-traditional venue beside the sea, the next in the series will be held in 2008 at the spectacular thermal resort of A Toxa, on the Galician coast of the Iberian Peninsula.
