1,216 research outputs found
Entropy Production in Collisions of Relativistic Heavy Ions -- a signal for Quark-Gluon Plasma phase transition?
Entropy production in the compression stage of heavy ion collisions is
discussed within three distinct macroscopic models (i.e. generalized RHTA,
geometrical overlap model and three-fluid hydrodynamics). We find that within
these models \sim 80% or more of the experimentally observed final-state
entropy is created in the early stage. It is thus likely followed by a nearly
isentropic expansion. We employ an equation of state with a first-order phase
transition. For low net baryon density, the entropy density exhibits a jump at
the phase boundary. However, the excitation function of the specific entropy
per net baryon, S/A, does not reflect this jump. This is due to the fact that
for final states (of the compression) in the mixed phase, the baryon density
\rho_B increases with \sqrt{s}, but not the temperature T. Calculations within
the three-fluid model show that a large fraction of the entropy is produced by
nuclear shockwaves in the projectile and target. With increasing beam energy,
this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the
entropy of the newly produced particles around midrapidity. Hadron ratios are
calculated for the entropy values produced initially at beam energies from 2 to
200 AGeV.Comment: 17 pages, 8 figures, uses epsfig.sty; Submitted to Nucl.Phys.
``Pressure Equilibration'' in Ultrarelativistic Heavy Ion Collisions
We study the time scale for pressure equilibration in heavy ion collisions at
AGS energies within the three-fluid hydrodynamical model and a microscopic
cascade model (UrQMD). We find that kinetic equilibrium is reached in both
models after a time of 5 fm/c (center-of-mass time). Thus, observables which
are sensitive to the early stage of the reaction differ considerably from the
expectations within the instant thermalization scenario (one-fluid
hydrodynamical model).Comment: to be published in GSI annual scientific report 1997, psfig style
file neede
The SLAC Polarized Electron Source
The SLAC PES, developed in the early 1990s for the SLC, has been in
continuous use since 1992, during which time it has undergone numerous
upgrades. The upgrades include improved cathodes with their matching laser
systems, modified activation techniques and better diagnostics. The source
itself and its performance with these upgrades will be described with special
attention given to recent high-intensity long-pulse operation for the E-158
fixed-target parity-violating experiment.Comment: 6 pages, 4 figures, Workshop on Polarized Electron Sources and
Polarimeters (PESP 2002), September 4-6, 2002, Danvers, M
Going Further with Point Pair Features
Point Pair Features is a widely used method to detect 3D objects in point
clouds, however they are prone to fail in presence of sensor noise and
background clutter. We introduce novel sampling and voting schemes that
significantly reduces the influence of clutter and sensor noise. Our
experiments show that with our improvements, PPFs become competitive against
state-of-the-art methods as it outperforms them on several objects from
challenging benchmarks, at a low computational cost.Comment: Corrected post-print of manuscript accepted to the European
Conference on Computer Vision (ECCV) 2016;
https://link.springer.com/chapter/10.1007/978-3-319-46487-9_5
Hydrodynamic Models for Heavy-Ion Collisions, and beyond
A generic property of a first-order phase transition in equilibrium, and in
the limit of large entropy per unit of conserved charge, is the smallness of
the isentropic speed of sound in the ``mixed phase''. A specific prediction is
that this should lead to a non-isotropic momentum distribution of nucleons in
the reaction plane (for energies around 40 AGeV in our model calculation). On
the other hand, we show that from present effective theories for low-energy QCD
one does not expect the thermal transition rate between various states of the
effective potential to be much larger than the expansion rate, questioning the
applicability of the idealized Maxwell/Gibbs construction. Experimental data
could soon provide essential information on the dynamics of the phase
transition.Comment: 10 Pages, 4 Figures. Presented at 241st WE-Heraeus Seminar: Symposium
on Fundamental Issues in Elementary Matter: In Honor and Memory of Michael
Danos, Bad Honnef, Germany, 25-29 Sep 200
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