1,355 research outputs found
On ultrafast magnetic flux dendrite propagation into thin superconducting films
We suggest a new theoretical approach describing the velocity of magnetic
flux dendrite penetration into thin superconducting films. The key assumptions
for this approach are based upon experimental observations. We treat a dendrite
tip motion as a propagating flux jump instability. Two different regimes of
dendrite propagation are found. A fast initial stage is followed by a slow
stage, which sets in as soon as a dendrite enters into the vortex-free region.
We find that the dendrite velocity is inversely proportional to the sample
thickness. The theoretical results and experimental data obtained by a
magneto-optic pump-probe technique are compared and excellent agreement between
the calculations and measurements is found.Comment: 4 pages, 4 figure
pi-/pi+ ratio in heavy ions collisions: Coulomb effect or chemical equilibration?
We calculate the pi-/pi+ ratio for Pb+Pb at CERN/SPS energies and for Au+Au
at BNL/AGS energies using a (3+1) dimensional hydrodynamical model. Without
consideration of Coulomb effect an enhancement of this ratio at low mt is found
compatible with that observed in these experiments. Our calculations are based
on previous (3+1) dimensional hydrodynamical simulations (HYLANDER), which
described many other aspects of experimental data. In this model the observed
enhancement is a consequence of baryon and strangeness conservation and of
chemical equilibration of the system and is caused by the decay of produced
hyperons, which leads to a difference in the total number of positive and
negative pions as well. Based on the same approach, we also present results for
the pi-/pi+ ratio for S+S (CERN/SPS) collisions, where we find a similar
effect. The absence of the enhancement of the pi-/pi+ ratio in the S+S data
presented by the NA44 Collaboration, if confirmed, could indicate that chemical
equilibration has not yet been estabilished in this reaction.Comment: 8 pages and 2 figures, submmited to Phys. Lett. B. This reviewed
version (Nov.29,1996) contains more details about the model simulated
efficiency considering the experimental detection conditions. Other small
modifications were mad
Hydrodynamical analysis of single inclusive spectra and Bose-Einstein correlations for at 160 AGeV
We present the first analysis of preliminary data for at 160
using 3+1-dimensional relativistic hydrodynamics. We find excellent agreement
with the rapidity spectra of negative hadrons and the correlation measurements.
The data indicates a large amount of stopping; of the invariant energy
of the collision is thermalized and of the baryons are contained in the
central fireball. Within our model this implies that a quark-gluon-plasma of
lifetime 3.4 was formed.Comment: 13 pages, 5 Postscript figures (attached to this file as compressed
and uuencoded Postscript file
Finger patterns produced by thermomagnetic instability in superconductors
A linear analysis of thermal diffusion and Maxwell equations is applied to
study the thermomagnetic instability in a type-II superconducting slab. It is
shown that the instability can lead to formation of spatially nonuniform
distributions of magnetic field and temperature. The distributions acquire a
finger structure with fingers perpendicular to the screening current direction.
We derive the criterion for the instability, and estimate its build-up time and
characteristic finger width. The fingering instability emerges when the
background electric field is larger than a threshold field, , and the
applied magnetic field exceeds a value . Numerical
simulations support the analytical results, and allow to follow the development
of the fingering instability beyond the linear regime. The fingering
instability may be responsible for the nucleation of dendritic flux patterns
observed in superconducting films using magneto-optical imaging.Comment: 8 pages, 6 figures, accepted to Phys. Rev. B; (new version: minor
changes
Space-time extensions from space-time densities and Bose-Einstein correlations
Using a (3+1)-dimensional solution of the relativistic Euler-equations for
at 160 , space-time extensions of kaon emission zones are
calculated from space-time densities and compared to the inverse widths of
two-kaon Bose-Einstein correlation functions. The comparison shows a
satisfactory agreement and it is concluded that because of the Gaussian shape
of the kaon correlation functions, the space-time parameters of the kaon source
can be calculated directly from space-time densities. In the case of intensity
interferometry of identical pions this simplification is not recommended when
applying Gaussian fits because of the present strong effects of resonance
decays. The whole discussion is based on the assumption that hadron emission in
ultra-relativistic heavy-ion collisions is purely chaotic or that coherence is
at least negligible.Comment: 11 pages, 2 figures (distributed on 3 Postscript files
Hydrodynamical analysis of symmetric nucleus-nucleus collisions at CERN/SPS energies
We present a coherent theoretical study of ultrarelativistic heavy-ion data
obtained at the CERN/SPS by the NA35/NA49 Collaborations using 3+1-dimensional
relativistic hydrodynamics. We find excellent agreement with the rapidity
spectra of negative hadrons and protons and with the correlation measurements
in two experiments: at 200 and at 160 (preliminary
results). Within our model this implies that for () a
quark-gluon-plasma of initial volume 174 (24 ) with a lifetime 3.4
(1.5 ) was formed. It is found that the Bose-Einstein correlation
measurements do not determine the maximal effective radii of the hadron sources
because of the large contributions from resonance decay at small momenta. Also
within this study we present an NA49 acceptance corrected two-pion
Bose-Einstein correlation function in the invariant variable, .Comment: 21 pages, 11 Postscript figures (1 File, 775654 Bytes, has to be
requested for submission via e.mail from [email protected]
Probing the equation of state in the AGS energy range with 3-d hydrodynamics
The effect of (i) the phase transition between a quark gluon plasma (QGP) and
a hadron gas and (ii) the number of resonance degrees of freedom in the
hadronic phase on the single inclusive distributions of 16 different types of
produced hadrons for Au+Au collisions at AGS energies is studied.
We have used an exact numerical solution of the relativistic hydrodynamical
equations without free parameters which, because of its 3-d character,
constitutes a considerable improvement over the classical Landau solution.
Using two different equations of state (eos) - one containing a phase
transition from QGP to the Hadronic Phase and two versions of a purely hadronic
eos - we find that the first one gives an overall better description of the
Au+Au experimental data at energies.
We reproduce and analyse measured meson and proton spectra and also make
predictions for anti-protons, deltas, anti-deltas and hyperons. The low m_t
enhancement in pi- spectra is explained by baryon number conservation and
strangeness equilibration.
We also find that negative kaon data are more sensitive to the eos, as well
as the K-/pi- ratio. All hyperons and deltas are sensitive to the presence of a
phase transition in the forward rapidity region. Anti-protons, Omegas and heavy
anti-baryons are sensitive in the whole rapidity range.Comment: 25 pages (.tex) and 9 figures (.ps
Thermal photon production in heavy ion collisions
Using a three-dimensional hydrodynamic simulation of the collision and an
equation of state containing a first order phase transition to the quark-gluon
plasma, we study thermal photon production for collisions at
AGeV and for collisions at AGeV. We obtain
surprisingly high rates of thermal photons even at the lower energy, suggesting
that, contrary to what was expected so far, photon production may be an
interesting topic for experimental search also at the Alternating Gradient
Synchrotron. When applied to the reaction at AGeV, our model can
reproduce preliminary data obtained by the WA80 Collaboration without having to
postulate the existence of an extremely long-lived mixed phase as was recently
proposed.Comment: 9 pages, figures are uudecoded compressed and tare
Tracing vs. Partial Evaluation: Comparing Meta-Compilation Approaches for Self-Optimizing Interpreters
Tracing and partial evaluation have been proposed as meta-compilation techniques for interpreters to make just-in-time compilation language-independent. They promise that programs executing on simple interpreters can reach performance of the same order of magnitude as if they would be executed on state-of-the-art virtual machines with highly optimizing just-in-time compilers built for a specific language. Tracing and partial evaluation approach this meta-compilation from two ends of a spectrum, resulting in different sets of tradeoffs. This study investigates both approaches in the context of self-optimizing interpreters, a technique for building fast abstract-syntax-tree interpreters. Based on RPython for tracing and Truffle for partial evaluation, we assess the two approaches by comparing the impact of various optimizations on the performance of an interpreter for SOM, an object-oriented dynamically-typed language. The goal is to determine whether either approach yields clear performance or engineering benefits. We find that tracing and partial evaluation both reach roughly the same level of performance. SOM based on meta-tracing is on average 3x slower than Java, while SOM based on partial evaluation is on average 2.3x slower than Java. With respect to the engineering, tracing has however significant benefits, because it requires language implementers to apply fewer optimizations to reach the same level of performance
On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions
Within a hydrodynamical approach we investigate the sensitivity of single
inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three
different equations of state of nuclear matter. Two of the equations of state
are based on lattice QCD results and include a phase transition to a
quark-gluon plasma. The third equation of state has been extracted from the
microscopic transport code RQMD under the assumption of complete local
thermalization. All three equations of state provide reasonable fits to data
taken by the NA44 and NA49 Collaborations. The initial conditions before the
evolution of the fireballs and the space-time evolution pictures differ
dramatically for the three equations of state when the same freeze-out
temperature is used in all calculations. However, the softest of the equations
of state results in transverse mass spectra that are too steep in the central
rapidity region. We conclude that the transverse particle momenta are
determined by the effective softness of the equation of state during the
fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of
the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm
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