Testing AdS/CFT Deviations from pQCD Heavy Quark Energy Loss with Pb+Pb at LHC

Heavy quark jet quenching in nuclear collisions at LHC is predicted and compared using the classical gravity AdS/CFT correspondence and Standard Model perturbative QCD. The momentum independence and inverse quark mass dependence of the drag coefficient in AdS/CFT differs substantially from the characteristic log(pT/M)/pT variation of the drag in QCD. We propose that the measurement of the momentum dependence of the double ratio of the nuclear modification factors of charm and bottom jets is a robust observable that can be used to search for strong coupling deviations from perturbative QCD predictions.Comment: 4 pages, 2 figure

Collisional Energy Loss of Non Asymptotic Jets in a QGP

We calculate the collisional energy loss suffered by a heavy (charm) quark created at a finite time within a Quark Gluon Plasma (QGP) in the classical linear response formalism as in Peigne {\it et al.} \cite{peigne}. We pay close attention to the problem of formulating a suitable current and the isolation of binding and radiative energy loss effects. We find that unrealistic large binding effects arising in previous formulations must be subtracted. The finite time correction is shown to be important only for very short length scales on the order of a Debye length. The overall energy loss is similar in magnitude to the energy loss suffered by a charge created in the asymptotic past. This result has significant implications for the relative contribution to energy loss from collisional and radiative sources and has important ramifications for the ``single electron puzzle'' at RHIC.Comment: 15 Pages, 11 figures, revte

Quenching and Tomography from RHIC to LHC

We compare fully perturbative and fully nonperturbative pictures of high-pT energy loss calculations to the first results from LHC. While over-suppressed compared to published ALICE data, parameter-free pQCD predictions based on the WHDG energy loss model constrained to RHIC data simultaneously describe well the preliminary CMS hadron suppression, ATLAS charged hadron v2, and ALICE D meson suppression; we also provide for future reference WHDG predictions for B meson RAA. However, energy loss calculations based on AdS/CFT also qualitatively describe well the RHIC pion and non-photonic electron suppression and LHC charged hadron suppression. We propose the double ratio of charm to bottom quark RAA will qualitatively distinguish between these two energy loss pictures.Comment: 4 pages, 3 figures. Proceedings for Quark Matter 201

Sensitivity of Azimuthal Jet Tomography to Early Time Energy-Loss at RHIC and LHC

We compute the jet path-length dependence of energy-loss for higher azimuthal harmonics of jet-fragments in a generalized model of energy-loss that can interpolate between pQCD and AdS/CFT limits and compare results with Glauber and CGC/KLN initial conditions. We find, however, that even the high-pT second moment is most sensitive to the poorly known early-time evolution during the first fm/c. Moreover, we demonstrate that quite generally the energy and density-dependence leads to an overquenching jet fragments relative to the first LHC $R_{AA}$-data, once the parameters of the energy-loss model are fixed from $R_{AA}$-data at RHIC.Comment: 4 pages, 2 figures, version accepted for publication in J. Phys. G: Nucl. Part. Phys. as conference proceedings for Quark Matter 2011, May 23 - May 28, Annecy, Franc

pQCD vs. AdS/CFT Tested by Heavy Quark Energy Loss

We predict the charm and bottom quark nuclear modification factors using weakly coupled pQCD and strongly coupled AdS/CFT drag methods. The log(pT/M_Q)/pT dependence of pQCD loss and the momentum independence of drag loss lead to different momentum dependencies for the R_{AA} predictions. This difference is enhanced by examining a new experimental observable, the double ratio of charm to bottom nuclear modification factors, R^{cb}=R^c_{AA}/R^b_{AA}. At LHC the weakly coupled theory predicts R^{cb} goes to 1; whereas the strongly coupled theory predicts R^{cb} .2 independent of pT. At RHIC the differences are less dramatic, as the production spectra are harder, but the drag formula is applicable to higher momenta, due to the lower temperature.Comment: 6 pages, 4 figures. Proceedings for the International Conference on Strangeness in Quark Matter (SQM 2007), Levoca, Slovakia, 24-29 June 200

Heavy-Quark Diffusion, Flow and Recombination at RHIC

We discuss recent developments in assessing heavy-quark interaction in the Quark-Gluon Plasma (QGP). While induced gluon radiation is expected to be the main energy-loss mechanism for fast-moving quarks, we focus on elastic scattering which prevails toward lower energies, evaluating both perturbative (gluon-exchange) and nonperturbative (resonance formation) interactions in the QGP. The latter are treated within an effective model for D- and B-meson resonances above T_c as motivated by current QCD lattice calculations. Pertinent diffusion and drag constants, following from a Fokker-Planck equation, are implemented into an expanding fireball model for Au-Au collisions at RHIC using relativistic Langevin simulations. Heavy quarks are hadronized in a combined fragmentation and coalescence framework, and resulting electron-decay spectra are compared to recent RHIC data. A reasonable description of both nuclear suppression factors and elliptic flow up to momenta of ~5 GeV supports the notion of a strongly interacting QGP created at RHIC. Consequences and further tests of the proposed resonance interactions are discussed.Comment: 8 pages, 14 figures, contribution to the proceedings for the "International Conference on Strangeness in Quark Matter 2006

Ratio of viscosity to entropy density in a strongly coupled one-component plasma

String theoretical arguments led to the hypothesis that the ratio of viscosity to entropy of any physical system has a lower bound. Strongly coupled systems usually have a small viscosity compared to weakly coupled plasmas in which the viscosity is proportional to the mean free path. In the case of a one-component plasma the viscosity as a function of the coupling strength shows a minimum. Here we show that the ratio of viscosity to entropy of a strongly coupled one-component plasma is always above the lower bound predicted by string theory.Comment: 5 pages, revised version to be published in Europhysics Letter

Angular hadron correlations probing the early medium evolution

Hard processes are a well calibrated probe to study heavy-ion collisions. However, the information to be gained from the nuclear suppression factor R_AA is limited, hene one has to study more differential observables to do medium tomography. The angular correlations of hadrons associated with a hard trigger appear suitable as they show a rich pattern when going from low p_T to high p_T. Of prime interest is the fate of away side partons with an in-medium pathlength O(several fm). At high p_T the correlations become dominated by the punchtrough of the away side parton with subsequent fragmentation. We discuss what information about the medium density can be gained from the data.Comment: Talk given at the 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14-20 Nov 200

Tomography of the Quark Gluon Plasma by Heavy Quarks

Using the recently published model \cite{Gossiaux:2008jv,goss2} for the collisional energy loss of heavy quarks in a Quark Gluon Plasma (QGP), based on perturbative QCD (pQCD), we study the centrality dependence of $R_{AA}$ and $R_{AA}(p_T^{min})$, %= \frac{dN_{AA}/dp_T}{ dN_{pp}/dp_T}$ measured by the Phenix collaboration, and compare our model with other approaches based on pQCD and on Anti de Sitter/ Conformal Field Theory (AdS/CFT)Comment: proceedings for SQM0

Quark Matter 2006: high-pT and jets

An overview of new experimental results on high-\pT{} particle production and jets in heavy ion collisions from the Quark Matter 2006 conference is presented.Comment: Presented at Quark Matter 200