High transverse momentum suppression and surface effects in Cu+Cu and Au+Au collisions within the PQM model

We study parton suppression effects in heavy-ion collisions within the Parton Quenching Model (PQM). After a brief summary of the main features of the model, we present comparisons of calculations for the nuclear modification and the away-side suppression factor to data in Au+Au and Cu+Cu collisions at 200 GeV. We discuss properties of light hadron probes and their sensitivity to the medium density within the PQM Monte Carlo framework.Comment: Comments: 6 pages, 8 figures. To appear in the proceedings of Hot Quarks 2006: Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions, Villasimius, Italy, 15-20 May 200

Direct photons in d+Au collisions at s_(NN)**(1/2)=200GeV with STAR

Results are presented of an ongoing analysis of direct photon production in s_(NN)=200GeV deuteron-gold collisions with the STAR experiment at RHIC. A significant excess of direct photons is observed near mid-rapidity 0<y<1 and found to be consistent with next-to-leading order pQCD calculations including the contribution from fragmentation photons.Comment: 4 pages, 4 figures, HotQuarks 200

Robustness of Sound Speed and Jet Quenching for Gauge/Gravity Models of Hot QCD

We probe the effectiveness and robustness of a simple gauge/gravity dual model of the QCD fireball that breaks conformal symmetry by constructing a family of similar geometries that solve the scalar/gravity equations of motion. This family has two parameters, one of which is associated to the temperature. We calculate two quantities, the speed of sound and the jet-quenching parameter. We find the speed of sound to be universal and robust over all the geometries when appropriate units are used, while the jet-quenching parameter varies significantly away from the conformal limit. We note that the overall structure of the jet-quenching depends strongly on whether the running scalar is the dilaton or not. We also discuss the variation of the scalar potential over our family of solutions, and truncate our results to where the associated error is small.Comment: 21 pages, 9 figures, LaTeX. v2:references added, minor correction to speed of sound; conclusions unchange

The gauge-string duality and heavy ion collisions

I review at a non-technical level the use of the gauge-string duality to study aspects of heavy ion collisions, with special emphasis on the trailing string calculation of heavy quark energy loss. I include some brief speculations on how variants of the trailing string construction could provide a toy model of black hole formation and evaporation. This essay is an invited contribution to "Forty Years of String Theory" and is aimed at philosophers and historians of science as well as physicists.Comment: 21 page

First Direct Measurement of Jets in sNN=200\sqrt{s_{NN}}=200 GeV Heavy Ion Collisions by STAR

We present the first measurement of reconstructed jets in ultra-relativistic heavy ion collisions. Utilizing the large coverage of the STAR Time Projection Chamber and Electromagnetic Calorimeter, we apply several modern jet reconstruction algorithms and background subtraction techniques and explore their systematic uncertainties in heavy ion events. The differential spectrum for inclusive jet production in central Au+Au collisions at $\sqrt {s_{NN}}= 200$ GeV is presented. In order to assess the jet reconstruction biases, this spectrum is compared with the jet cross section measured in $\sqrt{s}=200$ GeV p+p collisions scaled by the number of binary N-N collisions to account for nuclear geometric effects.Comment: Proceedings of the 3rd International Conference on Hard and Electro- Magnetic Probes of High-Energy Nuclear Collisions 8-14 June 2008, Illa da Toxa (Galicia-Spain

Stirring Strongly Coupled Plasma

We determine the energy it takes to move a test quark along a circle of radius L with angular frequency w through the strongly coupled plasma of N=4 supersymmetric Yang-Mills (SYM) theory. We find that for most values of L and w the energy deposited by stirring the plasma in this way is governed either by the drag force acting on a test quark moving through the plasma in a straight line with speed v=Lw or by the energy radiated by a quark in circular motion in the absence of any plasma, whichever is larger. There is a continuous crossover from the drag-dominated regime to the radiation-dominated regime. In the crossover regime we find evidence for significant destructive interference between energy loss due to drag and that due to radiation as if in vacuum. The rotating quark thus serves as a model system in which the relative strength of, and interplay between, two different mechanisms of parton energy loss is accessible via a controlled classical gravity calculation. We close by speculating on the implications of our results for a quark that is moving through the plasma in a straight line while decelerating, although in this case the classical calculation breaks down at the same value of the deceleration at which the radiation-dominated regime sets in.Comment: 27 pages LaTex, 5 figure

Longitudinal broadening of near side jets due to parton cascade

Longitudinal broadening along $\Delta\eta$ direction on near side in two-dimensional ($\Delta\phi \times \Delta\eta$) di-hadron correlation distribution has been studied for central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, within a dynamical multi-phase transport model. It was found that the longitudinal broadening is generated by a longitudinal flow induced by strong parton cascade in central Au+Au collisions, in comparison with p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV. The longitudinal broadening may shed light on the information about strongly interacting partonic matter at RHIC.Comment: 5 pages, 4 figures; accepted by Eur. Phys. J.

Strange prospects for LHC energies

Strange quark and hadron production will be studied at the Large Hadron Collider (LHC) energies in order to explore the properties of both pp and heavy-ion collisions. The ALICE experiment will be specifically efficient in the strange sector with the identification of baryons and mesons over a wide range of transverse momentum. Dedicated measurements are proposed for investigating chemical equilibration and bulk properties. Strange particles can also help to probe kinematical regions where hard processes and pQCD dominate. We try to anticipate here several ALICE analyses to be performed as the first Pb--Pb and pp data will be available.Comment: 5 pages, 2 figures. To appear in the proceedings of Hot Quarks 2006, Villasimius, Italy, 15-20 May 200

Families of IIB duals for nonrelativistic CFTs

We show that the recent string theory embedding of a spacetime with nonrelativistic Schrodinger symmetry can be generalised to a twenty one dimensional family of solutions with that symmetry. Our solutions include IIB backgrounds with no three form flux turned on, and arise as near horizon limits of branewave spacetimes. We show that there is a hypersurface in the space of these theories where an instability appears in the gravitational description, indicating a phase transition in the nonrelativistic field theory dual. We also present simple embeddings of duals for nonrelativistic critical points where the dynamical critical exponent can take many values z \neq 2.Comment: 1+25 pages. References adde

Chemical freeze-out temperature in hydrodynamical description of Au+Au collisions at sqrt(s_NN) = 200 GeV

We study the effect of separate chemical and kinetic freeze-outs to the ideal hydrodynamical flow in Au+Au collisions at RHIC (sqrt(s_NN) = 200 GeV energy). Unlike in earlier studies we explore how these effects can be counteracted by changes in the initial state of the hydrodynamical evolution. We conclude that the reproduction of pion, proton and antiproton yields necessitates a chemical freeze-out temperature of T = 150 MeV instead of T = 160 - 170 MeV motivated by thermal models. Unlike previously reported, this lower temperature makes it possible to reproduce the p_T-spectra of hadrons if one assumes very small initial time, tau_0 = 0.2 fm/c. However, the p_T-differential elliptic flow, v_2(p_T) remains badly reproduced. This points to the need to include dissipative effects (viscosity) or some other refinement to the model.Comment: 8 pages, 7 figures; Accepted for publication in European Physical Journal A; Added discussion about the effect of weak decays to chemical freeze-out temperature and a figure showing isentropic curves in T-mu plan