What to learn from dilepton transverse momentum spectra in heavy-ion collisions?

Recently the NA60 collaboration has presented high precision measurements of dimuon spectra double differential in invariant mass $M$ and transverse pair momentum $p_T$ in In-In collisions at $158 {\rm AGeV}$. While the $M$-dependence is important for an understanding of in-medium changes of light vector mesons and is $p_T$ integrated insensitive to collective expansion, the $p_T$-dependence arises from an interplay between emission temperature and collective transverse flow. This fact can be exploited to derive constraints on the evolution model and in particular on the contributions of different phases of the evolution to dimuon radiation into a given $M$ window. We present arguments that a thermalized evolution phase with $T > 170 {\rm MeV}$ leaves its imprint on the spectra.Comment: Contributed to 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14- 20 Nov 200

What does the rho-meson do? In-medium mass shift scenarios versus hadronic model calculations

The NA60 experiment has studied low-mass muon pair production in In-In collisions at $158 {\rm AGeV}$ with unprecedented precision. With these results there is hope that the in-medium modifications of the vector meson spectral function can be constrained more thoroughly than before. We investigate in particular what can be learned about collisional broadening by a hot and dense medium and what constrains the experimental results put on in-medium mass shift scenarios. The data show a clear indication of considerable in-medium broadening effects but disfavor mass shift scenarios where the $\rho$-meson mass scales with the square root of the chiral condensate. Scaling scenarios which predict at finite density a dropping of the $\rho$-meson mass that is stronger than that of the quark condensate are clearly ruled out since they are also accompanied by a sharpening of the spectral function.Comment: Proceeding contribution, Talk given by J. Ruppert at Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions (Hot Quarks 2006), Villasimius, Sardinia, Italy, 15-20 May 2006. To appear in EPJ

Hard and soft probe - medium interactions in a 3D hydro+micro approach at RHIC

We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-)strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. While the nuclear suppression factor $R_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of $R_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length.Comment: 5 pages, 4 figures, Quark Matter 2006 proceedings, to appear in Journal of Physics

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

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-

A comprehensive description of multiple observables in heavy-ion collisions at SPS

Combining and expanding on work from previous publications, a model for the evolution of ultrarelativistic heavy-ion collisions at the CERN SPS for 158 AGeV beam energy is presented. Based on the assumption of thermalization and a parametrization of the space-time expansion of the produced matter, this model is able to describe a large set of observables including hadronic momentum spectra, correlations and abundancies, the emission of real photons, dilepton radiation and the suppression pattern of charmonia. Each of these obervables provides unique capabilities to study the reaction dynamics and taken together they form a strong and consistent picture of the evolving system. Based on the emission of hard photons, we argue that a strongly interacting, hot and dense system with temperatures above 250 MeV has to be created early in the reaction. Such a system is bound to be different from hadronic matter and likely to be a quark-gluon plasma, and we find that this assumption is in line with the subsequent evolution of the system that is reflected in other observables.Comment: 21 pages, 10 figures, submitted to J. Phys.

Interpretation of Recent SPS Dilepton Data

We summarize our current theoretical understanding of in-medium properties of the electromagnetic current correlator in view of recent dimuon data from the NA60 experiment in In(158 AGeV)-In collisions at the CERN-SPS. We discuss the sensitivity of the results to space-time evolution models for the hot and dense partonic and hadronic medium created in relativistic heavy-ion collisions and the contributions from different sources to the dilepton-excess spectra.Comment: To appear in the proceedings of the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006) v2: references added, minor typos correcte

Systematic Comparison of Jet Energy-Loss Schemes in a 3D hydrodynamic medium

We utilize a 3D hydrodynamic model to provide the medium evolution for a systematic comparison of jet energy-loss calculations in the BDMPS/ASW, HT and AMY approaches. We find that the parameters of all three calculations can be adjusted to provide a good description of inclusive data on $R_{AA}$ versus transverse momentum. However, we do observe slight differences in their predictions for the azimuthal angular dependence of $R_{AA}$ vs. $p_T$. We also note that the value of the transport coefficient $\hat{q}$ needed in the three approaches to describe the data differs significantly.Comment: 4 pages, 4 figures, proceedings of Quark Matter 200

Theory and Phenomenology of Vector Mesons in Medium

Electromagnetic probes promise to be direct messengers of (spectral properties of) hot and dense matter formed in heavy-ion collisions, even at soft momentum transfers essential for characterizing possible phase transitions. We examine how far we have progressed toward this goal by highlighting recent developments, and trying to establish connections between lattice QCD, effective hadronic models and phenomenology of dilepton production.Comment: 8 pages latex incl. 12 ps/eps files; invited plenary talk at Quark Matter 2006 conference, Shanghai (China), Nov. 14-20, 200

Mach Cones in Quark Gluon Plasma

The experimental azimuthal dihadron distributions at RHIC show a double peak structure in the away side ($\Delta \phi = \pi \pm 1.2$ rad.) for intermediate $p_t$ particles. A variety of models have appeared trying to describe this modification. We will review most of them, with special emphasis in the Conical Flow scenario in which the observed shape is a consequence of the emission of sound by a supersonic high momentum particle propagating in the Quark Gluon Plasma.Comment: 8 pages, 3 figures, Invited plenary talk given at the 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14-20 Nov 200