224 research outputs found
Statistical Mechanics of semi-classical colored Objects
A microscopic model of deconfined matter based on color interactions between
semi-classical quarks is studied. A hadronization mechanism is imposed to
examine the properties and the disassembly of a thermalized quark plasma and to
investigate the possible existence of a phase transition from quark matter to
hadron matter.Comment: Submitted to Phys. Lett.
Direct photons in Pb+Pb at CERN-SPS from microscopic transport theory
Direct photon production in central Pb+Pb collisions at CERN-SPS energy is
calculated within the relativistic microscopic transport model UrQMD, and
within distinctly different versions of relativistic hydrodynamics. We find
that in UrQMD the local momentum distributions of the secondaries are strongly
elongated along the beam axis initially. Therefore, the pre-equilibrium
contribution dominates the photon spectrum at transverse momenta above GeV. The hydrodynamics prediction of a strong correlation between the
temperature and radial expansion velocities on the one hand and the slope of
the transverse momentum distribution of direct photons on the other hand thus
is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD
reveals that the initial conditions for the longitudinal expansion of the
photon source (the meson ``fluid'') resemble rather boostinvariance than
Landau-like flow.Comment: 14 pages, RevTex, 5 Encapsulated-PostScript Figure
Active control of the aerodynamic performance and tonal noise of axial turbomachines
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Two projects on axial fans are presented in this paper. The first aims at improving the aerodynamic performance and reducing the tip clearance noise caused by the rotating instability. This is achieved by injecting air into the radial gap between the impeller blade tips and the fan casing. Steady air injection with small mass flows results in remarkable reductions in the noise level along with improved aerodynamic performance. Larger injected mass flows give significant improvements in the aerodynamic performance at the expense of an increased overall noise level. The second project uses flow control actively to reduce the blade passage frequency (BPF) level. Experimental results are presented for steady jets injected into the main flow and cylindrical rods at axial positions downstream of the impeller blades. The method is successful for higher-order mode sound fields where the BPF level is reduced by up to 20.5 dB.DFB, SFB 557, Beeinflussung komplexer turbulenter Scherströmunge
The Origin of Transverse Flow at the SPS
We study the transverse expansion in central Pb+Pb collisions at the CERN
SPS. Strong collective motion of hadrons can be created. This flow is mainly
due to meson baryon rescattering. It allows to study the angular distribution
of intermediate mass meson baryon interactions.Comment: submitted to Phys. Lett.
Microscopic Models for Ultrarelativistic Heavy Ion Collisions
In this paper, the concepts of microscopic transport theory are introduced
and the features and shortcomings of the most commonly used ansatzes are
discussed. In particular, the Ultrarelativistic Quantum Molecular Dynamics
(UrQMD) transport model is described in great detail. Based on the same
principles as QMD and RQMD, it incorporates a vastly extended collision term
with full baryon-antibaryon symmetry, 55 baryon and 32 meson species. Isospin
is explicitly treated for all hadrons. The range of applicability stretches
from GeV/nucleon, allowing for
a consistent calculation of excitation functions from the intermediate energy
domain up to ultrarelativistic energies. The main physics topics under
discussion are stopping, particle production and collective flow.Comment: 129 pages, pagestyle changed using US letter (8.5x11 in) format. The
whole paper (13 Mb ps file) could also be obtained from
ftp://ftp.th.physik.uni-frankfurt.de/pub/urqmd/ppnp2.ps.g
Study of non-equilibrium effects and thermal properties of heavy ion collisions using a covariant approach
Non-equilibrium effects are studied using a full Lorentz-invariant formalism.
Our analysis shows that in reactions considered here, no global or local
equilibrium is reached. The heavier masses are found to be equilibrated more
than the lighter systems. The local temperature is extracted using hot Thomas
Fermi formalism generalized for the case of two interpenetrating pieces of
nuclear matter. The temperature is found to vary linearly with bombarding
energy and impact parameter whereas it is nearly independent of the mass of the
colliding nuclei. This indicates that the study of temperature with medium size
nuclei is also reliable. The maximum temperatures obtained in our approach are
in a nice agreement with earlier calculations of other approaches. A simple
parametrization of maximal temperature as a function of the bombarding energy
is also given.Comment: LaTex-file, 17 pages, 8 figures (available upon request), Journal of
Physics G20 (1994) 181
FACT - Long-term Monitoring of Bright TeV-Blazars
Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated
successfully on the Canary Island of La Palma. Apart from the proof of
principle for the use of G-APDs in Cherenkov telescopes, the major goal of the
project is the dedicated long-term monitoring of a small sample of bright TeV
blazars. The unique properties of G-APDs permit stable observations also during
strong moon light. Thus a superior sampling density is provided on time scales
at which the blazar variability amplitudes are expected to be largest, as
exemplified by the spectacular variations of Mrk 501 observed in June 2012.
While still in commissioning, FACT monitored bright blazars like Mrk 421 and
Mrk 501 during the past 1.5 years so far. Preliminary results including the Mrk
501 flare from June 2012 will be presented.Comment: 4 pages, 4 figures, presented at the 33rd ICRC (2013
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