615 research outputs found
An Interacting Parton Model for Quark and Anti-quark distributions in the Baryon
In this paper we study a 1+1 dimensional relativistic parton model for the
structure of baryons. The quarks and anti-quarks interact through a linear
potential. We obtain an analytic formula for the isospin averaged valence quark
distribution in the chiral and large Nc limits. The leading 1/Nc and non-zero
current quark mass corrections are estimated. Then we extend this model to
include `sea' and anti-quarks. We find that the anti-quark content is small at
a low value of Q^2. Using these distributions as initial conditions for Q^2
evolution, we compare with experimental measurements of the structure function
xF_3(x,Q^2) and find reasonable agreement. The only parameters we can adjust
are the fraction of baryon momentum carried by valence quarks and the initial
scale Q_0^2.Comment: 14 page
Sensitivity of High-Speed Lightwave System Receivers Using InAlAs Avalanche Photodiodes
Calculations based on a rigorous analytical model are carried out to compare the sensitivity of optical receivers that use InP and In0.52Al0.48As avalanche photodiodes (APDs). The model includes the effects of intersymbol interference, tunneling current, avalanche noise and its correlation with the stochastic avalanche duration, dead space, and transimpedance amplifier noise. For a 10-Gb/s system with a bit-error rate of 10-12, the optimum receiver sensitivity predicted for In0.52Al0.48As and InP APDs is -28.6 and -28.1 dBm, respectively, corresponding to a reduction of 11% in optical signal power for receivers using In0.52Al0.48As APDs. Thus, considering overall receiver sensitivity, the improvement offered by In0.52Al0.48As APDs over InP is modest
P09-15. Selection of higher avidity HLA-restricted T cell responses as a viral adaptation strategy
Loss of immune reactivity due to HIV mutational escape is well described. Data generated from a large population-based study (n>800) suggested that certain CD8 T cell epitopes are created as a result of HIV adaptation and are associated with enhanced viral replication. Here we sought to investigate the HLA-restricted T-cell responses associated with seven such adaptations
Parton Model from Bi-local Solitonic Picture of the Baryon in two-dimensions
We study a previously introduced bi-local gauge invariant reformulation of
two dimensional QCD, called 2d HadronDynamics. The baryon arises as a
topological soliton in HadronDynamics. We derive an interacting parton model
from the soliton model, thus reconciling these two seemingly different points
of view. The valence quark model is obtained as a variational approximation to
HadronDynamics. A succession of better approximations to the soliton picture
are obtained. The next simplest case corresponds to a system of interacting
valence, `sea' and anti-quarks. We also obtain this `embellished' parton model
directly from the valence quark system through a unitary transformation. Using
the solitonic point of view, we estimate the quark and anti-quark distributions
of 2d QCD. Possible applications to Deep Inelastic Structure Functions are
pointed out.Comment: 12 page
The central limit problem for random vectors with symmetries
Motivated by the central limit problem for convex bodies, we study normal
approximation of linear functionals of high-dimensional random vectors with
various types of symmetries. In particular, we obtain results for distributions
which are coordinatewise symmetric, uniform in a regular simplex, or
spherically symmetric. Our proofs are based on Stein's method of exchangeable
pairs; as far as we know, this approach has not previously been used in convex
geometry and we give a brief introduction to the classical method. The
spherically symmetric case is treated by a variation of Stein's method which is
adapted for continuous symmetries.Comment: AMS-LaTeX, uses xy-pic, 23 pages; v3: added new corollary to Theorem
Optimization of InP APDs for High-Speed Lightwave Systems
Calculations based on a rigorous analytical model are carried out to optimize the width of the indium phosphide avalanche region in high-speed direct-detection avalanche photodiode-based optical receivers. The model includes the effects of intersymbol interference (ISI), tunneling current, avalanche noise, and its correlation with the stochastic avalanche duration, as well as dead space. A minimum receiver sensitivity of -28 dBm is predicted at an optimal width of 0.18 mum and an optimal gain of approximately 13, for a 10 Gb/s communication system, assuming a Johnson noise level of 629 noise electrons per bit. The interplay among the factors controlling the optimum sensitivity is confirmed. Results show that for a given transmission speed, as the device width decreases below an optimum value, increased tunneling current outweighs avalanche noise reduction due to dead space, resulting in an increase in receiver sensitivity. As the device width increases above its optimum value, the receiver sensitivity increases as device bandwidth decreases, causing ISI to dominate avalanche noise and tunneling current shot noise
Generation and characterization of an inducible transgenic model for studying mouse esophageal biology
Background: To facilitate the in vivo study of esophageal (stem) cell biology in homeostasis and cancer, novel mouse models are necessary to elicit expression of candidate genes in a tissue-specific and inducible fashion. To this aim, we developed and studied a mouse model to allow labeling of esophageal cells with the histone 2B-GFP (H2B-GFP) fusion protein. Results: First, we generated a transgenic mouse model expressing the reverse tetracycline transactivator rtTA2-M2 under control of the promoter (ED-L2) of the Epstein-Barr virus (EBV) gene encoding the latent membrane protein-1 (LMP-1). The newly generated ED-L2-rtTA2-M2 (ED-L2-rtTA) mice were then bred with the previously developed tetO-HIST1H2BJ/GFP (tetO-H2B-GFP) model to assess inducibility and tissue-specificity. Expression of the H2B-GFP fusion protein was observed upon doxycycline induction but was restricted to the terminally differentiated cells above the basal cell layer. To achieve expression in the basal compartment of the esophagus, we ubsequently employed a different transgenic model expressing the reverse transactivator rtTA2S-M2 under the control of the ubiquitous, methylation-free CpG island of the human hnRNPA2B1-CBX3 gene (hnRNP-rtTA). Upon doxycycline administration to the compound hnRNP-rtTA/tetO-H2B-GFP mice, near-complete labeling of all esophageal cells was achieved. Pulse-chase experiments confirmed that complete turnover of the esophageal epithelium in the adult mouse is achieved within 710 days. Conclusions: We show that the esophagus-specific promoter ED-L2 is expressed only in the differentiated cells above the basal layer. oreover, we confirmed that esophageal turn-over in the adult mouse does not exceed 710 days
Orientability and energy minimization in liquid crystal models
Uniaxial nematic liquid crystals are modelled in the Oseen-Frank theory
through a unit vector field . This theory has the apparent drawback that it
does not respect the head-to-tail symmetry in which should be equivalent to
-. This symmetry is preserved in the constrained Landau-de Gennes theory
that works with the tensor .We study
the differences and the overlaps between the two theories. These depend on the
regularity class used as well as on the topology of the underlying domain. We
show that for simply-connected domains and in the natural energy class
the two theories coincide, but otherwise there can be differences
between the two theories, which we identify. In the case of planar domains we
completely characterise the instances in which the predictions of the
constrained Landau-de Gennes theory differ from those of the Oseen-Frank
theory
- …