963 research outputs found
Status of the Whipple Observatory Cerenkov air shower imaging telescope array
Recently the power of the Cerenkov imaging technique in Very High Energy gamma-ray astronomy was demonstrated by the detection of the Crab nebula at high statistical significance. In order to further develop this technique to allow the detection of weaker or more distant sources a second 10 m class reflector was constructed about 120 m from the original instrument. The addition of the second reflector will allow both a reduction in the energy threshold and an improvement in the rejection of the hadronic background. The design and construction of the second reflector, Gamma Ray Astrophysics New Imaging TElescope (GRANITE) is described
Alternative splicing and protein function
BACKGROUND: Alternative splicing is a major mechanism of generating protein diversity in higher eukaryotes. Although at least half, and probably more, of mammalian genes are alternatively spliced, it was not clear, whether the frequency of alternative splicing is the same in different functional categories. The problem is obscured by uneven coverage of genes by ESTs and a large number of artifacts in the EST data. RESULTS: We have developed a method that generates possible mRNA isoforms for human genes contained in the EDAS database, taking into account the effects of nonsense-mediated decay and translation initiation rules, and a procedure for offsetting the effects of uneven EST coverage. Then we computed the number of mRNA isoforms for genes from different functional categories. Genes encoding ribosomal proteins and genes in the category "Small GTPase-mediated signal transduction" tend to have fewer isoforms than the average, whereas the genes in the category "DNA replication and chromosome cycle" have more isoforms than the average. Genes encoding proteins involved in protein-protein interactions tend to be alternatively spliced more often than genes encoding non-interacting proteins, although there is no significant difference in the number of isoforms of alternatively spliced genes. CONCLUSION: Filtering for functional isoforms satisfying biological constraints and accountung for uneven EST coverage allowed us to describe differences in alternative splicing of genes from different functional categories. The observations seem to be consistent with expectations based on current biological knowledge: less isoforms for ribosomal and signal transduction proteins, and more alternative splicing of interacting and cell cycle proteins
Bosonization in Particle Physics
Path integral techniques in collective fields are shown to be a useful
analytical tool to reformulate a field theory defined in terms of microscopic
quark (gluon) degrees of freedom as an effective theory of collective boson
(meson) fields. For illustrations, the path integral bosonization approach is
applied to derive a (non)linear sigma model from a Nambu-Jona-Lasinio (NJL)
quark model. The method can be extended to include higher order derivative
terms in meson fields or heavy-quark symmetries. It is also approximately
applicable to QCD.Comment: 12 pages, LaTeX, uses lamuphys.sty, 5 LaTeX figures, talk given at
the Workshop "Field Theoretical Tools in Polymer and Particle Physics",
University Wuppertal, June 17-19, 199
Axial anomaly: the modern status
The modern status of the problem of axial anomaly in QED and QCD is reviewed.
Two methods of the derivation of the axial anomaly are presented: 1) by
splitting of coordinates in the expression for the axial current and 2) by
calculation of triangle diagrams, where the anomaly arises from the surface
terms in momentum space. It is demonstrated, that the equivalent formulation of
the anomaly can be given, as a sum rule for the structure function in
dispersion representation of three point function of AVV interaction. It is
argued, that such integral representation of the anomaly has some advantages in
the case of description of the anomaly by contribution of hadronic states in
QCD. The validity of the t'Hooft consistency condition is discussed. Few
examples of the physical application of the axial anomaly are given.Comment: 17 pages, 3 figures, to be published in International Journal of
Modern Physics A, few minor correction were done, two references were adde
Dynamical Chiral Symmetry Breaking in QED in a Magnetic Field: Toward Exact Results
We describe a (first, to the best of our knowledge) essentially soluble
example of dynamical symmetry breaking phenomenon in a 3+1 dimensional gauge
theory without fundamental scalar fields: QED in a constant magnetic field.Comment: 4 pages, 1 figure, REVTeX. Final version accepted for publication in
Physical Review Letter
The Burkholderia Genome Database: facilitating flexible queries and comparative analyses
Summary: As the genome sequences of multiple strains of a given bacterial species are obtained, more generalized bacterial genome databases may be complemented by databases that are focused on providing more information geared for a distinct bacterial phylogenetic group and its associated research community. The Burkholderia Genome Database represents a model for such a database, providing a powerful, user-friendly search and comparative analysis interface that contains features not found in other genome databases. It contains continually updated, curated and tracked information about Burkholderia cepacia complex genome annotations, plus other Burkholderia species genomes for comparison, providing a high-quality resource for its targeted cystic fibrosis research community
About the realization of chiral symmetry in QCD2
Two dimensional massless Quantum Chromodynamics presents many features which
resemble those of the true theory. In particular the spectrum consists of
mesons and baryons arranged in flavor multiplets without parity doubling. We
analyze the implications of chiral symmetry, which is not spontaneously broken
in two dimensions, in the spectrum and in the quark condensate. We study how
parity doubling, an awaited consequence of Coleman's theorem, is avoided due to
the dimensionality of space-time and confinement. We prove that a chiral phase
transition is not possible in the theory.Comment: 9 pages, latex, ftuv/92-
Effective Lagrangians in Dimensions
The failure of the the loop expansion and effective lagrangians in two
dimensions, which traditionally hinges on a power counting argument is
considered. We establish that the book keeping device for the loop expansion, a
role played by (the reciprocal of) the pion-decay constant itself vanishes for
, thereby going beyond the power counting argument. We point the
connection of our results to the distinct phases of the candidate for the
effective lagrangians, the non-linear sigma model, in , and
eventually for . In light of our results, we recall some of the relavant
features of the multi-flavor Schwinger and large as candidates
for the underlying theory in .Comment: 13 pages plain LaTeX, to be run twice. Replaced with expanded and
corrected version. One footnote adde
Anomalies without Massless Particles
Baryon and lepton number in the standard model are violated by anomalies,
even though the fermions are massive. This problem is studied in the context of
a two dimensional model. In a uniform background field, fermion production
arise from non-adiabatic behavior that compensates for the absence of massless
modes. On the other hand, for localized instanton-like configurations, there is
an adiabatic limit. In this case, the anomaly is produced by bound states which
travel across the mass gap. The sphaleron corresponds to a bound state at the
halfway point.Comment: (26 pages, 3 figures, uses harvmac and uufiles), UCSD/PTH 93-3
- …