2,271 research outputs found
Self-consistent treatment of the self-energy in nuclear matter
The influence of hole-hole propagation in addition to the conventional
particle-particle propagation, on the energy per nucleon and the momentum
distribution is investigated. The results are compared to the
Brueckner-Hartree-Fock (BHF) calculations with a continuous choice and
conventional choice for the single-particle spectrum. The Bethe-Goldstone
equation has been solved using realistic interactions. Also, the structure
of nucleon self-energy in nuclear matter is evaluated. All the self-energies
are calculated self-consistently. Starting from the BHF approximation without
the usual angle-average approximation, the effects of hole-hole contributions
and a self-consistent treatment within the framework of the Green function
approach are investigated. Using the self-consistent self-energy, the hole and
particle self-consistent spectral functions including the particle-particle and
hole-hole ladder contributions in nuclear matter are calculated using realistic
interactions. We found that, the difference in binding energy between both
results, i.e. BHF and self-consistent Green function, is not large. This
explains why is the BHF ignored the 2h1p contribution.Comment: Preprint 20 pages including 15 figures and one tabl
Scalar FCNC and rare top decays in a two Higgs doublet model "for the top"
In the so called two Higgs doublet model for the top-quark (T2HDM), first
suggested by Das and Kao, the top quark receives a special status, which endows
it with a naturally large mass, and also potentially gives rise to large flavor
changing neutral currents (FCNC) only in the up-quark sector. In this paper we
calculate the branching ratio (BR) for the rare decays t->ch and h->tc (h is a
neutral Higgs) in the T2HDM, at tree level and at 1-loop when it exceeds the
tree-level. We compare our results to predictions from other versions of 2HDM's
and find that the scalar FCNC in the T2HDM can play a significant role in these
decays. In particular, the 1-loop mediated decays can be significantly enhanced
in the T2HDM compared to the 2HDM of types I and II, in some instances reaching
BR~10^-4 which is within the detectable level at the LHC.Comment: added two references. 15 pages, 14 figure
Energy and Momentum densities of cosmological models, with equation of state , in general relativity and teleparallel gravity
We calculated the energy and momentum densities of stiff fluid solutions,
using Einstein, Bergmann-Thomson and Landau-Lifshitz energy-momentum complexes,
in both general relativity and teleparallel gravity. In our analysis we get
different results comparing the aforementioned complexes with each other when
calculated in the same gravitational theory, either this is in general
relativity and teleparallel gravity. However, interestingly enough, each
complex's value is the same either in general relativity or teleparallel
gravity. Our results sustain that (i) general relativity or teleparallel
gravity are equivalent theories (ii) different energy-momentum complexes do not
provide the same energy and momentum densities neither in general relativity
nor in teleparallel gravity. In the context of the theory of teleparallel
gravity, the vector and axial-vector parts of the torsion are obtained. We show
that the axial-vector torsion vanishes for the space-time under study.Comment: 15 pages, no figures, Minor typos corrected; version to appear in
International Journal of Theoretical Physic
Properties of asymmetric nuclear matter in different approaches
Properties of asymmetric nuclear matter are derived from various many-body
approaches. This includes phenomenological ones like the Skyrme Hartree-Fock
and relativistic mean field approaches, which are adjusted to fit properties of
nuclei, as well as more microscopic attempts like the Brueckner-Hartree-Fock
approximation, a self-consistent Greens function method and the so-called
approach, which are based on realistic nucleon-nucleon interactions
which reproduce the nucleon-nucleon phase shifts. These microscopic approaches
are supplemented by a density-dependent contact interaction to achieve the
empirical saturation property of symmetric nuclear matter. The predictions of
all these approaches are discussed for nuclear matter at high densities in
-equilibrium. Special attention is paid to behavior of the isovector
component of the effective mass in neutron-rich matter.Comment: 16 pages, 7 figure
Polymorphic members of the lag gene family mediate kin discrimination in Dictyostelium
Self and kin discrimination are observed in most kingdoms of life and are mediated by highly polymorphic plasma membrane proteins. Sequence polymorphism, which is essential for effective recognition, is maintained by balancing selection. Dictyostelium discoideum are social amoebas that propagate as unicellular organisms but aggregate upon starvation and form fruiting bodies with viable spores and dead stalk cells. Aggregative development exposes Dictyostelium to the perils of chimerism, including cheating, which raises questions about how the victims survive in nature and how social cooperation persists. Dictyostelids can minimize the cost of chimerism by preferential cooperation with kin, but the mechanisms of kin discrimination are largely unknown. Dictyostelium lag genes encode transmembrane proteins with multiple immunoglobulin (Ig) repeats that participate in cell adhesion and signaling. Here, we describe their role in kin discrimination. We show that lagB1 and lagC1 are highly polymorphic in natural populations and that their sequence dissimilarity correlates well with wild-strain segregation. Deleting lagB1 and lagC1 results in strain segregation in chimeras with wild-type cells, whereas elimination of the nearly invariant homolog lagD1 has no such consequences. These findings reveal an early evolutionary origin of kin discrimination and provide insight into the mechanism of social recognition and immunity
A new social gene in Dictyostelium discoideum, chtB
Background: Competitive social interactions are ubiquitous in nature, but their genetic basis is difficult to
determine. Much can be learned from single gene knockouts in a eukaryote microbe. The mutants can be
competed with the parent to discern the social impact of that specific gene. Dictyostelium discoideum is a social
amoeba that exhibits cooperative behavior in the construction of a multicellular fruiting body. It is a good model
organism to study the genetic basis of cooperation since it has a sequenced genome and it is amenable to genetic
manipulation. When two strains of D. discoideum are mixed, a cheater strain can exploit its social partner by
differentiating more spore than its fair share relative to stalk cells. Cheater strains can be generated in the lab or
found in the wild and genetic analyses have shown that cheating behavior can be achieved through many
pathways.
Results: We have characterized the knockout mutant chtB, which was isolated from a screen for cheater mutants
that were also able to form normal fruiting bodies on their own. When mixed in equal proportions with parental
strain cells, chtB mutants contributed almost 60% of the total number of spores. To do so, chtB cells inhibit wild
type cells from becoming spores, as indicated by counts and by the wild type cells’ reduced expression of the
prespore gene, cotB. We found no obvious fitness costs (morphology, doubling time in liquid medium, spore
production, and germination efficiency) associated with the cheating ability of the chtB knockout.
Conclusions: In this study we describe a new gene in D. discoideum, chtB, which when knocked out inhibits the
parental strain from producing spores. Moreover, under lab conditions, we did not detect any fitness costs
associated with this behavior
Nucleotide mutations associated with hepatitis B e antigen negativity
Copyright © 2005 Wiley-Liss, Inc., A Wiley CompanyArticleJOURNAL OF MEDICAL VIROLOGY. 76(2): 170-175 (2005)journal articl
Enhancement of the superconducting transition temperature in La2-xSrxCuO4 bilayers: Role of pairing and phase stiffness
The superconducting transition temperature, Tc, of bilayers comprising
underdoped La2-xSrxCuO4 films capped by a thin heavily overdoped metallic
La1.65Sr0.35CuO4 layer, is found to increase with respect to Tc of the bare
underdoped films. The highest Tc is achieved for x = 0.12, close to the
'anomalous' 1/8 doping level, and exceeds that of the optimally-doped bare
film. Our data suggest that the enhanced superconductivity is confined to the
interface between the layers. We attribute the effect to a combination of the
high pairing scale in the underdoped layer with an enhanced phase stiffness
induced by the overdoped film.Comment: Published versio
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