2,067 research outputs found
Low-lying continuum states of drip-line Oxygen isotopes
Low-lying continuum states of exotic oxygen isotopes are studied, by
introducing the Continuum-Coupled Shell Model (CCSM) characterized by an
infinite wall placed very far and by an interaction for continuum coupling
constructed in a close relation to realistic shell-model interaction. Neutron
emission spectra from exotic oxygen isotopes are calculated by the
doorway-state approach in heavy-ion multi-nucleon transfer reactions. The
results agree with experiment remarkably well, as an evidence that the
continuum effects are stronger than 1 MeV, consistently with the shell
evolution in exotic nuclei. The results by this CCSM doorway-state approach are
compared with calculations on neutron-scattering resonance peaks made within
the CCSM phase-shift approach and also with those obtained in the Gamow shell
model, by taking the same interaction. Remarkable similarities in peak energies
and certain differences in widths are then obtained.Comment: This paper has been withdrawn by the author due to the publication in
PTEP journal with considerable expansion. The re-submission to arXiv has been
given up due to the differences in style files, et
Gamow shell-model calculations of drip-line oxygen isotopes
We employ the Gamow shell model (GSM) to describe low-lying states of the
oxygen isotopes 24O and 25O. The many-body Schrodinger equation is solved
starting from a two-body Hamiltonian defined by a renormalized low-momentum
nucleon-nucleon (NN) interaction, and a spherical Berggren basis. The Berggren
basis treats bound, resonant, and continuum states on an equal footing, and is
therefore an appropriate representation of loosely bound and unbound nuclear
states near threshold. We show that such a basis is necessary in order to
obtain a detailed and correct description of the low-lying 1+ and 2+ excited
states in 24O. On the other hand, we find that a correct description of binding
energy systematics of the ground states is driven by proper treatment and
inclusion of many-body correlation effects. This is supported by the fact that
we get 25O unstable with respect to 24O in both oscillator and Berggren
representations starting from a 22O core. Furthermore, we show that the
structure of these loosely bound or unbound isotopes are strongly influenced by
the 1S0 component of the NN interaction. This has important consequences for
our understanding of nuclear stability.Comment: 5 pages, 3 figure
In-Medium Similarity Renormalization Group for Nuclei
We present a new ab-initio method that uses similarity renormalization group
(SRG) techniques to continuously diagonalize nuclear many-body Hamiltonians. In
contrast with applications of the SRG to two- and three-nucleon interactions in
free space, we perform the SRG evolution "in medium" directly in the -body
system of interest. The in-medium approach has the advantage that one can
approximately evolve -body operators using only two-body machinery
based on normal-ordering techniques. The method is nonperturbative and can be
tailored to problems ranging from the diagonalization of closed-shell nuclei to
the construction of effective valence shell-model Hamiltonians and operators.
We present first results for the ground-state energies of He, O and
Ca, which have accuracies comparable to coupled-cluster calculations.Comment: 4pages, 4 figures, to be published in PR
Discovering Restricted Regular Expressions with Interleaving
Discovering a concise schema from given XML documents is an important problem
in XML applications. In this paper, we focus on the problem of learning an
unordered schema from a given set of XML examples, which is actually a problem
of learning a restricted regular expression with interleaving using positive
example strings. Schemas with interleaving could present meaningful knowledge
that cannot be disclosed by previous inference techniques. Moreover, inference
of the minimal schema with interleaving is challenging. The problem of finding
a minimal schema with interleaving is shown to be NP-hard. Therefore, we
develop an approximation algorithm and a heuristic solution to tackle the
problem using techniques different from known inference algorithms. We do
experiments on real-world data sets to demonstrate the effectiveness of our
approaches. Our heuristic algorithm is shown to produce results that are very
close to optimal.Comment: 12 page
Analysis of nucleosome repositioning by yeast ISWI and Chd1 chromatin remodeling complexes
ISWI proteins form the catalytic core of a subset of ATP-dependent chromatin remodelling activities in eukaryotes from yeast to man. Many of these complexes have been found to reposition nucleosomes, but with different directionalities. We find that the yeast Isw1a, Isw2 and Chd1 enzymes preferentially move nucleosomes towards more central locations on short DNA fragments whereas Isw1b does not. Importantly, the inherent positioning properties of the DNA play an important role in determining where nucleosomes are relocated to by all of these enzymes. However, a key difference is that the Isw1a, Isw2 and Chd1 enzymes are unable to move nucleosomes to positions closer than 15 bp from a DNA end whereas Isw1b can. We also find that there is a correlation between the inability of enzymes to move nucleosomes close to DNA ends and the preferential binding to nucleosomes bearing linker DNA. These observations suggest that the accessibility of linker DNA together with the positioning properties of the underlying DNA play important roles in determining the outcome of remodelling by these enzymes
Chromosomes and Expression Mechanisms
Introduction
Whether one considers a single cell or a multicellular organism, a complex and precisely
coordinated series of regulatory events and communications is required to ensure its proper
configuration and function. One of the major goals in biology is to understand how cells
differentiate into specific types to perform their roles in vivo. Genome sequencing projects have
produced enormous amounts of data that are beginning to reveal the blue print of body plans for
various organisms. Despite this wealth of new information, we are still far from understanding
how cells differentiate. This is, in part, because we are not yet able to fully appreciate how this
genetic information is being read by the transcription machineries. It is widely accepted that
specific gene expression patterns are responsible for differentiation and maintenance of specific
cell types, with mistakes in these regulatory steps often leading to developmental defects and the
onset of cancers. Therefore, understanding the mechanisms of transcriptional control is a
necessary prerequisite to achieve this major goal in biology. To this end, we need to know more
about the substrate of transcription (chromatin), as well as the effectors of transcription
(transcription factors). The theme of this issue of Current Opinion in Genetics & Development is
the mechanism of transcriptional regulation, with an emphasis on latest topics in this rapidly
moving area of research. Because chromatin structure deeply affects transcription at multiple
stages, a significant portion of this issue is devoted to the mechanisms related to chromatin
regulation
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