Energy deposition in hard dihadron triggered events in heavy-ion collisions

The experimental observation of hadrons correlated back-to-back with a (semi-)hard trigger in heavy ion collisions has revealed a splitting of the away side correlation structure in a low to intermediate transverse momentum (P_T) regime. This is consistent with the assumption that energy deposited by the away side parton into the bulk medium produced in the collision excites a sonic shockwave (a Mach cone) which leads to away side correlation strength at large angles. A prediction following from assuming such a hydrodynamical origin of the correlation structure is that there is a sizeable elongation of the shockwave in rapidity due to the longitudinal expansion of the bulk medium. Using a single hadron trigger, this cannot be observed due to the unconstrained rapidity of the away side parton. Using a dihadron trigger, the rapidity of the away side parton can be substantially constrained and the longitudinal structure of the away side correlation becomes accessible. However, in such events several effects occur which change the correlation structure substantially: There is not only a sizeable contribution due to the fragmentation of the emerging away side parton, but also a systematic bias towards small energy deposition into the medium and hence a weak shockwave. In this paper, both effects are addressed.Comment: 5 pages, 2 figure

Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb-Au collisions at 158A GeV/c

Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Comment: 4 pages, 2 figures, contribution to the Quark Matter conference 200

Conical Emission in Heavy Ion Collisions

A broadened or double humped away-side structure was observed in 2-particle azimuthal jet-like correlations at RHIC and SPS. This modification can be explained by conical emission, from either Mach-cone shock waves or Cherenkov gluon radiation, and by other physics mechanisms, such as large angle gluon radiation, jets deflected by radial flow and path-length dependent energy loss. Three-particle jet-like correlations are studied for their power to distinguish conical emission from other mechanisms. This article discusses Mach-cone shock waves, Cherenkov gluon radiation and the experimental evidence for conical emission from RHIC and SPS.Comment: Talk given at QM2008, Jaipur, India. 8 pages, 7 figure

Three-particle cumulant Study of Conical Emission

We discuss the sensitivity of the three-particle azimuthal cumulant method for a search and study of conical emission in central relativistic $A+A$ collisions. Our study is based on a multi-component Monte Carlo model which include flow background, Gaussian mono-jets, jet-flow, and Gaussian conical signals. We find the observation of conical emission is hindered by the presence of flow harmonics of fourth order ($v_4$) but remains feasible even in the presence of a substantial background. We consider the use of probability cumulants for the suppression of 2$^{nd}$ order flow harmonics. We find that while probability cumulant significantly reduce $v_2^2$ contributions, they also complicate the cumulant of jets, and conical emission. The use of probability cumulants is therefore not particularly advantageous in searches for conical emission. We find the sensitivity of the (density) cumulant method depends inextricably on strengths of $v_2$, $v_4$, background and non-Poisson character of particle production. It thus cannot be expressed in a simple form, and without specific assumptions about the values of these parameters.Comment: 12 figures, accepted for publication in PR

Understanding jet quenching and medium response with di-hadron correlation

A brief review of the $p_T$ dependence of the dihadron correlations from RHIC is presented. We attempt to construct a consistent picture that can describe the data as a whole, focusing on the following important aspects, 1) the relation between jet fragmentation of survived jet and medium response to quenched jets, 2) the possible origin of the medium response and its relation to intermediate $p_T$ physics for single hadron production, 3) the connection between the near-side ridge and away-side cone, 4) and their relations to low energy results.Comment: 8 pages, 8 figures, presented at the 20th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, February 4-10, 2008. Updated with the published versio

ϕ\phi meson production and partonic collectivity at RHIC

New results on $\phi$-meson production and elliptic flow $v_{2}$ measurements from RHIC 2004 run (Run-IV) have been reviewed. In addition, the di-hadron correlation function between the trigged $\phi$ and $\Omega$ and the associated soft particles was simulated. Knowledge about these results are discussed.Comment: 8 pages, 7 figures; Invited talk in International Conference on Strangess in Quark Matter (SQM2006), UCLA, California, USA, March 26-31, 2006; to be publsihed in the Proceeding isuue of J. Phys.

Comparing different freeze-out scenarios in azimuthal hadron correlations induced by fast partons

I review the linearized hydrodynamical treatment of a fast parton traversing a perturbative quark-gluon plasma. Using numerical solutions for the medium's response to the fast parton, I obtain the medium's distribution function which is then used in a Cooper-Frye freeze-out prescription to obtain an azimuthal particle spectrum. Two different freeze-out scenarios are considered which yield significantly different results. I conclude that any meaningful comparison of azimuthal hadron correlation functions to RHIC data requires implementing a realistic freeze-out scenario in an expanding medium.Comment: Contribution to the Proceedings for 2008 Hot Quarks in Estes Park, CO, as accepted for publication in EPJ-

Shock waves in strongly coupled plasmas

Shock waves are supersonic disturbances propagating in a fluid and giving rise to dissipation and drag. Weak shocks, i.e., those of small amplitude, can be well described within the hydrodynamic approximation. On the other hand, strong shocks are discontinuous within hydrodynamics and therefore probe the microscopics of the theory. In this paper we consider the case of the strongly coupled N=4 plasma whose microscopic description, applicable for scales smaller than the inverse temperature, is given in terms of gravity in an asymptotically $AdS_5$ space. In the gravity approximation, weak and strong shocks should be described by smooth metrics with no discontinuities. For weak shocks we find the dual metric in a derivative expansion and for strong shocks we use linearized gravity to find the exponential tail that determines the width of the shock. In particular we find that, when the velocity of the fluid relative to the shock approaches the speed of light $v\to 1$ the penetration depth $\ell$ scales as $\ell\sim (1-v^2)^{1/4}$. We compare the results with second order hydrodynamics and the Israel-Stewart approximation. Although they all agree in the hydrodynamic regime of weak shocks, we show that there is not even qualitative agreement for strong shocks. For the gravity side, the existence of shock waves implies that there are disturbances of constant shape propagating on the horizon of the dual black holes.Comment: 47 pages, 8 figures; v2:typos corrected, references adde