Evidence for non-Gaussian tail in 3D pion emission source at the SPS

The NA49 experiment at CERN SPS has acquired a huge data set of Pb+Pb events over a broad range of energy and centrality during the last several years. This high statistics data set, coupled with a state-of-the-art analysis technique, allows for the first model-independent extraction and energy scan of 3D emission sources for pion pairs at SPS energies. These 3D pion emission sources provide new insights into the nature of a long-range source previously reported by PHENIX at RHIC. The new results indicate that the pion source displays significant non-Gaussian tails in the longitudinal direction at 40 and 158 AGeV and in the outward direction at 158 AGeV.Comment: Proceedings, QM06, Beijing, Chin

PHENIX measurements of 3D emission source functions in Au+Au collisions at sNN=200\sqrt{s_{NN}} = 200 GeV

A state-of-the-art 3D source imaging technique is used to extract the 3D two-pion source function in central and mid-central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. The source function indicates a previously unresolved non-Gaussian tail in the directions of the pion pair transverse momentum (out) and along the beam (long). Model comparisons give robust estimates for several characteristics of the emission source, including its transverse size, its mean proper breakup time $\tau$ and its emission duration $\Delta\tau$. These estimates are incompatible with the predictions for a first order phase transition. However, they point to significant relative emission times which could result from a crossover phase transition.Comment: Proceedings, Quark Matter 2008, Jaipur, Indi

Disappearance of Elliptic Flow: A New Probe for the Nuclear Equation of State

Using a relativistic hadron transport model, we investigate the utility of the elliptic flow excitation function as a probe for the stiffness of nuclear matter and for the onset of a possible quark-gluon-plasma (QGP) phase-transition at AGS energies 1 < E_Beam < 11 AGeV. The excitation function shows a strong dependence on the nuclear equation of state, and exhibits characteristic signatures which could signal the onset of a phase transition to the QGP.Comment: 11 pages, 4 Postscript figures, uses epsf.sty, submitted to Physical Review Letter

Has the QCD Critical Point been Signaled by Observations at RHIC ?

The shear viscosity to entropy ratio ($\eta/s$) is estimated for the hot and dense QCD matter created in Au+Au collisions at RHIC ($\sqrt{s_{NN}}=200$ GeV). A very low value is found $\eta/s \sim 0.1$, which is close to the conjectured lower bound ($1/4\pi$). It is argued that such a low value is indicative of thermodynamic trajectories for the decaying matter which lie close to the QCD critical end point.Comment: 4 pages, 3 figures. Revised version, accepted for publication in PR

Decomposition of Harmonic and Jet Contributions to Particle-pair Correlations at Ultra-relativistic Energies

Methodology is presented for analysis of two-particle azimuthal angle correlation functions obtained in collisions at ultra-relativistic energies. We show that harmonic and di-jet contributions to these correlation functions can be reliably decomposed by two techniques to give an accurate measurement of the jet-pair distribution. Results from detailed Monte Carlo simulations are used to demonstrate the efficacy of these techniques in the study of possible modifications to jet topologies in heavy ion reactions.Comment: Updated version to be published in PRC Rapid Com

A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on π−π\pi-\pi correlations at FAIR/AGS energies

HBT correlations of $\pi^--\pi^-$ pairs at FAIR/AGS energies are investigated by using the UrQMD transport model and the CRAB analyzing program. Three different possible sources (treatment of resonance lifetimes, a soft equation of state and non-Gaussian effects) to understand the HBT $R_O/R_S$ puzzle are investigated. Firstly, we find that different treatments of the resonance decay time can not resolve the HBT time-related puzzle, however it can modify the HBT radii at low transverse momenta to some extent to explain the data slightly. Secondly, with a soft equation of state with momentum dependence, the measured transverse momentum dependent HBT radii and $R_O/R_S$ ratio can be described fairly well. Thirdly, non-Gaussian effects are visible in the calculated correlation function. Using the Edgeworth expansion, one finds that the non-Gaussian effect is strongest in the longitudinal direction and weakest in the sideward direction.Comment: 18 pages, 6 figures. To be published in J.Phys.

Flow analysis from multiparticle azimuthal correlations

We present a new method for analyzing directed and elliptic flow in heavy ion collisions. Unlike standard methods, it separates the contribution of flow to azimuthal correlations from contributions due to other effects. The separation relies on a cumulant expansion of multiparticle azimuthal correlations, and includes corrections for detector inefficiencies. This new method allows the measurement of the flow of identified particles in narrow phase-space regions, and can be used in every regime, from intermediate to ultrarelativistic energies.Comment: 31 pages, revtex. Published version (references added

Effects of momentum conservation on the analysis of anisotropic flow

We present a general method for taking into account correlations due to momentum conservation in the analysis of anisotropic flow, either by using the two-particle correlation method or the standard flow vector method. In the latter, the correlation between the particle and the flow vector is either corrected through a redefinition (shift) of the flow vector, or subtracted explicitly from the observed flow coefficient. In addition, momentum conservation contributes to the reaction plane resolution. Momentum conservation mostly affects the first harmonic in azimuthal distributions, i.e., directed flow. It also modifies higher harmonics, for instance elliptic flow, when they are measured with respect to a first harmonic event plane such as one determined with the standard transverse momentum method. Our method is illustrated by application to NA49 data on pion directed flow.Comment: RevTeX 4, 10 pages, 1 eps figure. Version accepted for publication in Phys Rev

Anisotropic Collective Flow of Lambda Hyperons Produced in C + C Collisions at 4.2 AGeV/c

Features of anisotropic collective flow and spectral temperatures have been determined for lambda hyperons emitted from C + C collisions, at incident momentum of 4.2 AGeV/c, measured using the Propane Bubble Chamber of JINR at Dubna. Moreover, characteristics of protons and of negative pions, emitted from those collisions, have been determined and provided for comparison. The directed and elliptic flows of lambdas both agree in sign with the corresponding flows of protons. Parameters of the directed and elliptic flows for lambdas agree further, within errors, with the corresponding parameters for the co-produced protons. This contrasts an earlier finding by the E895 Collaboration of the directed flow being significantly weaker for lambdas than protons, in the much heavier Au + Au system, at comparable incident momentum. Particle spectral temperatures in the C + C collisions have been determined focusing independently on either center-of-mass energy, transverse energy or transverse momentum distributions. For either protons or negative pions, the temperatures were found to be approximately the same, no matter whether the emission of those particles was associated with lambda production or not. Results of the measurements have been compared to the results of simulations within the Quark-Gluon String Model.Comment: 25 pages, 5 figures, submitted to Nuclear Physics A; in revision, the discussion of results has been expanded and some deficiencies of figures and text have been correcte