661 research outputs found

    Measurements of Ks, Lambda and Xi from Au+Au collisions at \sqrt{s_NN} = 7.7, 11.5 and 39 GeV in STAR

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    We report on the measurements of Ks, Lambda and Xi spectra at mid-rapidity (|y|<0.5) in the most central (0-5%) Au+Au collisions at \sqrt{s_NN} = 7.7, 11.5 and 39 GeV from the STAR experiment. The extracted yields and the corresponding data from Pb+Pb collisions measured by the NA49 and CERES experiments at SPS are consistent. The Lambda, Anti-Lambda, Xi- and Xi+ to pion ratios agree well with the predictions from a statistical hadronization model at all three energies.Comment: 6 pages, 4 figures, to appear in the proceedings for Strangeness in Quark Matter 2011, Cracow, Poland, September 18-24, 201

    Elliptic Flow Analysis at RHIC: Fluctuations vs. Non-Flow Effects

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    The cumulant method is applied to study elliptic flow (v2v_2) in Au+Au collisions at s=200\sqrt{s}=200AGeV, with the UrQMD model. In this approach, the true event plane is known and both the non-flow effects and event-by-event spatial (ŌĶ\epsilon) and v2v_2 fluctuations exist. Qualitatively, the hierarchy of v2v_2's from two, four and six-particle cumulants is consistent with the STAR data, however, the magnitude of v2v_2 in the UrQMD model is only 60% of the data. We find that the four and six-particle cumulants are good measures of the real elliptic flow over a wide range of centralities except for the most central and very peripheral events. There the cumulant method is affected by the v2v_2 fluctuations. In mid-central collisions, the four and six-particle cumulants are shown to give a good estimation of the true differential v2v_2, especially at large transverse momentum, where the two-particle cumulant method is heavily affected by the non-flow effects.Comment: 7 pages, 7 figures, revtex 4; The discussion on the non-flow effects is extended, a new figure (Fig.3) on v2-eccentricity correlation is added, accepted for publication in Phys. Rev.

    Directed and elliptic flow in heavy ion collisions from Ebeam=90E_{\rm beam}=90 MeV/nucleon to Ec.m.=200E_{\rm c.m.}=200 GeV/nucleon

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    Recent data from the NA49 experiment on directed and elliptic flow for Pb+Pb reactions at CERN-SPS are compared to calculations with a hadron-string transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. The rapidity and transverse momentum dependence of the directed and elliptic flow, i.e. v1v_1 and v2v_2, are investigated. The flow results are compared to data at three different centrality bins. Generally, a reasonable agreement between the data and the calculations is found. Furthermore, the energy excitation functions of v1v_1 and v2v_2 from Ebeam=90AE_{\rm beam}=90A MeV to Ecm=200AE_{\rm cm}=200A GeV are explored within the UrQMD framework and discussed in the context of the available data. It is found that, in the energy regime below Ebeam‚ȧ10AE_{\rm beam}\leq 10A GeV, the inclusion of nuclear potentials is necessary to describe the data. Above 40A40A GeV beam energy, the UrQMD model starts to underestimate the elliptic flow. Around the same energy the slope of the rapidity spectra of the proton directed flow develops negative values. This effect is known as the third flow component ("antiflow") and cannot be reproduced by the transport model. These differences can possibly be explained by assuming a phase transition from hadron gas to quark gluon plasma at about 40A40A GeV.Comment: 19 pages, minor changes and modified title as published in PR

    DD correlations as a sensitive probe for thermalization in high-energy nuclear collisions

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    We propose to measure azimuthal correlations of heavy-flavor hadrons to address the status of thermalization at the partonic stage of light quarks and gluons in high-energy nuclear collisions. In particular, we show that hadronic interactions at the late stage cannot significantly disturb the initial back-to-back azimuthal correlations of DDbar pairs. Thus, a decrease or the complete absence of these initial correlations does indicate frequent interactions of heavy-flavor quarks and also light partons in the partonic stage, which are essential for the early thermalization of light partons

    Deformable Generator Network: Unsupervised Disentanglement of Appearance and Geometry

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    We present a deformable generator model to disentangle the appearance and geometric information for both image and video data in a purely unsupervised manner. The appearance generator network models the information related to appearance, including color, illumination, identity or category, while the geometric generator performs geometric warping, such as rotation and stretching, through generating deformation field which is used to warp the generated appearance to obtain the final image or video sequences. Two generators take independent latent vectors as input to disentangle the appearance and geometric information from image or video sequences. For video data, a nonlinear transition model is introduced to both the appearance and geometric generators to capture the dynamics over time. The proposed scheme is general and can be easily integrated into different generative models. An extensive set of qualitative and quantitative experiments shows that the appearance and geometric information can be well disentangled, and the learned geometric generator can be conveniently transferred to other image datasets to facilitate knowledge transfer tasks.Comment: version
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