4,123 research outputs found
Sea quark transverse momentum distributions and dynamical chiral symmetry breaking
Recent theoretical studies have provided new insight into the intrinsic
transverse momentum distributions of valence and sea quarks in the nucleon at a
low scale. The valence quark transverse momentum distributions (q - qbar) are
governed by the nucleon's inverse hadronic size R^{-1} ~ 0.2 GeV and drop
steeply at large p_T. The sea quark distributions (qbar) are in large part
generated by non-perturbative chiral-symmetry-breaking interactions and extend
up to the scale rho^{-1} ~ 0.6 GeV. These findings have many implications for
modeling the initial conditions of perturbative QCD evolution of TMD
distributions (starting scale, shape of p_T distributions, coordinate-space
correlation functions). The qualitative difference between valence and sea
quark intrinsic p_T distributions could be observed experimentally, by
comparing the transverse momentum distributions of selected hadrons in
semi-inclusive deep-inelastic scattering, or those of dileptons produced in pp
and pbar-p scattering.Comment: 9 pages, 5 figures. Proceedings of QCD Evolution Workshop, Jefferson
Lab, May 6-10, 201
Non-equilibrium dynamics of an active colloidal "chucker"
We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal
particle which emits smaller solute particles from its surface, isotropically
and at a constant rate k_c. We find that the diffusion constant of the chucker
increases for small k_c, as recently predicted theoretically. At large k_c the
chucker diffuses more slowly due to crowding effects. We compare our simulation
results to those of a "point particle" Langevin dynamics scheme in which the
solute concentration field is calculated analytically, and in which
hydrodynamic effects can be included albeit in an approximate way. By
simulating the dragging of a chucker, we obtain an estimate of its apparent
mobility coefficient which violates the fluctuation-dissipation theorem. We
also characterise the probability density profile for a chucker which sediments
onto a surface which either repels or absorbs the solute particles, and find
that the steady state distributions are very different in the two cases. Our
simulations are inspired by the biological example of
exopolysaccharide-producing bacteria, as well as by recent experimental,
simulation and theoretical work on phoretic colloidal "swimmers".Comment: re-submission after referee's comment
First-principles prediction of a decagonal quasicrystal containing boron
We interpret experimentally known B-Mg-Ru crystals as quasicrystal
approximants. These approximant structures imply a deterministic decoration of
tiles by atoms that can be extended quasiperiodically. Experimentally observed
structural disorder corresponds to phason (tile flip) fluctuations.
First-principles total energy calculations reveal that many distinct tilings
lie close to stability at low temperatures. Transfer matrix calculations based
on these energies suggest a phase transition from a crystalline state at low
temperatures to a high temperature state characterized by tile fluctuations. We
predict BMgRu forms a decagonal quasicrystal that is
metastable at low temperatures and may be thermodynamically stable at high
temperatures.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
A complementary view on the growth of directory trees
Trees are a special sub-class of networks with unique properties, such as the level distribution which has often been overlooked. We analyse a general tree growth model proposed by Klemm etal.[Phys. Rev. Lett. 95, 128701 (2005)] to explain the growth of user-generated directory structures in computers. The model has a single parameter q which interpolates between preferential attachment and random growth. Our analysis results in three contributions: first, we propose a more efficient estimation method for q based on the degree distribution, which is one specific representation of the model. Next, we introduce the concept of a level distribution and analytically solve the model for this representation. This allows for an alternative and independent measure of q. We argue that, to capture real growth processes, the q estimations from the degree and the level distributions should coincide. Thus, we finally apply both representations to validate the model with synthetically generated tree structures, as well as with collected data of user directories. In the case of real directory structures, we show that q measured from the level distribution are incompatible with q measured from the degree distribution. In contrast to this, we find perfect agreement in the case of simulated data. Thus, we conclude that the model is an incomplete description of the growth of real directory structures as it fails to reproduce the level distribution. This insight can be generalised to point out the importance of the level distribution for modeling tree growt
Sustainable growth in complex networks
Based on the empirical analysis of the dependency network in 18 Java
projects, we develop a novel model of network growth which considers both: an
attachment mechanism and the addition of new nodes with a heterogeneous
distribution of their initial degree, . Empirically we find that the
cumulative degree distributions of initial degrees and of the final network,
follow power-law behaviors: , and
, respectively. For the total number of links as a
function of the network size, we find empirically ,
where is (at the beginning of the network evolution) between 1.25 and
2, while converging to for large . This indicates a transition from
a growth regime with increasing network density towards a sustainable regime,
which revents a collapse because of ever increasing dependencies. Our
theoretical framework is able to predict relations between the exponents
, , , which also link issues of software engineering and
developer activity. These relations are verified by means of computer
simulations and empirical investigations. They indicate that the growth of real
Open Source Software networks occurs on the edge between two regimes, which are
either dominated by the initial degree distribution of added nodes, or by the
preferential attachment mechanism. Hence, the heterogeneous degree distribution
of newly added nodes, found empirically, is essential to describe the laws of
sustainable growth in networks.Comment: 5 pages, 2 figures, 1 tabl
Active Brownian particles with velocity-alignment and active fluctuations
We consider a model of active Brownian particles with velocity-alignment in
two spatial dimensions with passive and active fluctuations. Hereby, active
fluctuations refers to purely non-equilibrium stochastic forces correlated with
the heading of an individual active particle. In the simplest case studied
here, they are assumed as independent stochastic forces parallel (speed noise)
and perpendicular (angular noise) to the velocity of the particle. On the other
hand, passive fluctuations are defined by a noise vector independent of the
direction of motion of a particle, and may account for example for thermal
fluctuations.
We derive a macroscopic description of the active Brownian particle gas with
velocity-alignment interaction. Hereby, we start from the individual based
description in terms of stochastic differential equations (Langevin equations)
and derive equations of motion for the coarse grained kinetic variables
(density, velocity and temperature) via a moment expansion of the corresponding
probability density function.
We focus here in particular on the different impact of active and passive
fluctuations on the onset of collective motion and show how active fluctuations
in the active Brownian dynamics can change the phase-transition behaviour of
the system. In particular, we show that active angular fluctuation lead to an
earlier breakdown of collective motion and to emergence of a new bistable
regime in the mean-field case.Comment: 5 figures, 22 pages, submitted to New Journal of Physic
Black-pigmented anaerobic bacteria associated with ovine periodontitis
Periodontitis is a polymicrobial infectious disease that causes occlusion change, tooth loss, difficulty in rumination, and premature culling of animals. This study aimed to detect species of the genera Porphyromonas and Prevotella present in the periodontal pocket of sheep with lesions deeper than 5mm (n=14) and in the gingival sulcus of animals considered periodontally healthy (n=20). The presence of microorganisms was evaluated by polymerase chain reaction (PCR) using specific primers for Porphyromonas asaccharolytica, Porphyromonas endodontalis, Porphyromonas gingivalis, Porphyromonas gulae, Prevotella buccae, Prevotella intermedia, Prevotella loescheii, Prevotella melaninogenica, Prevotella nigrescens, Prevotella oralis, and Prevotella tannerae. Prevalence and risk analysis were performed using Student's t-test and Spearman's correlation. Among the Prevotella and Porphyromonas species detected in the periodontal lesions of sheep, P. melaninogenica (85.7%), P. buccae (64.3%), P. gingivalis (50%), and P. endodontalis (50%) were most prevalent. P. gingivalis (15%) and P. oralis (10%) prevailed in the gingival sulcus. P. gulae and P. tannerae were not detected in the 34 samples studied. Data evaluation by t-test verified that occurrence of P. asaccharolytica, P. endodontalis, P. gingivalis, P. buccae, P. intermedia, P. melalinogenica, and P. nigrescens correlated with sheep periodontitis. The findings of this study will be an important contribution to research on pathogenesis of sheep periodontitis and development of its control measures
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