842 research outputs found

    VLA Observations of the Gravitational Lens System Q2237+0305

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    We report observations of the four-image gravitational lens system Q2237+0305 with the VLA at 20 cm and 3.6 cm. The quasar was detected at both frequencies (\approx 0.7 mJy) with a flat spectrum. All four lensed images are clearly resolved at 3.6 cm, and the agreement of the radio and optical image positions is excellent. No radio emission is detected from the lensing galaxy, and any fifth lensed quasar image must be fainter than \sim 20% of the A image flux density. Since the optical quasar images are variable and susceptible to extinction, radio flux ratios provide the best measurement of the macrolensing magnification ratios. The radio B/A and C/A image flux ratios are consistent with the observed range of optical variations, but the D/A ratio is consistently higher in the radio than in the optical. The radio ratios are consistent with magnification ratios predicted by lens models, and weaken alternative interpretations for Q2237+0305. More accurate radio ratios can distinguish between the models, as well as improve our understanding of both microlensing and extinction in this system.Comment: 1 postscript file, 13 pages. To appear in AJ (1996.09), Submitted 1996.03.13, Accepted 1996.05.2

    Jets propagation through a hadron-string medium

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    Di-jet correlations in nucleus-nucleus collisions are studied within the Hadron-String-Dynamics (HSD) transport approach taking into account the reaction of the medium on the jet energy loss nonperturbatively. A comparison with the STAR and PHENIX data in central Au+Au collisions at the RHIC energy s=200\sqrt{s}=200 GeV is performed differentially, i.e. with respect to correlations in azimuthal angle Δϕ\Delta \phi and pseudorapidity Δη\Delta \eta. The HSD results do not show enough suppression for the `away-side' jets in accordance with earlier perturbative studies. Furthermore, the `Mach-cone' structure for the angle distribution in the `away-side' jet as well as `ridge' long rapidity correlations in the `near-side' jet - observed by the STAR and PHOBOS Collaborations - are not seen in the HSD results, thus suggesting a partonic origin.Comment: 7 pages, 3 figure

    Viscosity in the excluded volume hadron gas model

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    The shear viscosity η\eta in the van der Waals excluded volume hadron-resonance gas model is considered. For the shear viscosity the result of the non-relativistic gas of hard-core particles is extended to the mixture of particles with different masses, but equal values of hard-core radius r. The relativistic corrections to hadron average momenta in thermal equilibrium are also taken into account. The ratio of the viscosity η\eta to the entropy density s is studied. It monotonously decreases along the chemical freeze-out line in nucleus-nucleus collisions with increasing collision energy. As a function of hard-core radius r, a broad minimum of the ratio η/s≈0.3\eta/s\approx 0.3 near r≈0.5r \approx 0.5 fm is found at high collision energies. For the charge-neutral system at T=Tc=180T=T_c=180 MeV, a minimum of the ratio η/s≅0.24\eta/s\cong 0.24 is reached for r≅0.53r\cong 0.53 fm. To justify a hydrodynamic approach to nucleus-nucleus collisions within the hadron phase the restriction from below, r ≥ 0.2r~ \ge ~0.2 fm, on the hard-core hadron radius should be fulfilled in the excluded volume hadron-resonance gas.Comment: 12 pages, 3 figure

    Fluctuations and Correlations in Nucleus-Nucleus Collisions within Transport Models

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    Particle number fluctuations and correlations in nucleus-nucleus collisions at SPS and RHIC energies are studied within microscopic transport approaches. In this review we focus on the Hadron-String-Dynamics (HSD) and Ultra-relativistic-Quantum-Molecular-Dynamics (UrQMD) models The obtained results are compared with the available experimental data as well as with the statistical models and the model of independent sources. In particular the role of the experimental centrality selection and acceptance is discussed in detail for a variety of experimental fluctuations and correlation observables with the aim to extract information on the critical point in the (T,μB)(T,\mu_B) plane of strongly interacting matter

    Multiplicity Fluctuations in Au+Au Collisions at RHIC

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    The preliminary data of the PHENIX collaboration for the scaled variances of charged hadron multiplicity fluctuations in Au+Au at s=200\sqrt{s}=200 GeV are analyzed within the model of independent sources. We use the HSD transport model to calculate the participant number fluctuations and the number of charged hadrons per nucleon participant in different centrality bins. This combined picture leads to a good agreement with the PHENIX data and suggests that the measured multiplicity fluctuations result dominantly from participant number fluctuations. The role of centrality selection and acceptance is discussed separately.Comment: 7 pages, 3 figures, submitted to Phys. Rev. C (Rapid Communication

    Forward-backward correlations in nucleus-nucleus collisions: baseline contributions from geometrical fluctuations

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    We discuss the effects of initial collision geometry and centrality bin definition on correlation and fluctuation observables in nucleus-nucleus collisions. We focus on the forward-backward correlation coefficient recently measured by the STAR Collaboration in Au+Au collisions at RHIC. Our study is carried out within two models: the Glauber Monte Carlo code with a `toy' wounded nucleon model and the hadron-string dynamics (HSD) transport approach. We show that strong correlations can arise due to averaging over events in one centrality bin. We, furthermore, argue that a study of the dependence of correlations on the centrality bin definition as well as the bin size may distinguish between these `trivial' correlations and correlations arising from `new physics'.Comment: 12 pages, 6 figure
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