658 research outputs found
Confirmation of Anomalous Dynamical Arrest in attractive colloids: a molecular dynamics study
Previous theoretical, along with early simulation and experimental, studies
have indicated that particles with a short-ranged attraction exhibit a range of
new dynamical arrest phenomena. These include very pronounced reentrance in the
dynamical arrest curve, a logarithmic singularity in the density correlation
functions, and the existence of `attractive' and `repulsive' glasses. Here we
carry out extensive molecular dynamics calculations on dense systems
interacting via a square-well potential. This is one of the simplest systems
with the required properties, and may be regarded as canonical for interpreting
the phase diagram, and now also the dynamical arrest. We confirm the
theoretical predictions for re-entrance, logarithmic singularity, and give the
first direct evidence of the coexistence, independent of theory, of the two
coexisting glasses. We now regard the previous predictions of these phenomena
as having been established.Comment: 15 pages,15 figures; submitted to Phys. Rev.
A Study of Sequence Distribution of a Painted Globule as a Model for Proteins with Good Folding Properties
In this paper we present a method to study the folding structure of a simple model consisting of two kinds of monomers, hydrophobic and hydrophilic. This method has three main steps: an efficient simulation method to bring an open sequence of homopolymer to a folded state, the application of a painting method called regular hull to the folded globule and the refolding process of the obtained copolymer sequence. This study allows us to suggest a theoretical function of disorder distribution for copolymer sequences that give rise to a compacted and well micro-phase separated globule
A Study of Sequence Distribution of a Painted Globule as a Model for Proteins with Good Folding Properties
In this paper we present a method to study the folding structure of a simple model consisting of two kinds of monomers, hydrophobic and hydrophilic. This method has three main steps: an efficient simulation method to bring an open sequence of homopolymer to a folded state, the application of a painting method called regular hull to the folded globule and the refolding process of the obtained copolymer sequence. This study allows us to suggest a theoretical function of disorder distribution for copolymer sequences that give rise to a compacted and well micro-phase separated globule
Variation of the cross section for e+e- --> W+H- in the Minimal Supersymmetric Standard Model
We study the loop-induced process e+e- --> W+H- in the Minimal Supersymmetric
Standard Model (MSSM). This process allows the charged Higgs boson to be
produced in e+e- collisions when its mass is larger than half the
center-of-mass energy, so that e+e- --> H+H- is kinematically forbidden. By
scanning over the MSSM parameters subject to experimental constraints we
examine the range of values possible for this cross section. We find that, in
regions of parameter space where this cross section is large enough to be of
interest, the contributions from supersymmetric particles typically increase
the cross section by 50-100% compared to the non-supersymmetric two Higgs
doublet model result. Choosing a few typical MSSM parameter sets, we show the
regions in the m_{H^{\pm}}-tan(beta) plane in which at least 10 W^{\pm}H^{\mp}
events would be produced at the e+e- collider for m_{H^{\pm}} >= sqrt(s)/2. We
also show that including radiative corrections to the MSSM Higgs sector has
only a small effect on the cross section.Comment: 5 pages, 4 figures, v2: minor changes; v3: extensive changes to text
and figures, version to appear in PR
Slowed Relaxational Dynamics Beyond the Fluctuation-Dissipation Theorem
To describe the slow dynamics of a system out of equilibrium, but close to a
dynamical arrest, we generalize the ideas of previous work to the case where
time-translational invariance is broken. We introduce a model of the dynamics
that is reasonably general, and show how all of the unknown parameters of this
model may be related to the observables or to averages of the noise. One result
is a generalisation of the Fluctuation Dissipation Theorem of type two (FDT2),
and the method is thereby freed from this constraint. Significantly, a
systematic means of implementing the theory to higher order is outlined. We
propose the simplest possible closure of these generalized equations, following
the same type of approximations that have been long known for the equilibrium
case of Mode Coupling Theory (MCT). Naturally, equilibrium MCT equations are
found as a limit of this generalized formalism. %We indicate that, within the
same general %framework, it should be possible to make higher level
approximations, %leading to more general applicability.Comment: 14 pages, Physica A (in press
Collapse of a semiflexible polymer in poor solvent
We investigate the dynamics and the pathways of the collapse of a single,
semiflexible polymer in a poor solvent via 3-D Brownian Dynamics simulations.
Earlier work indicates that the condensation of semiflexible polymers
generically proceeds via a cascade through metastable racquet-shaped,
long-lived intermediates towards the stable torus state. We investigate the
rate of decay of uncollapsed states, analyze the preferential pathways of
condensation, and describe likelihood and lifespan of the different metastable
states. The simulation are performed with a bead-stiff spring model with
excluded volume interaction and exponentially decaying attractive potential.
The semiflexible chain collapse is studied as functions of the three relevant
length scales of the phenomenon, i.e., the total chain length , the
persistence length and the condensation length , where is a measure of the attractive potential per unit
length. Two dimensionless ratios, and , suffice to describe
the decay rate of uncollapsed states, which appears to scale as . The condensation sequence is described in terms of the time series
of the well separated energy levels associated with each metastable collapsed
state. The collapsed states are described quantitatively through the spatial
correlation of tangent vectors along the chain. We also compare the results
obtained with a locally inextensible bead-rod chain and with a phantom
bead-spring model. Finally, we show preliminary results on the effects of
steady shear flow on the kinetics of collapse.Comment: 9 pages, 8 figure
Evidence for Unusual Dynamical Arrest Scenario in Short Ranged Colloidal Systems
Extensive molecular dynamics simulation studies of particles interacting via
a short ranged attractive square-well (SW) potential are reported. The
calculated loci of constant diffusion coefficient in the
temperature-packing fraction plane show a re-entrant behavior, i.e. an increase
of diffusivity on cooling, confirming an important part of the high
volume-fraction dynamical-arrest scenario earlier predicted by theory for
particles with short ranged potentials. The more efficient localization
mechanism induced by the short range bonding provides, on average, additional
free volume as compared to the hard-sphere case and results in faster dynamics.Comment: 4 pages, 3 figure
Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines
Within the mode-coupling theory for ideal glass-transitions, the mean-squared
displacement and the correlation function for density fluctuations are
evaluated for a colloidal liquid of particles interacting with a square-well
potential for states near the crossing of the line for transitions to a gel
with the line for transitions to a glass. It is demonstrated how the dynamics
is ruled by the interplay of the mechanisms of arrest due to hard-core
repulsion and due to attraction-induced bond formation as well as by a nearby
higher-order glass-transition singularity. Application of the universal
relaxation laws for the slow dynamics near glass-transition singularities
explains the qualitative features of the calculated time dependence of the
mean-squared displacement, which are in accord with the findings obtained in
molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66,
041402 (2002)]. Correlation functions found by photon-correlation spectroscopy
in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can
be interpreted qualitatively as a crossover from gel to glass dynamics.Comment: 13 pages, 12 figure
An horizon scan of biogeography
The opportunity to reflect broadly on the accomplishments, prospects, and reach of a field may present itself relatively infrequently. Each biennial meeting of the International Biogeography Society showcases ideas solicited and developed largely during the preceding year, by individuals or teams from across the breadth of the discipline. Here, we highlight challenges, developments, and opportunities in biogeography from that biennial synthesis. We note the realized and potential impact of rapid data accumulation in several fields, a renaissance for inter-disciplinary research, the importance of recognizing the evolution-ecology continuum across spatial and temporal scales and at different taxonomic, phylogenetic and functional levels, and re-exploration of classical assumptions and hypotheses using new tools. However, advances are taxonomically and geographically biased, and key theoretical frameworks await tools to handle, or strategies to simplify, the biological complexity seen in empirical systems. Current threats to biodiversity require unprecedented integration of knowledge and development of predictive capacity that may enable biogeography to unite its descriptive and hypothetico-deductive branches and establish a greater role within and outside academia
Higgs-Boson Production Induced by Bottom Quarks
Bottom quark-induced processes are responsible for a large fraction of the
LHC discovery potential, in particular for supersymmetric Higgs bosons.
Recently, the discrepancy between exclusive and inclusive Higgs boson
production rates has been linked to the choice of an appropriate bottom
factorization scale. We investigate the process kinematics at hadron colliders
and show that it leads to a considerable decrease in the bottom factorization
scale. This effect is the missing piece needed to understand the corresponding
higher order results. Our results hold generally for charged and for neutral
Higgs boson production at the LHC as well as at the Tevatron. The situation is
different for single top quark production, where we find no sizeable
suppression of the factorization scale. Turning the argument around, we can
specify how large the collinear logarithms are, which can be resummed using the
bottom parton picture.Comment: 18 page
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