Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Sat, 05 Dec 2009 08:38:20 +0200 Sat, 05 Dec 2009 08:38:20 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/6516 Rezension zu: Peter Woit : Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics. Verlag Vintage, London 2006, 290 Seiten, 27,89 Euro. Marcus Bleicher review http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/6516 Tue, 12 May 2009 08:38:20 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/927 Schwarze Löcher – das sind im Allgemeinen alles verschlingende, gigantisch schwere astronomische Objekte mit bis zu einigen Milliarden Sonnenmassen. Am Frankfurt Institute for Advanced Studies (FIAS) und am Institut für Theoretische Physik sind in den vergangenen fünf Jahren eine ganz neue Art von Schwarzen Löchern theoretisch vorhergesagt worden, die genau das Gegenteil der astronomisch gemessenen Giganten darstellen, nämlich winzig kleine Schwarze Löcher, so genannte »mini black holes«. Auftreten könnten sie, wenn im kommenden Jahr der neue Teilchenbeschleuniger am CERN in Genf in Betrieb genommen wird. Marcus Bleicher; Horst Stöcker article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/927 Mon, 18 Jun 2007 09:07:52 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/948 The effects of the onset of deconfinement on longitudinal and transverse flow are studied. First, we analyze longitudinal pion spectra from Elab = 2A GeV to √sNN = 200 GeV within Landau’s hydrodynamical model and the UrQMD transport approach. From the measured data on the widths of the pion rapidity spectra, we extract the sound velocity c2s in the early stage of the reactions. It is found that the sound velocity has a local minimum (indicating a softest point in the equation of state, EoS) at Ebeam = 30A GeV. This softening of the EoS is compatible with the assumption of the formation of a mixed phase at the onset of deconfinement. Furthermore, the energy excitation function of elliptic flow (v2) from Ebeam = 90A MeV to √sNN = 200 GeV is explored within the UrQMD framework and discussed in the context of the available data. The transverse flow should also be sensitive to changes in the equation of state. Therefore, the underestimation of elliptic flow by the UrQMD model calculation above Elab = 30A GeV might also be explained by assuming a phase transition from a hadron gas to the quark gluon plasma around this energy. This would be consistent with the model calculations, indicating a transition from hadronic matter to “string matter” in this energy range. Hannah Petersen; Marcus Bleicher article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/948 Thu, 14 Jun 2007 13:17:03 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/1689 Based on the UrQMD transport model, the transverse momentum and the rapidity dependence of the Hanbury-Brown-Twiss (HBT) radii R_L, R_O, R_S as well as the cross term R_OL at SPS energies are investigated and compared with the experimental NA49 and CERES data. The rapidity dependence of the R_L, R_O, R_S is weak while the R_OL is significantly increased at large rapidities and small transverse momenta. The HBT "life-time" issue (the phenomenon that the calculated sqrt R_O^2-R_S^2 value is larger than the correspondingly extracted experimental data) is also present at SPS energies. Qingfeng Li; Marcus Bleicher; Xianglei Zhu; Horst Stöcker preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/1689 Fri, 16 Feb 2007 14:41:50 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2173 We examine experimental signatures of TeV-mass black hole formation in heavy ion collisions at the LHC. We find that the black hole production results in a complete disappearance of all very high p_T (> 500 GeV) back-to-back correlated di-jets of total mass M > M_f ~ 1 TeV. We show that the subsequent Hawking-decay produces multiple hard mono-jets and discuss their detection. We study the possibility of cold black hole remnant (BHR) formation of mass ~ M_f and the experimental distinguishability of scenarios with BHRs and those with complete black hole decay. Finally we point out that a Heckler-Kapusta-Hawking plasma may form from the emitted mono-jets. In this context we present new simulation data of Mach shocks and of the evolution of initial conditions until the freeze-out. Barbara Betz; Marcus Bleicher; Ulrich Harbach; Thomas J. Humanic; Benjamin Koch; Horst Stöcker preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2173 Tue, 31 Oct 2006 10:01:29 +0100 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/2172 Tue, 31 Oct 2006 09:40:57 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3418 We study forward-backward charge fluctuations to probe the correlations among produced particles in ultra relativistic heavy ion collisions. We develop a model that describes the forward-backward dynamical fluctuations and apply it to interpret the recent PHOBOS data. Within the present model, the dynamical fluctuations are related to the particle production mechanism via cluster decay and to long range correlations between the forward and backward rapidity hemispheres. We argue that with a tight centrality cut, PHOBOS may see a strong decrease of the dynamical fluctuations. Within the present model, this deterioration of the correlation among the produced hadrons can be interpreted as a sign for the production of a hot, dense and interacting medium. Mohamed Abdel-Aziz; Marcus Bleicher preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/3418 Wed, 14 Jun 2006 12:50:26 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2743 We demonstrate the occurrence of canonical suppression associated with the conservation of an U(1)-charge in current transport models. For this study a pion gas is simulated within two different transport approaches by incorporating inelastic and volume-limited collisions pi pi leftrightarrow K bar-K for the production of kaon pairs. Both descriptions can dynamically account for the suppression in the yields of rare strange particles in a limited box, being in full accordance with a canonical statistical description. Oliver Fochler; Sascha Vogel; Marcus Bleicher; Carsten Greiner; Peter Koch-Steinheimer; Zhe Xu preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2743 Fri, 12 May 2006 10:23:09 +0200 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/2758 Thu, 11 May 2006 13:17:05 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2775 Several observables of unbound nucleons which are to some extent sensitive to the medium modifications of nucleon-nucleon elastic cross sections in neutron-rich intermediate energy heavy ion collisions are investigated. The splitting effect of neutron and proton effective masses on cross sections is discussed. It is found that the transverse flow as a function of rapidity, the Q_zz as a function of momentum, and the ratio of halfwidths of the transverse to that of longitudinal rapidity distribution R_t/l are very sensitive to the medium modifications of the cross sections. The transverse momentum distribution of correlation functions of two-nucleons does not yield information on the in-medium cross section. Qingfeng Li; Zhuxia Li; Sven Soff; Marcus Bleicher; Horst Stöcker preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2775 Wed, 10 May 2006 09:28:02 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2774 The Lee-Yang zeroes method is applied to study elliptic flow (v_2) in Au+Au collisions at sqrt s =200 A GeV, with the UrQMD model. In this transport approach, the true event plane is known and both the nonflow effects and event-by-event v_2 fluctuations exist. Although the low resolutions prohibit the application of the method for most central and peripheral collisions, the integral and differential elliptic flow from the Lee-Yang zeroes method agrees with the exact v_2 values very well for semi-central collisions. Xianglei Zhu; Marcus Bleicher; Horst Stöcker preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2774 Wed, 10 May 2006 09:24:16 +0200 http://publikationen.stub.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.stub.uni-frankfurt.de/frontdoor/index/index/docId/2773 Tue, 09 May 2006 15:59:18 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2779 The pion source as seen through HBT correlations at RHIC energies is investigated within the UrQMD approach. We find that the calculated transverse momentum, centrality, and system size dependence of the Pratt-HBT radii R_L and R_S are reasonably well in line with experimental data. The predicted R_O values in central heavy ion collisions are larger as compared to experimental data. The corresponding quantity sqrt R_O^2-R_S^2 of the pion emission source is somewhat larger than experimental estimates. Qingfeng Li; Marcus Bleicher; Horst Stöcker preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2779 Tue, 09 May 2006 15:46:37 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2778 The effects of classical gravitational radiation in models with large extra dimensions are investigated for ultra high energy cosmic rays (CRs). The cross sections are implemented into a simulation package (SENECA) for high energy hadron induced CR air showers. We predict that gravitational radiation from quasi-elastic scattering could be observed at incident CR energies above 10^9 GeV for a setting with more than two extra dimensions. It is further shown that this gravitational energy loss can alter the energy reconstruction for CR energies E_CR > 5 10^9 GeV. Benjamin Koch; Hans-Joachim Drescher; Marcus Bleicher preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2778 Tue, 09 May 2006 15:42:07 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2777 We discuss modifications of the neutrino-nucleon cross section due to a minimal length scale. It is shown that the enhancement of the nu-N cross section due to new physics is suppressed. Especially the potential observation rate of micro black holes at neutrino telescopes is strongly reduced. Ulrich Harbach; Marcus Bleicher preprint http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2777 Tue, 09 May 2006 15:36:23 +0200