Time Dependence of Chemical Freeze-out in Relativistic Heavy Ion Collisions

We investigate chemical and thermal freeze-out time dependencies for strange particle production for CERN SPS heavy ion collisions in the framework of a dynamical hadronic transport code. We show that the Lambda yield changes considerably after hadronization in the case of Pb+Pb collisions, whereas for smaller system sizes (e.g. S+S) the direct particle production dominates over production from inelastic rescattering. Chemical freeze-out times for strange baryons in Pb+Pb are smaller than for non-strange baryons, but they are still sufficiently long for hadronic rescattering to contribute significantly to the final Lambda yield. Based on inelastic and elastic cross section estimates we expect the trend of shorter freeze-out times (chemical and kinetic), and thus less particle production after hadronization, to continue for multi-strange baryons.Comment: 10 pages, 7 postscript figure

Dibaryons with Strangeness: their Weak Nonleptonic Decay using SU(3) Symmetry and how to find them in Relativistic Heavy-Ion Collisions

Weak SU(3) symmetry is successfully applied to the weak hadronic decay amplitudes of octet hyperons. Weak nonmesonic and mesonic decays of various dibaryons with strangeness, their dominant decay modes, and lifetimes are calculated. Production estimates for BNL's Relativistic Heavy-Ion Collider are presented employing wave function coalescence. Signals for detecting strange dibaryon states in heavy-ion collisions and revealing information about the unknown hyperon-hyperon interactions are outlined.Comment: 4 pages, 2 figures, uses RevTeX, discussion about the model of the weak decay and experimental signals extended, references update

Energy dependence of kaon production in central Pb+Pb collisions

Recent results from the NA49 experiment on the energy dependence of charged kaon production in central Pb+Pb collisions are presented. First results from the new data at 80 AGeV beam energy are compared with those from lower and higher energies. A difference in the energy dependence of the / and / ratios is observed. The / ratio shows a non-monotonic behaviour with a maximum near 40 AGeV.Comment: 8 pages, 7 figures, proceedings of talk at SQM2001, Frankfurt, Germany, to appear in J. Phys.

Strangeness enhancements at central rapidity in 40 A GeV/c Pb-Pb collisions

Results are presented on neutral kaon, hyperon and antihyperon production in Pb-Pb and p-Be interactions at 40 GeV/c per nucleon. The enhancement pattern follows the same hierarchy as seen in the higher energy data - the enhancement increases with the strangeness content of the hyperons and with the centrality of collision. The centrality dependence of the Pb-Pb yields and enhancements is steeper at 40 than at 158 A GeV/c. The energy dependence of strangeness enhancements at mid-rapidity is discussed.Comment: 15 pages, 10 figures and 3 tables. Presented at International Conference on Strangeness in Quark Matter (SQM2009), Buzios, Rio de Janeiro, Brazil, 27 Sept - 2 Oct 2009. Submitted to J.Phys.G: Nucl.Part.Phys, one reference adde

The K/pi ratio from condensed Polyakov loops

We perform a field-theoretical computation of hadron production in large systems at the QCD confinement phase transition associated with restoration of the Z(3) global symmetry. This occurs from the decay of a condensate for the Polyakov loop. From the effective potential for the Polyakov loop, its mass just below the confinement temperature T_c is in between the vacuum masses of the pion and that of the kaon. Therefore, due to phase-space restrictions the number of produced kaons is roughly an order of magnitude smaller than that of produced pions, in agreement with recent results from collisions of gold ions at the BNL-RHIC. From its mass, we estimate that the Polyakov loop condensate is characterized by a (spatial) correlation scale of 1/m_\ell ~ 1/2 fm. For systems of deconfined matter of about that size, the free energy may not be dominated by a condensate for the Polyakov loop, and so the process of hadronization may be qualitatively different as compared to large systems. In that vein, experimental data on hadron abundance ratios, for example K/pi, in high-multiplicity pp events at high energies should be very interesting.Comment: 7 pages, 4 figures; discussion of the two-point function of Polyakov Loops in small versus large systems adde

Measurements of the transverse-momentum-dependent cross sections of J /ψ production at mid-rapidity in proton+proton collisions at s =510 and 500 GeV with the STAR detector

We present measurements of the differential cross sections of inclusive J/ψ meson production as a function of transverse momentum (pTJ/ψ) using the μ+μ- and e+e- decay channels in proton+proton collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the μ+μ- channel is for

Measurement of inclusive J/ψ suppression in Au+Au collisions at sNN=200 GeV through the dimuon channel at STAR

J/ψ suppression has long been considered a sensitive signature of the formation of the Quark-Gluon Plasma (QGP) in relativistic heavy-ion collisions. In this letter, we present the first measurement of inclusive J/ψ production at mid-rapidity through the dimuon decay channel in Au+Au collisions at sNN=200 GeV with the STAR experiment. These measurements became possible after the installation of the Muon Telescope Detector was completed in 2014. The J/ψ yields are measured in a wide transverse momentum (pT) range of 0.15 GeV/c to 12 GeV/c from central to peripheral collisions. They extend the kinematic reach of previous measurements at RHIC with improved precision. In the 0-10% most central collisions, the J/ψ yield is suppressed by a factor of approximately 3 for pT>5 GeV/c relative to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The J/ψ nuclear modification factor displays little dependence on pT in all centrality bins. Model calculations can qualitatively describe the data, providing further evidence for the color-screening effect experienced by J/ψ mesons in the QGP

Charge-dependent pair correlations relative to a third particle in p + Au and d + Au collisions at RHIC

Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions – the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in p+Au and d+Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-ion collisions. This bears important implications for the interpretation of the heavy-ion data

Observation of Excess J/ψ Yield at Very Low Transverse Momenta in Au+Au Collisions at sqrt[s_{NN}]=200 GeV and U+U Collisions at sqrt[s_{NN}]=193 GeV.

We report on the first measurements of J/ψ production at very low transverse momentum (p_{T}<0.2  GeV/c) in hadronic Au+Au collisions at sqrt[s_{NN}]=200  GeV and U+U collisions at sqrt[s_{NN}]=193  GeV. Remarkably, the inferred nuclear modification factor of J/ψ at midrapidity in Au+Au (U+U) collisions reaches about 24 (52) for p_{T}<0.05  GeV/c in the 60%-80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the dN/dt distribution of J/ψ for the very low p_{T} range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semicentral collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/ψ yield observed at extremely low p_{T} originates from coherent photon-nucleus interactions. In particular, coherently produced J/ψ's in violent hadronic collisions may provide a novel probe of the quark-gluon plasma

Resonances and fluctuations of strange particle in 200 GeV Au-Au collisions

We perform an analysis of preliminary data on strange particles yields and fluctuations within the Statistical hadronization model. We begin by describing the theoretical disagreements between different statistical models currently on the market. We then show how the simultaneous analysis of yields and fluctuations can be used to differentiate between the different models, and determine if one of them can be connected to underlying physics. We perform a study on a RHIC 200 GeV data sample that includes stable particles, resonances, and the event-by-event fluctuation of the $K/\pi$ ratio. We show that the equilibrium statistical model can not describe the fluctuation, unless an unrealistically small volume is assumed. Such small volume then makes it impossible to describe the total particle multiplicity. The non-equilibrium model,on the other hand, describes both the $K/\pi$ fluctuation and yields acceptably due to the extra boost to the $\pi$ fluctuation provided by the high pion chemical potential. $\Lambda(1520)$ and $K^*$ abundance is described within error bars, but the $\Sigma^*$ is under-predicted to $\sim$ 1.5 standard deviations. We suggest further measurements that have the potential to test the non-equilibrium model, as well as gauge the effect of re-interactions between hadronization and freeze-out.Comment: References added, equations corrected. As accepted for publication by Journal of Physics