6,904 research outputs found
Selected highlights from the study of mesons
We provide a brief review of recent progress in the study of mesons using
QCD's Dyson-Schwinger equations. Along the way we touch on aspects of
confinement and dynamical chiral symmetry breaking but in the main focus upon:
exact results for pseudoscalar mesons, including aspects of the eta-eta'
problem; a realisation that the so-called vacuum condensates are actually an
intrinsic, localised property of hadrons; an essentially nonperturbative
procedure for constructing a symmetry-preserving Bethe-Salpeter kernel, which
has enabled a demonstration that dressed-quarks possess momentum-dependent
anomalous chromo- and electromagnetic moments that are large at infrared
momenta, and resolution of a longstanding problem in understanding the
mass-splitting between rho- and a1-mesons such that they are now readily seen
to be parity partners in the meson spectrum; features of electromagnetic form
factors connected with charged and neutral pions; and computation and
explanation of valence-quark distribution functions in pseudoscalar mesons. We
argue that in solving QCD, a constructive feedback between theory and extant
and forthcoming experiments will enable constraints to be placed on the
infrared behaviour of QCD's beta-function, the nonperturbative quantity at the
core of hadron physics.Comment: 28 pages, 15 figures, 2 tables. Version to appear in the Chinese
Journal of Physic
Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite LaPrOsSb
We study the influence of inelastic rare-earth impurity scattering on
electron-phonon mediated superconductivity and mass renormalization in
(LaPr)OsSb compounds. Solving the strong coupling
Eliashberg equations we find that the dominant quadrupolar component of the
inelastic scattering on Pr impurities yields an enhancement of the
superconducting transition temperature T in LaOsSb and
increases monotonically as a function of Pr concentration. The calculated
results are in good agreement with the experimentally observed T
dependence. Our analysis suggests that phonons and quadrupolar excitations
cause the attractive electron interaction which results in the formation of
Cooper pairs and singlet superconductivity in PrOsSb.Comment: 5 pages,4 figures, revised title suggested by editor, original fig.4
and fig.5 combined together, discussion added before conclusio
Exposing strangeness: projections for kaon electromagnetic form factors
A continuum approach to the kaon and pion bound-state problems is used to
reveal their electromagnetic structure. For both systems, when used with parton
distribution amplitudes appropriate to the scale of the experiment, Standard
Model hard-scattering formulae are accurate to within 25% at momentum transfers
GeV. There are measurable differences between the
distribution of strange and normal matter within the kaons, e.g. the ratio of
their separate contributions reaches a peak value of at GeV. Its subsequent -evolution is accurately described by the hard
scattering formulae. Projections for kaon and pion form factors at timelike
momenta beyond the resonance region are also presented. These results and
projections should prove useful in planning next-generation experiments.Comment: 7 pages, 4 figure
Flavour symmetry breaking and meson masses
The axial-vector Ward-Takahashi identity is used to derive mass formulae for
neutral pseudoscalar mesons. Flavour symmetry breaking entails non-ideal
flavour content for these states. Adding that the \eta^\prime is not a
Goldstone mode, exact chiral-limit relations are developed from the identity.
They connect the dressed-quark propagator to the topological susceptibility. It
is confirmed that in the chiral limit the \eta^\prime mass is proportional to
the matrix element which connects this state to the vacuum via the topological
susceptibility. The implications of the mass formulae are illustrated using an
elementary dynamical model, which includes an Ansatz for that part of the
Bethe-Salpeter kernel related to the non-Abelian anomaly. In addition to the
current-quark masses, the model involves two parameters, one of which is a
mass-scale. It is employed in an analysis of pseudoscalar- and vector-meson
bound-states. While the effects of SU(N_f=2) and SU(N_f=3) flavour symmetry
breaking are emphasised, the five-flavour spectra are described. Despite its
simplicity, the model is elucidative and phenomenologically efficacious; e.g.,
it predicts \eta-\eta^\prime mixing angles of ~ (-15 degrees) and \pi^0-\eta
angles of ~ 1 degree.Comment: 11 pages, 2 figure
Flavour symmetry breaking in the kaon parton distribution amplitude
We compute the kaon's valence-quark (twist-two parton) distribution amplitude
(PDA) by projecting its Poincare'-covariant Bethe-Salpeter wave-function onto
the light-front. At a scale \zeta=2GeV, the PDA is a broad, concave and
asymmetric function, whose peak is shifted 12-16% away from its position in
QCD's conformal limit. These features are a clear expression of
SU(3)-flavour-symmetry breaking. They show that the heavier quark in the kaon
carries more of the bound-state's momentum than the lighter quark and also that
emergent phenomena in QCD modulate the magnitude of flavour-symmetry breaking:
it is markedly smaller than one might expect based on the difference between
light-quark current masses. Our results add to a body of evidence which
indicates that at any energy scale accessible with existing or foreseeable
facilities, a reliable guide to the interpretation of experiment requires the
use of such nonperturbatively broadened PDAs in leading-order, leading-twist
formulae for hard exclusive processes instead of the asymptotic PDA associated
with QCD's conformal limit. We illustrate this via the ratio of kaon and pion
electromagnetic form factors: using our nonperturbative PDAs in the appropriate
formulae, at spacelike-, which compares
satisfactorily with the value of inferred in annihilation
at .Comment: 7 pages, 2 figures, 3 table
Tailored composite wings with elastically produced chordwise camber
Four structural concepts were created which produce chordwise camber deformation that results in enhanced lift. A wing box can be tailored to utilize each of these with composites. In attempting to optimize the aerodynamic benefits, researchers found that there are two optimum designs that are of interest. There is a weight optimum which corresponds to the maximum lift per unit structural weight. There is also a lift optimum that corresponds to maximum absolute lift. Experience indicates that a large weight penalty accompanies the transition from weight to lift optimum designs. New structural models, the basic deformation mechanisms that are utilized, and typical analytical results are presented. It appears that lift enhancements of sufficient magnitude can be produced to render this type of wing tailoring of practical interest
Topological Quantum Optics in Two-Dimensional Atomic Arrays
We demonstrate that two-dimensional atomic emitter arrays with subwavelength
spacing constitute topologically protected quantum optical systems where the
photon propagation is robust against large imperfections while losses
associated with free space emission are strongly suppressed. Breaking
time-reversal symmetry with a magnetic field results in gapped photonic bands
with non-trivial Chern numbers and topologically protected, long-lived edge
states. Due to the inherent nonlinearity of constituent emitters, such systems
provide a platform for exploring quantum optical analogues of interacting
topological systems.Comment: 11 pages and 9 figures; paper updated to match published versio
A Decision Support System for Building ARIMA Models: Using A Relational Database As A Knowledge Base
Tyrosine phosphorylation is required for Fc receptor-mediated phagocytosis in mouse macrophages
Although Fc receptor-mediated phagocytosis is accompanied by a variety of transmembrane signaling events, not all signaling events are required for particle ingestion. For example, Fc receptor-mediated phagocytosis in mouse inflammatory macrophages (Di Virgilio, F., B. C. Meyer, S. Greenberg, and S. C. Silverstein. 1988. J. Cell Biol. 106:657; Greenberg, S., J. El Khoury, F. Di Virgilio, and S. C. Silverstein. 1991. J. Cell Biol. 113:757) and neutrophils (Della Bianca, V., M. Grzeskowiak, and F. Rossi. 1990. J. Immunol. 144:1411) occurs in the absence of cytosolic calcium transients. We sought to identify transmembrane signaling events that are essential for phagocytosis. Here we show that tyrosine phosphorylation is an early event after Fc receptor ligation in mouse inflammatory macrophages, and that the formation of tyrosine phosphoproteins coincides temporally with the appearance of F-actin beneath phagocytic cups. The distribution of tyrosine phosphoproteins that accumulated beneath phagocytic cups was punctate and corresponded to areas of high ligand density on the surface of the antibody-coated red blood cells, which provided the phagocytic stimulus. A tyrosine kinase inhibitor, genistein, but not several inhibitors of protein kinase C, blocked the appearance of tyrosine phosphoproteins as assessed by immunofluorescence, the focal accumulation of F-actin beneath immunoglobulin G-opsonized particles, and the ingestion of these particles as well. We suggest that tyrosine phosphorylation is a critical signaling event that underlies Fc receptor-mediated phagocytosis in mouse macrophages, and is necessary for the engulfment per se
Identification of the Cell Fate Gene Stalky in Dictyostelium
AbstractUsing insertional mutagenesis, we have isolated a “stalky” mutant in which cells destined to become spores end up as stalk cells. Similar mutants were previously observed after chemical mutagenesis, but the affected gene could not be isolated. Our mutant, like the previous ones, is in stkA. Its defect is cell-autonomous and not overcome by overexpressing cAMP-dependent protein kinase. stkA is strongly expressed in the prespore region of aggregates but not in the anterior prestalk zone. The mutant expresses normal levels of prespore-cell transcripts but fails to produce the spore transcript spiA. stkA encodes a predicted 99 kDa protein (STKA) with two putative C4 zinc fingers, one of which is a GATA-type finger, indicating that it may be a transcription factor. This conclusion is supported by localization of STKA in the nucleus
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