20,938 research outputs found
Quark-Gluon-Plasma Formation at SPS Energies?
By colliding ultrarelativistic ions, one achieves presently energy densities
close to the critical value, concerning the formation of a quark-gluon-plasma.
This indicates the importance of fluctuations and the necessity to go beyond
the investigation of average events. Therefore, we introduce a percolation
approach to model the final stage ( fm/c) of ion-ion collisions, the
initial stage being treated by well-established methods, based on strings and
Pomerons. The percolation approach amounts to finding high density domains, and
treating them as quark-matter droplets. In this way, we have a {\bf realistic,
microscopic, and Monte--Carlo based model which allows for the formation of
quark matter.} We find that even at SPS energies large quark-matter droplets
are formed -- at a low rate though. In other words: large quark-matter droplets
are formed due to geometrical fluctuation, but not in the average event.Comment: 7 Pages, HD-TVP-94-6 (1 uuencoded figure
Semihard Interactions in Nuclear Collisions Based on a Unified Approach to High Energy Scattering
Our ultimate goal is the construction of a model for interactions of two
nuclei in the energy range between several tens of GeV up to several TeV per
nucleon in the centre-of-mass system. Such nuclear collisions are very complex,
being composed of many components, and therefore some strategy is needed to
construct a reliable model. The central point of our approach is the
hypothesis, that the behavior of high energy interactions is universal
(universality hypothesis). So, for example, the hadronization of partons in
nuclear interactions follows the same rules as the one in electron-positron
annihilation; the radiation of off-shell partons in nuclear collisions is based
on the same principles as the one in deep inelastic scattering. We construct a
model for nuclear interactions in a modular fashion. The individual modules,
based on the universality hypothesis, are identified as building blocks for
more elementary interactions (like e^+ e^-, lepton-proton), and can therefore
be studied in a much simpler context. With these building blocks under control,
we can provide a quite reliable model for nucleus-nucleus scattering, providing
in particular very useful tests for the complicated numerical procedures using
Monte Carlo techniques.Comment: 10 pages, no figures; Proc. of the ``Workshop on Nuclear Matter in
Different Phases and Transitions'', Les Houches, France, March 31 - April 10,
199
Models for RHIC and LHC: New Developments
We outline inconsistencies in presently used models for high energy nuclear
scattering, which make their application quite unreliable. Many "successes" are
essentially based on an artificial freedom of parameters, which does not exist
when the models are constructed properly.
The problem is the fact that any multiple scattering theory requires an
appropriate treatment of the energy sharing between the individual
interactions, which is technically very difficult to implement. Lacking a
satisfying solution to this problem, it has been simply ignored.
We introduce a fully self-consistent formulation of the multiple-scattering
scheme. Inclusion of soft and hard components - very crucial at high energies -
appears in a "natural way", providing a smooth transition from soft to hard
physics.
We can show that the effect of appropriately considering energy conservation
has a big influence on the results, and MUST therefore be included in any
serious calculation.Comment: talk given at the ``15thInternational Conference on Ultrarelativistic
Nucleus-Nucleus Collisions'', Quark Matter 2001, Stony Brook, USA, January
15-20, 200
The Nexus Model
The interpretation of experimental results at RHIC and in the future also at
LHC requires very reliable and realistic models.
Considerable effort has been devoted to the development of such models during
the past decade, many of them being heavily used in order to analyze data.
There are, however, serious inconsistencies in the above-mentioned approaches.
In this paper, we will introduce a fully self-consistent formulation of the
multiple-scattering scheme in the framework of a Gribov-Regge type effective
theory.Comment: Invited talk given at the International Workshop on the Physics of
the Quark Gluon Plasma, Palaiseau, France, September 4-7, 200
On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars
About a quarter of all post-asymptotic giant branch (AGB) stars are
hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated
H-deficient stars by a (very) late thermal pulse scenario where the
hydrogen-rich envelope is mixed with the helium-rich intershell layer.
Depending on the particular time at which the final flash occurs, the entire
hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars
and their evolution are yet not understood. A small group of very hot,
helium-dominated stars is formed by O(He)-type stars. We performed a detailed
spectral analysis of ultraviolet and optical spectra of four O(He) stars by
means of state-of-the-art non-LTE model-atmosphere techniques. We determined
effective temperatures, surface gravities, and the abundances of H, He, C, N,
O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss
rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss.
The comparison with evolutionary models shows that the status of the O(He)
stars remains uncertain. Their abundances match predictions of a double helium
white dwarf merger scenario, suggesting that they might be the progeny of the
compact and of the luminous helium-rich sdO-type stars. The existence of
planetary nebulae that do not show helium enrichment around every other O(He)
star, precludes a merger origin for these stars. These stars must have formed
in a different way, for instance via enhanced mass-loss during their post-AGB
evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant
or AGB star. A helium-dominated stellar evolutionary sequence exists, that may
be fed by different types of mergers or CE scenarios. It appears likely, that
all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&
Effect of long range forces on the interfacial profiles in thin binary polymer films
We study the effect of surface fields on the interfacial properties of a
binary polymer melt confined between two parallel walls. Each wall attracts a
different component of the blend by a non-retarded van der Waals potential. An
interface which runs parallel to the surfaces is stabilized in the center of
the film. Using extensive Monte Carlo simulations we study the interfacial
properties as a function of the film thickness, the strength of the surface
forces and the lateral size over which the profiles across the film are
averaged. We find evidence for capillary wave broadening of the apparent
interfacial profiles. However, the apparent interfacial width cannot be
described quantitatively by a simple logarithmic dependence on the film
thickness. The Monte Carlo simulations reveal that the surface fields give rise
to an additional reduction of the intrinsic interfacial width and an increase
of the effective interfacial tension upon decreasing the film thickness. These
modifications of the intrinsic interfacial properties are confirmed by
self-consistent field calculations. Taking account of the thickness dependence
of the intrinsic interfacial properties and the capillary wave broadening, we
can describe our simulation results quantitatively.Comment: to appear in J.Chem.Phy
Microcanonical Treatment of Hadronizing the Quark-Gluon Plasma
We recently introduced a completely new way to study ultrarelativistic
nuclear scattering by providing a link between the string model approach and a
statistical description. A key issue is the microcanonical treatment of
hadronizing individual quark matter droplets. In this paper we describe in
detail the hadronization of these droplets according to n-body phase space, by
using methods of statistical physics, i.e. constructing Markov chains of hadron
configurations.Comment: Complete paper enclosed as postscript file (uuencoded
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