1,977 research outputs found
Gold-plated processes at photon colliders
We review the most important topics and objectives of the physics program of
the gamma-gamma, gamma-electron collider (photon collider) option for an e+e-
linear collider.Comment: 36 pages, Latex, 11 figures(ps,eps), Talk at Intern. Workshop on High
Energy Photon Colliders; June 14-17, 2000, DESY, Hamburg, Germany; to be
published in Nucl. Instr. and Methods
Endogenous transforming growth factor β1 suppresses inflammation and promotes survival in adult CNS
Transforming growth factor β1 (TGFβ1) is a pleiotropic cytokine with potent neurotrophic and immunosuppressive properties that is upregulated after injury, but also expressed in the normal nervous system. In the current study, we examined the regulation of TGFβ1 and the effects of TGFβ1 deletion on cellular response in the uninjured adult brain and in the injured and regenerating facial motor nucleus. To avoid lethal autoimmune inflammation within 3 weeks after birth in TGFβ1-deficient mice, this study was performed on a T- and B-cell-deficient RAG2-/- background. Compared with wild-type siblings, homozygous deletion of TGFβ1 resulted in an extensive inflammatory response in otherwise uninjured brain parenchyma. Astrocytes increased in GFAP and CD44 immunoreactivity; microglia showed proliferative activity, expression of phagocytosis-associated markers [αXβ2, B7.2, and MHC1 (major histocompatibility complex type 1)], and reduced branching. Ultrastructural analysis revealed focal blockade of axonal transport, perinodal damming of axonal organelles, focal demyelination, and myelin debris in granule-rich, phagocytic microglia. After facial axotomy, absence of TGFβ1 led to a fourfold increase in neuronal cell death (52 vs 13%), decreased central axonal sprouting, and significant delay in functional recovery. It also interfered with the microglial response, resulting in a diminished expression of early activation markers [ICAM1 (intercellular adhesion molecule 1), α6β1, and αMβ2] and reduced proliferation. In line with axonal and glial findings in the otherwise uninjured CNS, absence of endogenous TGFβ1 also caused an ∼10% reduction in the number of normal motoneurons, pointing to an ongoing and potent trophic role of this anti-inflammatory cytokine in the normal as well as in the injured brain. Copyright © 2007 Society for Neuroscience
Evolution of the universality class in slightly diluted (1>p>0.8) Ising systems
The crossover of a pure (undiluted) Ising system (spin per site probability
p=1) to a diluted Ising system (spin per site probability p<0.8) is studied by
means of Monte Carlo calculations with p ranging between 1 and 0.8 at intervals
of 0.025. The evolution of the self averaging is analyzed by direct
determination of the normalized square widths for magnetization and
susceptibility as a function of p. We find a monotonous and smooth evolution
from the pure to the randomly diluted universality class. The p-dependent
transition is found to be independent of the size (L). This property is very
convenient for extrapolation towards the randomly diluted universality class
avoiding complications resulting from finite size effects.Comment: 15 pages, 6 figures, RevTe
Colloids in light fields: particle dynamics in random and periodic energy landscapes
The dynamics of colloidal particles in potential energy landscapes have
mainly been investigated theoretically. In contrast, here we discuss the
experimental realization of potential energy landscapes with the help of light
fields and the observation of the particle dynamics by video microscopy. The
experimentally observed dynamics in periodic and random potentials are compared
to simulation and theoretical results in terms of, e.g. the mean-squared
displacement, the time-dependent diffusion coefficient or the non-Gaussian
parameter. The dynamics are initially diffusive followed by intermediate
subdiffusive behaviour which again becomes diffusive at long times. How
pronounced and extended the different regimes are, depends on the specific
conditions, in particular the shape of the potential as well as its roughness
or amplitude but also the particle concentration. Here we focus on dilute
systems, but the dynamics of interacting systems in external potentials, and
thus the interplay between particle-particle and particle-potential
interactions, is also mentioned briefly. Furthermore, the observed dynamics of
dilute systems resemble the dynamics of concentrated systems close to their
glass transition, with which it is compared. The effect of certain potential
energy landscapes on the dynamics of individual particles appears similar to
the effect of interparticle interactions in the absence of an external
potential
Friction Drag on a Particle Moving in a Nematic Liquid Crystal
The flow of a liquid crystal around a particle does not only depend on its
shape and the viscosity coefficients but also on the direction of the
molecules. We studied the resulting drag force on a sphere moving in a nematic
liquid crystal (MBBA) in a low Reynold's number approach for a fixed director
field (low Ericksen number regime) using the computational artificial
compressibility method. Taking the necessary disclination loop around the
sphere into account, the value of the drag force anisotropy
(F_\perp/F_\parallel=1.50) for an exactly computed field is in good agreement
with experiments (~1.5) done by conductivity diffusion measurements. We also
present data for weak anchoring of the molecules on the particle surface and of
trial fields, which show to be sufficiently good for most applications.
Furthermore, the behaviour of the friction close to the transition point
nematic isotropic and for a rod-like and a disc-like liquid crystal will be
given.Comment: 23 pages RevTeX, including 3 PS figures, 1 PS table and 1 PS-LaTeX
figure; Accepted for publication in Phys. Rev.
The potential energy landscape of a model glass former: thermodynamics, anharmonicities, and finite size effects
It is possible to formulate the thermodynamics of a glass forming system in
terms of the properties of inherent structures, which correspond to the minima
of the potential energy and build up the potential energy landscape in the
high-dimensional configuration space. In this work we quantitatively apply this
general approach to a simulated model glass-forming system. We systematically
vary the system size between N=20 and N=160. This analysis enables us to
determine for which temperature range the properties of the glass former are
governed by the regions of the configuration space, close to the inherent
structures. Furthermore, we obtain detailed information about the nature of
anharmonic contributions. Moreover, we can explain the presence of finite size
effects in terms of specific properties of the energy landscape. Finally,
determination of the total number of inherent structures for very small systems
enables us to estimate the Kauzmann temperature
Effective Lagrangians for physical degrees of freedom in the Randall-Sundrum model
We derive the second variation Lagrangian of the Randall-Sundrum model with
two branes, study its gauge invariance and diagonalize it in the unitary gauge.
We also show that the effective four-dimensional theory looks different on
different branes and calculate the observable mass spectra and the couplings of
the physical degrees of freedom of 5-dimensional gravity to matter.Comment: 22 pages, LaTeX, typos correcte
Mode-coupling theory for multiple-time correlation functions of tagged particle densities and dynamical filters designed for glassy systems
The theoretical framework for higher-order correlation functions involving
multiple times and multiple points in a classical, many-body system developed
by Van Zon and Schofield [Phys. Rev. E 65, 011106 (2002)] is extended here to
include tagged particle densities. Such densities have found an intriguing
application as proposed measures of dynamical heterogeneities in structural
glasses. The theoretical formalism is based upon projection operator techniques
which are used to isolate the slow time evolution of dynamical variables by
expanding the slowly-evolving component of arbitrary variables in an infinite
basis composed of the products of slow variables of the system. The resulting
formally exact mode-coupling expressions for multiple-point and multiple-time
correlation functions are made tractable by applying the so-called N-ordering
method. This theory is used to derive for moderate densities the leading mode
coupling expressions for indicators of relaxation type and domain relaxation,
which use dynamical filters that lead to multiple-time correlations of a tagged
particle density. The mode coupling expressions for higher order correlation
functions are also succesfully tested against simulations of a hard sphere
fluid at relatively low density.Comment: 15 pages, 2 figure
Multiple-Point and Multiple-Time Correlations Functions in a Hard-Sphere Fluid
A recent mode coupling theory of higher-order correlation functions is tested
on a simple hard-sphere fluid system at intermediate densities. Multi-point and
multi-time correlation functions of the densities of conserved variables are
calculated in the hydrodynamic limit and compared to results obtained from
event-based molecular dynamics simulations. It is demonstrated that the mode
coupling theory results are in excellent agreement with the simulation results
provided that dissipative couplings are included in the vertices appearing in
the theory. In contrast, simplified mode coupling theories in which the
densities obey Gaussian statistics neglect important contributions to both the
multi-point and multi-time correlation functions on all time scales.Comment: Second one in a sequence of two (in the first, the formalism was
developed). 12 pages REVTeX. 5 figures (eps). Submitted to Phys.Rev.
Dynamical heterogeneities in a supercooled Lennard-Jones liquid
We present the results of a large scale molecular dynamics computer
simulation study in which we investigate whether a supercooled Lennard-Jones
liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian
parameter for the self part of the van Hove correlation function and use it to
identify ``mobile'' particles. We find that these particles form clusters whose
size grows with decreasing temperature. We also find that the relaxation time
of the mobile particles is significantly shorter than that of the bulk, and
that this difference increases with decreasing temperature.Comment: 8 pages of RevTex, 4 ps figure
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