8,296 research outputs found
Finite-Volume Two-Pion Amplitudes in the I=0 Channel
We perform a calculation in one-loop chiral perturbation theory of the
two-pion matrix elements and correlation functions of an I=0 scalar operator,
in finite and infinite volumes for both full and quenched QCD. We show that
major difficulties arise in the quenched theory due to the lack of unitarity.
Similar problems are expected for quenched lattice calculations of amplitudes with . Our results raise the important question
of whether it is consistent to study amplitudes beyond leading
order in chiral perturbation theory in quenched or partially quenched QCD.Comment: Version to appear on Phys. Lett. B, with only very minor and
stylistic change
A note on the power divergence in lattice calculations of amplitudes at
In this note, we clarify a point concerning the power divergence in lattice
calculations of decay amplitudes. There have been
worries that this divergence might show up in the Minkowski amplitudes at
with all the mesons at rest. Here we demonstrate, via an
explicit calculation in leading-order Chiral Perturbation Theory, that the
power divergence is absent at the above kinematic point, as predicted by CPS
symmetry.Comment: 5 pages, 2 figure
Arithmetic Properties of Overpartition Pairs
Bringmann and Lovejoy introduced a rank for overpartition pairs and
investigated its role in congruence properties of , the number of
overpartition pairs of n. In particular, they applied the theory of Klein forms
to show that there exist many Ramanujan-type congruences for the number
. In this paper, we shall derive two Ramanujan-type identities and
some explicit congruences for . Moreover, we find three ranks as
combinatorial interpretations of the fact that is divisible by
three for any n. We also construct infinite families of congruences for
modulo 3, 5, and 9.Comment: 19 page
Phylogenetic Analysis of Cell Types using Histone Modifications
In cell differentiation, a cell of a less specialized type becomes one of a
more specialized type, even though all cells have the same genome.
Transcription factors and epigenetic marks like histone modifications can play
a significant role in the differentiation process. In this paper, we present a
simple analysis of cell types and differentiation paths using phylogenetic
inference based on ChIP-Seq histone modification data. We propose new data
representation techniques and new distance measures for ChIP-Seq data and use
these together with standard phylogenetic inference methods to build
biologically meaningful trees that indicate how diverse types of cells are
related. We demonstrate our approach on H3K4me3 and H3K27me3 data for 37 and 13
types of cells respectively, using the dataset to explore various issues
surrounding replicate data, variability between cells of the same type, and
robustness. The promising results we obtain point the way to a new approach to
the study of cell differentiation.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
Neutral B Meson Mixing and Heavy-light Decay Constants from Quenched Lattice QCD
We present high-statistics results for neutral -meson mixing and
heavy-light-meson leptonic decays in the quenched approximation from
tadpole-improved clover actions at and . We consider
quantities such as , , and the full
matrix elements as well as the corresponding SU(3)-breaking
ratios. These quantities are important for determining the CKM matrix element
.Comment: LATTICE98(heavyqk). Revised version. Typos in the second and third
equations corrected. Very small changes to text. Results unchange
Panel discussion on chiral extrapolation of physical observables
This is an approximate reconstruction of the panel discussion on chiral
extrapolation of physical observables. The session consisted of brief
presentations from panelists, followed by responses from the panel, and
concluded with questions and comments from the floor with answers from
panelists. In the following, the panelists have summarized their statements,
and the ensuing discussion has been approximately reconstructed from notes.Comment: Lattice2002(plenary) 15 pages, 3 figure
Nano-scale modeling and elastic properties of a typical CSH (I) structure based on DFT and Molecular Dynamics Methods
International audience Les silicates de calcium hydratés (C-S-H) sont les constituants principaux de la pâte de ciment et ont donc une grande influence sur les propriétés mécaniques des matériaux cimentaires. Le modèle de tobermorite-11Å (formule chimique: Ca4Si6O14(OH)4•2H2O) est d'abord considéré comme configuration initiale pour décrire ces hydrates. Ce modèle est alors étudié par DFT (Density Functional Theory) et Dynamique Moléculaire. Les constantes élastiques sont calculées et comparées à des valeurs expérimentales. Un Silicate de Calcium Hydraté amorphe est obtenu par le biais d'une modélisation par Dynamique Moléculaire d'un processus de recuit de la tobermorite-11Å avec utilisation d'un potentiel de Born-Huggins-Meyer (BMH). Des tests uniaxiaux de traction et de compression d'un silicate de calcium hydraté amorphe (avec un rapport Ca/Si de 0,67), à une certaine vitesse de déformation, sont modélisés. Les courbes contrainte-déformation sont analysées. Les résultats montrent que: (1) les coefficients élastiques Cij sont obtenus dans la plage de pression de confinement 0-1GPa pour vérifier la fiabilité du modèle par comparaison avec des résultats de la littérature. (2) Un modèle de super-cellule à l'échelle nano montre des propriétés mécaniques isotropes (3) Après recuit pour obtenir un C-S-H (I) amorphe, le module de Young est en moyenne d'environ 21,4 GPa. </div
Estimating true evolutionary distances under the DCJ model
Motivation: Modern techniques can yield the ordering and strandedness of genes on each chromosome of a genome; such data already exists for hundreds of organisms. The evolutionary mechanisms through which the set of the genes of an organism is altered and reordered are of great interest to systematists, evolutionary biologists, comparative genomicists and biomedical researchers. Perhaps the most basic concept in this area is that of evolutionary distance between two genomes: under a given model of genomic evolution, how many events most likely took place to account for the difference between the two genomes? Results: We present a method to estimate the true evolutionary distance between two genomes under the ‘double-cut-and-join' (DCJ) model of genome rearrangement, a model under which a single multichromosomal operation accounts for all genomic rearrangement events: inversion, transposition, translocation, block interchange and chromosomal fusion and fission. Our method relies on a simple structural characterization of a genome pair and is both analytically and computationally tractable. We provide analytical results to describe the asymptotic behavior of genomes under the DCJ model, as well as experimental results on a wide variety of genome structures to exemplify the very high accuracy (and low variance) of our estimator. Our results provide a tool for accurate phylogenetic reconstruction from multichromosomal gene rearrangement data as well as a theoretical basis for refinements of the DCJ model to account for biological constraints. Availability: All of our software is available in source form under GPL at http://lcbb.epfl.ch Contact: [email protected]
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