59,762 research outputs found
Novel non-equilibrium critical behavior in unidirectionally coupled stochastic processes
Phase transitions from an active into an absorbing, inactive state are
generically described by the critical exponents of directed percolation (DP),
with upper critical dimension d_c = 4. In the framework of single-species
reaction-diffusion systems, this universality class is realized by the combined
processes A -> A + A, A + A -> A, and A -> \emptyset. We study a hierarchy of
such DP processes for particle species A, B,..., unidirectionally coupled via
the reactions A -> B, ... (with rates \mu_{AB}, ...). When the DP critical
points at all levels coincide, multicritical behavior emerges, with density
exponents \beta_i which are markedly reduced at each hierarchy level i >= 2.
This scenario can be understood on the basis of the mean-field rate equations,
which yield \beta_i = 1/2^{i-1} at the multicritical point. We then include
fluctuations by using field-theoretic renormalization group techniques in d =
4-\epsilon dimensions. In the active phase, we calculate the fluctuation
correction to the density exponent for the second hierarchy level, \beta_2 =
1/2 - \epsilon/8 + O(\epsilon^2). Monte Carlo simulations are then employed to
determine the values for the new scaling exponents in dimensions d<= 3,
including the critical initial slip exponent. Our theory is connected to
certain classes of growth processes and to certain cellular automata, as well
as to unidirectionally coupled pair annihilation processes. We also discuss
some technical and conceptual problems of the loop expansion and their possible
interpretation.Comment: 29 pages, 19 figures, revtex, 2 columns, revised Jan 1995: minor
changes and additions; accepted for publication in Phys. Rev.
Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers
In antiferromagnetically coupled superlattices grown on (001) faces of cubic
substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or
Fe/Cr, the magnetic states evolve under competing influence of bilinear and
biquadratic exchange interactions, surface-enhanced four-fold in-plane
anisotropy, and specific finite-size effects. Using phenomenological
(micromagnetic) theory, a comprehensive survey of the magnetic states and
reorientation transitions has been carried out for multilayer systems with even
number of ferromagnetic sub-layers and magnetizations in the plane. In
two-layer systems (N=2) the phase diagrams in dependence on components of the
applied field in the plane include ``swallow-tail'' type regions of
(metastable) multistate co-existence and a number of continuous and
discontinuous reorientation transitions induced by radial and transversal
components of the applied field. In multilayers (N \ge 4) noncollinear states
are spatially inhomogeneous with magnetization varying across the multilayer
stack. For weak four-fold anisotropy the magnetic states under influence of an
applied field evolve by a complex continuous reorientation into the saturated
state. At higher anisotropy they transform into various inhomogeneous and
asymmetric structures. The discontinuous transitions between the magnetic
states in these two-layers and multilayers are characterized by broad ranges of
multi-phase coexistence of the (metastable) states and give rise to specific
transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio
Noise-induced behaviors in neural mean field dynamics
The collective behavior of cortical neurons is strongly affected by the
presence of noise at the level of individual cells. In order to study these
phenomena in large-scale assemblies of neurons, we consider networks of
firing-rate neurons with linear intrinsic dynamics and nonlinear coupling,
belonging to a few types of cell populations and receiving noisy currents.
Asymptotic equations as the number of neurons tends to infinity (mean field
equations) are rigorously derived based on a probabilistic approach. These
equations are implicit on the probability distribution of the solutions which
generally makes their direct analysis difficult. However, in our case, the
solutions are Gaussian, and their moments satisfy a closed system of nonlinear
ordinary differential equations (ODEs), which are much easier to study than the
original stochastic network equations, and the statistics of the empirical
process uniformly converge towards the solutions of these ODEs. Based on this
description, we analytically and numerically study the influence of noise on
the collective behaviors, and compare these asymptotic regimes to simulations
of the network. We observe that the mean field equations provide an accurate
description of the solutions of the network equations for network sizes as
small as a few hundreds of neurons. In particular, we observe that the level of
noise in the system qualitatively modifies its collective behavior, producing
for instance synchronized oscillations of the whole network, desynchronization
of oscillating regimes, and stabilization or destabilization of stationary
solutions. These results shed a new light on the role of noise in shaping
collective dynamics of neurons, and gives us clues for understanding similar
phenomena observed in biological networks
Dominant next-to-leading order QCD corrections to Higgs plus three jet production in vector-boson fusion
We present the calculation of the dominant next to leading order QCD
corrections to Higgs boson production in association with three jets via vector
boson fusion in the form of a NLO parton-level Monte Carlo program. QCD
corrections to integrated cross sections are modest, while the shapes of some
kinematical distributions change appreciably at NLO. Scale uncertainties are
shown to be reduced at NLO for the total cross section and for distributions.
We consider a central jet veto at the LHC and analyze the veto probability for
typical vector boson fusion cuts. Scale uncertainties of the veto probability
are sufficiently small at NLO for precise Higgs coupling measurements at the
LHC.Comment: 40 pages, 17 figures, 2 tables, published versio
More is different: 50 years of nuclear BCS
Many of the concepts which are at the basis of the development associated
with a quantitative treatment of the variety of phenomena associated with the
spontaneous breaking of gauge symmetry in nuclei have been instrumental in
connection with novel studies of soft matter, namely of protein evolution and
protein folding. Although the route to these subjects and associated
development does not necessarily imply the nuclear physics connection, such a
connection has proven qualitatively and quantitatively inspiring. In particular
to model protein evolution in terms of the alignment of quasispins displaying
twenty different projections, one for each of the twenty amino acids occurring
in nature, and the associated symmetry breaking in information (sequence)
space. Emergent properties of the corresponding phase transition are domain
walls which stabilize local elementary structures (LES), few groups of 10-20
aminoacids which become structured already in the denatured state provide the
molecular recognition directing protein folding. In fact, their docking is
closely related to the transition state of the process. While the two-step, yes
or no, folding process, does not provide direct information concerning LES, one
can force LES from virtual to become real, observable final state entities.
Getting again inspiration from the nuclear case (virtual processes contributing
to pair correlations can be forced to become real with the help of a probe
which itself changes particle number by two), one would expect that to make
real virtual LES, that is segments of the protein which already at an early
stage of the folding process flicker in and out of their native conformation,
one needs a probe which itself displays a similar behaviour. Peptides
displaying a sequence identical to LES are such probes.Comment: Contribution to the Volume 50 years of Nuclear BCS edited by World
Scientifi
Transformation of UML Behavioral Diagrams to Support Software Model Checking
Unified Modeling Language (UML) is currently accepted as the standard for
modeling (object-oriented) software, and its use is increasing in the aerospace
industry. Verification and Validation of complex software developed according
to UML is not trivial due to complexity of the software itself, and the several
different UML models/diagrams that can be used to model behavior and structure
of the software. This paper presents an approach to transform up to three
different UML behavioral diagrams (sequence, behavioral state machines, and
activity) into a single Transition System to support Model Checking of software
developed in accordance with UML. In our approach, properties are formalized
based on use case descriptions. The transformation is done for the NuSMV model
checker, but we see the possibility in using other model checkers, such as
SPIN. The main contribution of our work is the transformation of a non-formal
language (UML) to a formal language (language of the NuSMV model checker)
towards a greater adoption in practice of formal methods in software
development.Comment: In Proceedings FESCA 2014, arXiv:1404.043
Fold-Hopf Bursting in a Model for Calcium Signal Transduction
We study a recent model for calcium signal transduction. This model displays
spiking, bursting and chaotic oscillations in accordance with experimental
results. We calculate bifurcation diagrams and study the bursting behaviour in
detail. This behaviour is classified according to the dynamics of separated
slow and fast subsystems. It is shown to be of the Fold-Hopf type, a type which
was previously only described in the context of neuronal systems, but not in
the context of signal transduction in the cell.Comment: 13 pages, 5 figure
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