731 research outputs found
Static and dynamic heterogeneities in irreversible gels and colloidal gelation
We compare the slow dynamics of irreversible gels, colloidal gels, glasses
and spin glasses by analyzing the behavior of the so called non-linear
dynamical susceptibility, a quantity usually introduced to quantitatively
characterize the dynamical heterogeneities. In glasses this quantity typically
grows with the time, reaches a maximum and then decreases at large time, due to
the transient nature of dynamical heterogeneities and to the absence of a
diverging static correlation length. We have recently shown that in
irreversible gels the dynamical susceptibility is instead an increasing
function of the time, as in the case of spin glasses, and tends asymptotically
to the mean cluster size. On the basis of molecular dynamics simulations, we
here show that in colloidal gelation where clusters are not permanent, at very
low temperature and volume fractions, i.e. when the lifetime of the bonds is
much larger than the structural relaxation time, the non-linear susceptibility
has a behavior similar to the one of the irreversible gel, followed, at higher
volume fractions, by a crossover towards the behavior of glass forming liquids.Comment: 9 pages, 3 figure
Dynamic fluctuations in ascending heart-to-brain communication under mental stress
Dynamical information exchange between central and autonomic nervous systems, as referred to functional brain-heart interplay, occurs during emotional and physical arousal. It is well documented that physical and mental stress lead to sympathetic activation. Nevertheless, the role of autonomic inputs in nervous system-wise communication under mental stress is yet unknown. In this study, we estimated the causal and bidirectional neural modulations between electroencephalogram (EEG) oscillations and peripheral sympathetic and parasympathetic activities using a recently proposed computational framework for a functional brain-heart interplay assessment, namely the sympathovagal synthetic data generation model. Mental stress was elicited in 37 healthy volunteers by increasing their cognitive demands throughout three tasks associated with increased stress levels. Stress elicitation induced an increased variability in sympathovagal markers, as well as increased variability in the directional brain-heart interplay. The observed heart-to-brain interplay was primarily from sympathetic activity targeting a wide range of EEG oscillations, whereas variability in the efferent direction seemed mainly related to EEG oscillations in the c band. These findings extend current knowledge on stress physiology, which mainly referred to top-down neural dynamics. Our results suggest that mental stress may not cause an increase in sympathetic activity exclusively as it initiates a dynamic fluctuation within brain-body networks including bidirectional interactions at a brain-heart level. We conclude that directional brain-heart interplay measurements may provide suitable biomarkers for a quantitative stress assessment and bodily feedback may modulate the perceived stress caused by increased cognitive demand
Tri-Resonant Leptogenesis
We present a class of leptogenesis models where the light neutrinos acquire
their observed mass through a symmetry-motivated construction. We consider an
extension of the Standard Model, which includes three singlet neutrinos which
have mass splittings comparable to their decay widths. We show that this
tri-resonant structure leads to an appreciable increase in the observed CP
asymmetry over that found previously in typical bi-resonant models. To analyse
such tri-resonant scenarios, we solve a set of coupled Boltzmann equations,
crucially preserving the variations in the relativistic degrees of freedom. We
highlight the fact that small variations at high temperatures can have major
implications for the evolution of the baryon asymmetry when the singlet
neutrino mass scale is below GeV. We then illustrate how this variation
can significantly affect the ability to find successful leptogenesis at these
low masses. Finally, the parameter space for viable leptogenesis is delineated,
and comparisons are made with current and future experiments.Comment: 16 pages, 5 figures, conference proceedings for Corfu Summer
Institute 2022, School and Workshops on Elementary Particle Physics and
Gravity, August 28 - September 8, 2022, Corfu, Greec
Effects of Mass Media and Cultural Drift in a Model for Social Influence
In the context of an extension of Axelrod's model for social influence, we
study the interplay and competition between the cultural drift, represented as
random perturbations, and mass media, introduced by means of an external
homogeneous field. Unlike previous studies [J. C. Gonz\'alez-Avella {\it et
al}, Phys. Rev. E {\bf 72}, 065102(R) (2005)], the mass media coupling proposed
here is capable of affecting the cultural traits of any individual in the
society, including those who do not share any features with the external
message. A noise-driven transition is found: for large noise rates, both the
ordered (culturally polarized) phase and the disordered (culturally fragmented)
phase are observed, while, for lower noise rates, the ordered phase prevails.
In the former case, the external field is found to induce cultural ordering, a
behavior opposite to that reported in previous studies using a different
prescription for the mass media interaction. We compare the predictions of this
model to statistical data measuring the impact of a mass media vasectomy
promotion campaign in Brazil.Comment: 10 pages, 3 figures; minor changes; added references. To appear in
IJMP
Tri-Resonant Leptogenesis in a Seesaw Extension of the Standard Model
We study a class of leptogenesis models where the light neutrinos acquire
their observed small masses by a symmetry-motivated construction. This class of
models may naturally include three nearly degenerate heavy Majorana neutrinos
that can strongly mix with one another and have mass differences comparable to
their decay widths. We find that such a tri-resonant heavy neutrino system can
lead to leptonic CP asymmetries which are further enhanced than those obtained
in the usual bi-resonant approximation. Moreover, we solve the Boltzmann
equations by paying special attention to the temperature dependence of the
relativistic degrees of freedom of the plasma. The latter results in
significant corrections to the evolution equations for the heavy neutrinos and
the lepton asymmetry that have been previously ignored in the literature. We
show the importance of these corrections to accurately describe the dynamical
evolution of the baryon-to-photon ratio for heavy neutrino masses at
and below GeV, and demonstrate that successful leptogenesis at lower
masses can be significantly affected by the variation of the relativistic dofs.
The parameter space for the leptogenesis model is discussed, and it could be
probed in future experimental facilities searching for charged lepton flavour
violation and heavy neutrinos in future -boson factories.Comment: 42 pages, 10 figures, additional references included, inclusion of
additional clarifying comments, to appear in JHE
Contact processes with long-range interactions
A class of non-local contact processes is introduced and studied using
mean-field approximation and numerical simulations. In these processes
particles are created at a rate which decays algebraically with the distance
from the nearest particle. It is found that the transition into the absorbing
state is continuous and is characterized by continuously varying critical
exponents. This model differs from the previously studied non-local directed
percolation model, where particles are created by unrestricted Levy flights. It
is motivated by recent studies of non-equilibrium wetting indicating that this
type of non-local processes play a role in the unbinding transition. Other
non-local processes which have been suggested to exist within the context of
wetting are considered as well.Comment: Accepted with minor revisions by Journal of Statistical Mechanics:
Theory and experiment
Spatial signal amplification in cell biology: a lattice-gas model for self-tuned phase ordering
Experiments show that the movement of eukaryotic cells is regulated by a
process of phase separation of two competing enzymes on the cell membrane, that
effectively amplifies shallow external gradients of chemical attractant.
Notably, the cell is able to self-tune the final enzyme concentrations to an
equilibrium state of phase coexistence, for a wide range of the average
attractant concentration. We propose a simple lattice model in which, together
with a short-range attraction between enzymes, a long-range repulsion naturally
arises from physical considerations, that easily explains such observed
behavior
Pacman percolation: a model for enzyme gel degradation
We study a model for the gel degradation by an enzyme, where the gel is
schematized as a cubic lattice, and the enzyme as a random walker, that cuts
the bonds over which it passes. The model undergoes a (reverse) percolation
transition, which for low density of enzymes falls in a universality class
different from random percolation. In particular we have measured a gel
fraction critical exponent beta=1.0+-0.1, in excellent agreement with
experiments made on the real system.Comment: 4 pages, 7 eps figure
The cryogenic system for the SLAC E158 experiment
E158 is a fixed target experiment at SLAC in which high energy (up to 48 GeV) polarized electrons are scattered off the unpolarized electrons in a 1.5 m long liquid hydrogen target. The total volume of liquid hydrogen in the system is 47.1. The beam can deposit as much as 700 W into the liquid hydrogen. Among the requirements for the system are: that density fluctuations in the liquid hydrogen be kept to a minimum, that the target can be moved out of the beam line while cold and replaced to within 2 mm and that the target survive lifetime radiation doses of up to 1×106 Gy. The cryogenic system for the experiment consists of the target itself, the cryostat containing the target, a refurbished CTI 4000 refrigerator providing more than 1 kW of cooling at 20 K and associated transfer lines and valve boxes. This paper discusses the requirements, design, construction, testing and operation of the cryogenic system. The unique features of the design associated with hydrogen safety and the high radiation field in which the target resides are also covered
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