Physics – Nuclear Physics – Nuclear Theory
Scientific paper
2011-12-31
Physics
Nuclear Physics
Nuclear Theory
8 pages, 7 figures. The range of the LHC predictions is extended up to p_T = 100 GeV, additional references and comments are a
Scientific paper
The nuclear modification factor, $R_{AA}$, of jet fragments in 2.76 ATeV Pb+Pb collisions observed by the ALICE and CMS Collaborations at the Large Hadron Collider (LHC) suggests that the Quark-Gluon Plasma (QGP) may be less opaque to jets than expected from fixed jet-medium coupling extrapolations from the Relativistic Hadron Collider (RHIC). We evaluate the effective reduction factor of the jet-medium coupling from a combined fit to the available data on $R_{AA}(\sqrt{s},p_T,b)$ and the elliptic flow $v_2(\sqrt{s},p_T,b)$ at $\sqrt{s}=0.2,2.76$ ATeV in the transverse momentum range $p_T\sim 5-100$ GeV and a broad centrality (impact parameter, $b$) range. We compare Glauber and Color Glass Condensate (CGC) initial conditions using a simple analytic energy-loss model ($dE/dx=- \kappa E^{a} x^z T^{z-a+2}$) that can interpolate between weakly-coupled tomographic and strongly-coupled holographic jet energy-loss models. Extrapolation from RHIC to LHC assuming a fixed jet-medium coupling, $\kappa$, and a monotonic density dependence, leads to an overquenching of $R_{AA}$ at LHC for both Glauber and CGC initial conditions and both $z=1,2$ path-length dependencies. The fit to the $p_T$ and centrality dependence of $R_{AA}$ at LHC requires a reduced coupling $\kappa_{LHC}\approx (0.6\pm0.1) \kappa_{RHIC}$ with either initial geometry and either path-length dependence. Interpreted in terms of weakly-coupled jet tomography, this implies a modest reduction of $\alpha_{LHC}\approx 0.9 \alpha_{RHIC}$ at temperatures $T_{LHC}\approx 1.3 T_{RHIC}$. However, in terms of strongly-coupled gravity dual holography, the fit implies a more rapid reduction of the effective t'Hooft coupling, $\lambda_{LHC}\sim 0.5\, \lambda_{RHIC}$. Surprisingly, unlike at RHIC, the preliminary LHC data on the high-$p_T$ $R_{AA}(p_T,b)$ appears to be inconsistent with CGC initial geometries.
Betz Barbara
Gyulassy Miklos
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