Dynamics of Particle Production in Relativistic Nuclear Collisions

Saturation models for particle production in relativistic nuclear collisions are discussed. In particular, I show that the predictions from the high density QCD for the qualitative shape of $dN/dy$ are very sensitive to the form of the unintegrated gluon distribution.Comment: 4 pages, 4 figures, Proceedings for Quark Matter 200

Jet Fragmentation in Vacuum and Medium with gamma-hadron Correlations in PHENIX

Jet fragmentation in p+p and Au+Au collisions is studied via back-to-back correlations of direct photons and charged hadrons. The direct photon correlations are obtained by statical subtraction of the background from decay photons. Results on the nuclear modification to the associated charged hadron yields are reviewed. Further studies of jet fragmentation in p+p using isolated direct photons are also presented. A kT-smeared LO pQCD calculation is used to interpret the data. The sensitivity of the data to the underlying fragmentation function is tested and the results are found to be compatible with expectations of a sample dominated by quark jet fragmentation.Comment: Talk presented at Hot Quarks 2010, June 21-26, La Londe Les Maures, France. To be published in Journal of Physics: Conference Series (JPCS

Stopping and Baryon Transport in Heavy Ion Reactions

In this report I will give an experimental overview on nuclear stopping in hadron collisions, and relate observations to understanding of baryon transport. Baryon number transport is not only evidenced via net-proton distributions but also by the enhancement of strange baryons near mid-rapidity. Although the focus is on high-energy data obtained from pp and heavy ions from RHIC, relevant data from SPS and ISR will be considered. A discussion how the available data at higher energy relates and gives information on baryon junction, quark-diquark breaking will be made.Comment: 8 pages, 7 figures. Invited plenary talk for the 5'th international conference on Physics and Astrophysics of Quark Gluon Plasma, February 8-12, 2005, Salt Lake City, Kolkata, Indi

Elliptic flow of the dilute Fermi gas: From kinetics to hydrodynamics

We use the Boltzmann equation in the relaxation time approximation to study the expansion of a dilute Fermi gas at unitarity. We focus, in particular, on the approach to the hydrodynamic limit. Our main finding are: i) In the regime that has been studied experimentally hydrodynamic effects beyond the Navier-Stokes approximation are small, ii) mean field corrections to the Boltzmann equation are not important, iii) experimental data imply that freezeout occurs very late, that means that the relaxation time remains smaller than the expansion time during the entire evolution of the system, iv) the experimental results also imply that the bulk viscosity is significantly smaller than the shear viscosity of the system.Comment: 18 pages, 6 figure

Study of Cronin effect and nuclear modification of strange particles in d-Au and Au-Au collisions at 200 GeV in PHENIX

Effects of strangeness on nuclear modification in d-Au and Au-Au collisions at 200 GeV are studied, in order to quantify the effects of quark content and mass. Measurements of ratios of the yields in central collisions to the yields in peripheral collisions are performed for lambda baryon and phi meson. Found results show little dependence of particle suppression or enhancement on mass and strange content, but rather prominent difference in nuclear modification between mesons and baryons.Comment: 4 pages, 4 figures, proceedings of the Seventeenth International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2004

Beam-Energy and System-Size Dependence of Dynamical Net Charge Fluctuations

We present measurements of net charge fluctuations in $Au + Au$ collisions at $\sqrt{s_{NN}} =$ 19.6, 62.4, 130, and 200 GeV, $Cu + Cu$ collisions at $\sqrt{s_{NN}} =$ 62.4, 200 GeV, and $p + p$ collisions at $\sqrt{s} =$ 200 GeV using the net charge dynamical fluctuations measure $\nu_{+-,dyn}$. The dynamical fluctuations are non-zero at all energies and exhibit a rather modest dependence on beam energy. We find that at a given energy and collision system, net charge dynamical fluctuations violate $1/N_{ch}$ scaling, but display approximate $1/N_{part}$ scaling. We observe strong dependence of dynamical fluctuations on the azimuthal angular range and pseudorapidity widths.Comment: 4 pages, 4 figures, presented at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, February 4-10, 200

Measurement of the Transverse Single Spin Asymmetry of pp + p↑p^\uparrow →\rightarrow η\eta + XX at s\sqrt{s} = 200 GeV

The measurement of transverse single spin asymmetries ($A_N$) provides insight into the structure of the nucleon. Several mechanisms have been proposed that attempt to explain $A_N$ based on QCD, and additional measurements of $A_N$ for different processes further constrain these models. Using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC), we study transversely polarized p+p collisions. Results from PHENIX and other experiments show significant asymmetries in the forward region, which could be due to contributions from both the Sivers and the Collins effects. Studying the species as well as the kinematic dependencies of these transverse single spin asymmetries will help to disentangle the origin of the observed asymmetries. Therefore, measurements of $A_N$ with inclusive $\eta$ mesons at forward rapidities are an important tool for the understanding of these asymmetries. In 2008, the PHENIX experiment collected 5.2 pb$^{-1}$ integrated luminosity in $p^\uparrow+p$ collisions at $\sqrt{s}$ = 200 GeV. The status of the asymmetry analysis of $\eta$ mesons at forward rapidity will be shown.Comment: 4 pages, 3 figure

Has the Quark-Gluon Plasma been seen?

Data from the first three years of running at RHIC are reviewed and put into context with data obtained previously at the AGS and SPS and with the physics question of creation of a quark-gluon plasma in high energy heavy ion collisions. Also some very recent and still preliminary data from run4 are included.Comment: plenary paper, Lepton-Photon 2005, Uppsala, Swede

Investigation of high pt_{t} events in Nucleus-Nucleus collisions using the Hijing event generator

In recent years lot of interest has been observed in the nucleus-nucleus collisions at RHIC energies in phenomena related to high $p_{t}$ physics \cite{ref1}. The suppression of high $p_{t}$ particles and disappearance of back-to-back jets compared to the scaling with number of binary nucleon-nucleon collisions indicates that a nearly perfect liquid is produced in these collisions. Results on self shadowing of high $p_{t}$ events are presented using hadron multiplicity associated to high $p_{t}$ and unbiased events in nucleus-nucleus collisions \cite{ref2} obtained from the hijing event generator.Comment: 4 pages, 3 figures, Proceedings of the poster presented at Quark Matter 200

Relativistic Heavy-Ion Physics: Experimental Overview

The field of relativistic heavy-ion physics is reviewed with emphasis on new results and highlights from the first run of the Relativistic Heavy-Ion Collider at BNL and the 15 year research programme at the SPS at CERN and the AGS at BNL.Comment: 15 pages, 9 figures. Invited Talk at the Fourth International Conference on Physics and Astrophysics of Quark Gluon Plasma, (ICPAQGP-2001) Jaipur, India, November 26-30, 200