Jets in heavy ion collisions

High energetic particles traversing a dense medium lose a sizable part of their energy in form of gluon radiation. As a result, the rate of high-$p_t$ particles is expected to be suppressed in heavy ion collisions with respect to the proton case. Recent experimental data from RHIC strongly support this scenario. This allows to study the properties of the medium by the amount of jet quenching it produces. The angular dependence of the radiation is modified in the medium in a characteristic way. This provides another tool to study the medium properties in a more differential measurement.Comment: 4 pages, 3 postscript figures. Contributed to 38th Rencontres de Moriond on QCD and Hadronic Interactions, Les Arcs, France, 22-29 Mar 200

Medium Modification of the Jet Properties

In the case that a dense medium is created in a heavy ions collision, high-E_t jets are expected to be broadened by medium-modified gluon emission. This broadening is directly related, through geometry, to the energy loss measured in inclusive high-p_t particle suppression. We present here the modifications of jet observables due to the presence of a medium for the case of azimuthal jet energy distributions and k_t-differential multiplicities inside the jets.Comment: 4 pages, 3 postscript figures. Proceedings for Quark Matter 200

Gluon distributions in nuclei at small x: guidance from different models

Different approaches to gluon shadowing at small x are reviewed. Some available results relevant for RHIC and LHC are compared.Comment: 6 pages, LaTeX2e, uses enclosed cernrep.cls, one eps figure enclosed using graphicx, contribution to the Yellow Report on Hard Probes in Heavy Ion Collisions at the LH

Baryon and Antibaryon production at RHIC energies in the Dual Parton Model

We compute the mid-rapidity densities of pions, kaons, baryons and antibaryons in $Au$--$Au$ collisions at $\sqrt{s}$ = 130 GeV in the Dual Parton Model supplemented with final state interactions, and we present a comparison with available data.Comment: 4 pages, 2 figures. Talk given at XXXVII Rencontres de Moriond, Les Arcs, France, March 200

Hyperon Enhancement in the Dual Parton Model

We review the two sources of hyperon enhancement in the dual parton model: strings originating from diquark-antidiquark pairs in the nucleon sea and net baryons containing two or three sea quarks with a yield controlled by the observed stopping. We show that adding final state interactions (including strangeness exchange reactions as well as the inverse reactions required by detailed balance) with a single averaged cross-section $\sigma=0.2$ mb, we can explain the observed hyperon enhancement in PbPb collisions at CERN SPS.Comment: 6 pages, 2 eps-figure

Update on Counting Valence Quarks at RHIC

We update our former analysis of the Nuclear Modification Factors (NMF) for different hadron species at RHIC and LHC. This update is motivated by the new experimental data from STAR which presents differences with the preliminary data used to fix some of the parameters in our model. The main change is the use of AKK fragmentation functions for the hard part of the spectrum and minor adjustments of the coalescence (soft) contribution. We confirm that observation of the NMF for the f_0 meson can shed light on its quark composition.Comment: 4 pages, 4 figure

The scale dependent nuclear effects in parton distributions for practical applications

The scale dependence of the ratios of parton distributions in a proton of a nucleus $A$ and in the free proton, $R_i^A(x,Q^2)=f_{i/A}(x,Q^2)/f_i(x,Q^2)$, is studied within the framework of the lowest order leading-twist DGLAP evolution. By evolving the initial nuclear distributions obtained with the GRV-LO and CTEQ4L sets at a scale $Q_0^2$, we show that the ratios $R_i^A(x,Q^2)$ are only moderately sensitive to the choice of a specific modern set of free parton distributions. We propose that to a good first approximation, this parton distribution set-dependence of the nuclear ratios $R_i^A(x,Q^2)$ can be neglected in practical applications. With this result, we offer a numerical parametrization of $R_i^A(x,Q^2)$ for all parton flavours $i$ in any $A>2$, and at any $10^{-6}\le x \le 1$ and any $Q^2\ge 2.25$ GeV$^2$ for computing cross sections of hard processes in nuclear collisions.Comment: 14 pages, including 4 eps-figure

Universal geometrical scaling of the elliptic flow

The presence of scaling variables in experimental observables provide very valuable indications of the dynamics underlying a given physical process. In the last years, the search for geometric scaling, that is the presence of a scaling variable which encodes all geometrical information of the collision as well as other external quantities as the total energy, has been very active. This is motivated, in part, for being one of the genuine predictions of the Color Glass Condensate formalism for saturation of partonic densities. Here we extend these previous findings to the case of experimental data on elliptic flow. We find an excellent scaling for all centralities and energies, from RHIC to LHC, with a simple generalization of the scaling previously found for other observables and systems. Interestingly the case of the photons, difficult to reconcile in most formalisms, nicely fit the scaling curve. We discuss the possible interpretations of this finding in terms of initial or final state effects.Comment: 6 pages, 4 figures, accepted for publication in Phys Rev