842 research outputs found
VLA Observations of the Gravitational Lens System Q2237+0305
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
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
GeV is performed differentially, i.e. with respect to
correlations in azimuthal angle and pseudorapidity .
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
The shear viscosity 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 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 near fm is found at high collision energies. For the
charge-neutral system at MeV, a minimum of the ratio is reached for fm. To justify a hydrodynamic approach to
nucleus-nucleus collisions within the hadron phase the restriction from below,
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
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 plane of
strongly interacting matter
Multiplicity Fluctuations in Au+Au Collisions at RHIC
The preliminary data of the PHENIX collaboration for the scaled variances of
charged hadron multiplicity fluctuations in Au+Au at 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
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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