271 research outputs found
Teleparallel Gravity and Dimensional Reductions of Noncommutative Gauge Theory
We study dimensional reductions of noncommutative electrodynamics on flat
space which lead to gauge theories of gravitation. For a general class of such
reductions, we show that the noncommutative gauge fields naturally yield a
Weitzenbock geometry on spacetime and that the induced diffeomorphism invariant
field theory can be made equivalent to a teleparallel formulation of gravity
which macroscopically describes general relativity. The Planck length is
determined in this setting by the Yang-Mills coupling constant and the
noncommutativity scale. The effective field theory can also contain
higher-curvature and non-local terms which are characteristic of string theory.
Some applications to D-brane dynamics and generalizations to include the
coupling of ordinary Yang-Mills theory to gravity are also described.Comment: 31 pages LaTeX; References adde
Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides
We present scanning tunneling spectroscopy and microscopy measurements at low
temperatures in the borocarbide materials RNi2B2C (R=Y, Lu). The characteristic
strong coupling structure due to the pairing interaction is unambiguously
resolved in the superconducting density of states. It is located at the
superconducting gap plus the energy corresponding to a phonon mode identified
in previous neutron scattering experiments. These measurements also show that
this mode is coupled to the electrons through a highly anisotropic
electron-phonon interaction originated by a nesting feature of the Fermi
surface. Our experiments, from which we can extract a large electron-phonon
coupling parameter lambda (between 0.5 and 0.8), demonstrate that this
anisotropic electron-phonon coupling has an essential contribution to the
pairing interaction. The tunneling spectra show an anisotropic s-wave
superconducting gap function.Comment: 5 pages, 3 figure
Acute biphenotypic leukaemia: immunophenotypic and cytogenetic analysis
The incidence of acute biphenotypic leukaemia has ranged from less than 1% to almost 50% in various reports in the literature. This wide variability may be attributed to a number of reasons including lack of consistent diagnostic criteria, use of various panels of antibodies, and the failure to recognize the lack of lineage specificity of some of the antibodies used. The morphology, cytochemistry, immunophenotype and cytogenetics of acute biphenotypic leukaemias from our institution were studied. The diagnostic criteria took into consideration the morphology of the analysed cells, light scatter characteristics, and evaluation of antibody fluorescence histograms in determining whether the aberrant marker expression was arising from leukaemic blasts or differentiated bone marrow elements. Fifty-two of 746 cases (7%) fulfilled our criteria for acute biphenotypic leukaemias. These included 30 cases of acute lymphoblastic leukaemia (ALL) expressing myeloid antigens, 21 cases of acute myelogenous leukaemia (AML) expressing lymphoid markers, and one case of ALL expressing both B- and T-cell associated antigens. The acute biphenotypic leukaemia cases consisted of four major immunophenotypic subgroups: CD2± AML (11), CD19± AML (8), CD13 and/or CD33± ALL (24), CD11b± ALL (5) and others (4). Chromosomal analysis was carried out in 42/52 of the acute biphenotypic leukaemia cases; a clonal abnormality was found in 31 of these 42 cases. This study highlights the problems encountered in the diagnosis of acute biphenotypic leukaemia, some of which may be reponsible for the wide variation in the reported incidence of this leukaemia. We suggest that the use of strict, uniform diagnostic criteria may help in establishing a more consistent approach towards diagnosis of this leukaemic entity. We also suggest that biphenotypic leukaemia is comprised of biologically different groups of leukaemia based on immunophenotypic and cytogenetic findings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73301/1/j.1365-2141.1993.tb03024.x.pd
Understanding Galaxy Formation and Evolution
The old dream of integrating into one the study of micro and macrocosmos is
now a reality. Cosmology, astrophysics, and particle physics intersect in a
scenario (but still not a theory) of cosmic structure formation and evolution
called Lambda Cold Dark Matter (LCDM) model. This scenario emerged mainly to
explain the origin of galaxies. In these lecture notes, I first present a
review of the main galaxy properties, highlighting the questions that any
theory of galaxy formation should explain. Then, the cosmological framework and
the main aspects of primordial perturbation generation and evolution are
pedagogically detached. Next, I focus on the ``dark side'' of galaxy formation,
presenting a review on LCDM halo assembling and properties, and on the main
candidates for non-baryonic dark matter. It is shown how the nature of
elemental particles can influence on the features of galaxies and their
systems. Finally, the complex processes of baryon dissipation inside the
non-linearly evolving CDM halos, formation of disks and spheroids, and
transformation of gas into stars are briefly described, remarking on the
possibility of a few driving factors and parameters able to explain the main
body of galaxy properties. A summary and a discussion of some of the issues and
open problems of the LCDM paradigm are given in the final part of these notes.Comment: 50 pages, 10 low-resolution figures (for normal-resolution, DOWNLOAD
THE PAPER (PDF, 1.9 Mb) FROM http://www.astroscu.unam.mx/~avila/avila.pdf).
Lectures given at the IV Mexican School of Astrophysics, July 18-25, 2005
(submitted to the Editors on March 15, 2006
Dynamic protein methylation in chromatin biology
Post-translational modification of chromatin is emerging as an increasingly important regulator of chromosomal processes. In particular, histone lysine and arginine methylation play important roles in regulating transcription, maintaining genomic integrity, and contributing to epigenetic memory. Recently, the use of new approaches to analyse histone methylation, the generation of genetic model systems, and the ability to interrogate genome wide histone modification profiles has aided in defining how histone methylation contributes to these processes. Here we focus on the recent advances in our understanding of the histone methylation system and examine how dynamic histone methylation contributes to normal cellular function in mammals
Diagnosis of biliary tract and ampullary carcinomas
Diagnostic methods for biliary tract carcinoma and the efficacy of these methods are discussed. Neither definite methods for early diagnosis nor specific markers are available in this disease. When this disease is suspected on the basis of clinical symptoms and risk factors, hemato-biochemical examination and abdominal ultrasonography are performed and, where appropriate, enhanced computed tomography (CT) and/or magnetic resonance cholangiopancreatography (MRCP) is carried out. Diagnoses of extrahepatic bile duct cancer and ampullary carcinoma are often made based on the presence of obstructive jaundice. Although rare, abdominal pain and pyrexia, as well as abnormal findings of the hepatobiliary system detected by hemato-biochemical examination, serve as a clue to making a diagnosis of these diseases. On the other hand, the early diagnosis of gallbladder cancer is scarcely possible on the basis of clinical symptoms, so when this cancer is found with the onset of abdominal pain and jaundice, it is already advanced at the time of detection, thus making a cure difficult. When gallbladder cancer is suspected, enhanced CT is carried out. Multidetector computed tomography (MDCT), in particular — one of the methods of enhanced CT — is useful for decision of surgical criteria, because MDCT shows findings such as localization and extension of the tumor, and the presence or absence of remote metastasis. Procedures such as magnetic resonance imaging, endoscopic ultrasonography, bile duct biopsy, and cholangioscopy should be carried out taking into account indications for these procedures in individual patients. However, direct biliary tract imaging is necessary for making a precise diagnosis of the horizontal extension of bile duct cancer
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