Latest documents http://publikationen.ub.uni-frankfurt.de/index/index/ Tue, 31 Oct 2006 09:40:57 +0100 Tue, 31 Oct 2006 09:40:57 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2172 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. Kai Oliver Schweda; Xianglei Zhu; Marcus Bleicher; Shengli Huang; Horst Stöcker; Nu Xu; Pengfei Zhuang preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2172 Tue, 31 Oct 2006 09:40:57 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2758 The transverse momentum dependence of the anisotropic flow v_2 for pi, K, nucleon, Lambda, Xi and Omega is studied for Au+Au collisions at sqrt s_NN = 200 GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured v_2, they both predict the particle type dependence of v_2, as observed by the RHIC experiments: v_2 exhibits a hadron-mass hierarchy (HMH) in the low p_T region and a number-of-constituent-quark (NCQ) dependence in the intermediate p_T region. The failure of the hadronic models to reproduce the absolute magnitude of the observed v_2 indicates that transport calculations of heavy ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependencies observed in heavy-ion collisions at intermediate p_T are related to the hadronic cross sections in vacuum rather than to the hadronization process itself, as suggested by quark recombination models. Yang Lu; Marcus Bleicher; Fuming Liu; Zuhua Liu; Paul Sorensen; Horst Stöcker; Nu Xu; Xianglei Zhu preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2758 Thu, 11 May 2006 13:17:05 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2773 We propose to measure 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, shown on the example of azimuthal correlations of D-Dbar pairs. We show that hadronic interactions at the late stage can not disturb these correlations significantly. Thus, a decrease or the complete absence of these initial correlations indicates frequent interactions of heavy-flavor quarks in the partonic stage. Therefore, early thermalization of light quarks is likely to be reached. PACS numbers: 25.75.-q Xianglei Zhu; Marcus Bleicher; Shengli Huang; Kai Oliver Schweda; Horst Stöcker; Hermine K. Wöhri; Nu Xu; Pengfei Zhuang preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2773 Tue, 09 May 2006 15:59:18 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3143 Using a microscopic transport model together with a coalescence after-burner, we study the formation of deuterons in Au + Au central collisions at s = 200 AGeV . It is found that the deuteron transverse momentum distributions are strongly a ected by the nucleon space-momentum correlations, at the moment of freeze-out, which are mostly determined by the number of rescatterings. This feature is useful for studying collision dynamics at ultrarelativistic energies. B. Monreal; Steffen A. Bass; Marcus Bleicher; S. Esumi; Walter Greiner; Qingfeng Li; H. Liu; W.J. Llope; Raffaele Mattiello; Sergey Y. Panitkin; I. Sakrejda; Raimond Snellings; Heinz Sorge; Christian Spieles; Horst Stöcker; J. Thomas; S. Voloshin; F. Wang; Nu Xu preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3143 Thu, 01 Dec 2005 10:14:29 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3205 In ultra-relativistic heavy ion collisions, early stage multiple scatterings may lead to an increase of the color electric field strength. Consequently, particle production - especially heavy quark (and di-quark) production - is greatly enhanced according to the Schwinger mechanism. We test this idea via the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) for Au+Au collisions at the full RHIC energy (ps = 200 AGeV). Relative to p+p collisions, a factor of 60, 20 and 7 enhancement respectively, for (sss), (ss), and , (s) is predicted for a model with increased color electric field strength. Marcus Bleicher; Walter Greiner; Horst Stöcker; Nu Xu preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3205 Tue, 22 Nov 2005 10:34:03 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3355 We study the collective flow of open charm mesons and charmonia in Au + Au collisions at s = 200 GeV within the hadron-string-dynamics (HSD) transport approach. The detailed studies show that the coupling of D, mesons to the light hadrons leads to comparable directed and elliptic flow as for the light mesons. This also holds approximately for J/ mesons since more than 50% of the final charmonia for central and midcentral collisions stem from D + induced reactions in the transport calculations. The transverse momentum spectra of D, mesons and J/ s are only very moderately changed by the (pre-)hadronic interactions in HSD, which can be traced back to the collective flow generated by elastic interactions with the light hadrons. PACS-Nr. 25.75.-q, 13.60.Le, 14.40.Lb, 14.65.Dw Elena L. Bratkovskaya; Wolfgang Cassing; Horst Stöcker; Nu Xu article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3355 Tue, 08 Nov 2005 11:14:07 +0100 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3924 The ultrarelativistic quantum molecular dynamics model (UrQMD) is used to study global observables in central reactions of Au+Au at sqrt[s]=200A GeV at the Relativistic Heavy Ion Collider (RHIC). Strong stopping governed by massive particle production is predicted if secondary interactions are taken into account. The underlying string dynamics and the early hadronic decoupling implies only small transverse expansion rates. However, rescattering with mesons is found to act as a source of pressure leading to additional flow of baryons and kaons, while cooling down pions. Marcus Bleicher; Steffen A. Bass; Larissa V. Bravina; Walter Greiner; Sven Soff; Horst Stöcker; Nu Xu; Eugene E. Zabrodin article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/3924 Wed, 21 Sep 2005 18:13:01 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/4817 We calculate the antibaryon-to-baryon ratios, anti-p/p, anti-Lambda/Lambda, anti-Xi/Xi, and anti-Omega/Omega for Au+Au collisions at RHIC (sqrt{s}_{NN}=200 GeV). The effects of strong color fields associated with an enhanced strangeness and diquark production probability and with an effective decrease of formation times are investigated. Antibaryon-to-baryon ratios increase with the color field strength. The ratios also increase with the strangeness content |S|. The netbaryon number at midrapidity considerably increases with the color field strength while the netproton number remains roughly the same. This shows that the enhanced baryon transport involves a conversion into the hyperon sector (hyperonization) which can be observed in the (Lambda - anti-Lambda)/(p - anti-p) ratio. Sven Soff; Jørgen Randrup; Horst Stöcker; Nu Xu preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/4817 Wed, 13 Apr 2005 16:56:07 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/4830 We investigate the effects of strong color fields and of the associated enhanced intrinsic transverse momenta on the phi-meson production in ultrarelativistic heavy ion collisions at RHIC. The observed consequences include a change of the spectral slopes, varying particle ratios, and also modified mean transverse momenta. In particular, the composition of the production processes of phi-mesons, that is, direct production vs. coalescence-like production, depends strongly on the strength of the color fields and intrinsic transverse momenta and thus represents a sensitive probe for their measurement. Sven Soff; Srikumar Kesavan; Jørgen Randrup; Horst Stöcker; Nu Xu preprint http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/4830 Wed, 13 Apr 2005 12:07:38 +0200