929 research outputs found
Lepton-Flavour Violation in Ordinary and Supersymmetric Grand Unified Theories
By an explicit calculation we show that in ordinary SU(5) logarithmic
divergence in the amplitude of cancels among diagrams and
remaining finite part is suppressed by at least . In SUSY SU(5),
when the effect of flavour changing wave function renormalization is taken into
account such logarithmic correction disappears, provided a condition is met
among SUSY breaking masses. In SUGRA-inspired SUSY GUT the remaining
logarithmic effect is argued not to be taken as a prediction of the theory.Comment: 8 pages, LaTeX209 file, using axodraw.st
On P Systems as a Modelling Tool for Biological Systems
We introduce a variant of P systems where rules have associated
a real number providing a measure for the âintrinsic reactivityâof
the rule and roughly corresponding to the kinetic coefficient which, in
bio-chemistry, is usually associated to each molecular reaction. The behaviour
of these P systems is then defined according to a strategy which,
in each step, randomly selects the next rule to be applied depending upon
a certain distribution of probabilities. As an application, we present a P
system model of the quorum sensing regulatory networks of the bacterium
Vibrio Fischeri. In this respect, a formalisation of the network
in terms of P systems is provided and some simulation results concerning
the behaviour of a colony of such bacteria are reported. We also
briefly describe the implementation techniques adopted by pointing out
the generality of our approach which appears to be fairly independent
from the particular choice of P system variant and the language used to
implement it.Ministerio de Ciencia y TecnologĂa TIC2002-04220-C03-0
The M 31 double nucleus probed with OASIS and HST. A natural m=1 mode?
We present observations with the adaptive optics assisted integral field
spectrograph OASIS of the M 31 double nucleus at a spatial resolution better
than 0.5 arcsec FWHM. These data are used to derive the two-dimensional stellar
kinematics within the central 2 arcsec. Archival WFPC2/HST images are revisited
to perform a photometric decomposition of the nuclear region. We also present
STIS/HST kinematics obtained from the archive. The luminosity distribution of
the central region is well separated into the respective contributions of the
bulge, the nucleus including P1 and P2, and the so-called UV peak. We then show
that the axis joining P1 and P2, the two local surface brightness maxima, does
not coincide with the kinematic major-axis, which is also the major-axis of the
nuclear isophotes (excluding P1). We also confirm that the velocity dispersion
peak is offset by ~ 0.2 arcsec from the UV peak, assumed to mark the location
of the supermassive black hole. The newly reduced STIS/HST velocity and
dispersion profiles are then compared to OASIS and other published kinematics.
We find significant offsets with previously published data. Simple parametric
models are then built to successfully reconcile all the available kinematics.
We finally interpret the observations using new N-body simulations. The nearly
keplerian nuclear disk of M31 is subject to a natural m=1 mode, with a very
slow pattern speed (3 km/s/pc for M_BH = 7 10^7~\Msun), that can be maintained
during more than a thousand dynamical times. The resulting morphology and
kinematics of the mode can reproduce the M~31 nuclear-disk photometry and mean
stellar velocity, including the observed asymmetries. It requires a central
mass concentration and a cold disk system representing between 20 and 40% of
its mass. Abridged..Comment: 21 pages. accepted for publication in A&
Evolution of central pattern generators for the control of a five-link bipedal walking mechanism
Central pattern generators (CPGs), with a basis is neurophysiological
studies, are a type of neural network for the generation of rhythmic motion.
While CPGs are being increasingly used in robot control, most applications are
hand-tuned for a specific task and it is acknowledged in the field that generic
methods and design principles for creating individual networks for a given task
are lacking. This study presents an approach where the connectivity and
oscillatory parameters of a CPG network are determined by an evolutionary
algorithm with fitness evaluations in a realistic simulation with accurate
physics. We apply this technique to a five-link planar walking mechanism to
demonstrate its feasibility and performance. In addition, to see whether
results from simulation can be acceptably transferred to real robot hardware,
the best evolved CPG network is also tested on a real mechanism. Our results
also confirm that the biologically inspired CPG model is well suited for legged
locomotion, since a diverse manifestation of networks have been observed to
succeed in fitness simulations during evolution.Comment: 11 pages, 9 figures; substantial revision of content, organization,
and quantitative result
Raman Scattering Study of Ba-doped C60 with t1g States
Raman spectra are reported for Ba doped fullerides, BaxC60(x=3,4,and 6). The
lowest frequency Hg modes split into five components for Ba4C60 and Ba6C60 even
at room temperature, allowing us a quantitative analysis based on the
electron-phonon couping theory. For the superconducting Ba4C60, the density of
states at the Fermi energy was derived as 7 eV-1, while the total value of
electron-phonon coupling \lambda was found to be 1.0, which is comparable to
that of K3C60. The tangential Ag(2) mode, which is known as a sensitive probe
for the degree of charge transfer on C60 molecule, shows a remarkable shift
depending on the Ba concentration, being roughly consistent with the full
charge transfer from Ba to C60. An effect of hybridization between Ba and C60
\pi orbitals is also discussed.Comment: 15 pages, 6 figures submitted to Phys. Rev. B (December 1,1998
Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826
We present new high-resolution observations of the nucleus of the
counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with
the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve
0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas
disk which is truncated at an outer radius of 700pc. The total molecular gas
mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm
spirals, which develop at different radii in the disk. The distribution and
kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of
different types of m=1 perturbations in the gas. Although dominated by
rotation, the gas kinematics are perturbed by streaming motions related to the
m=1 instabilities. The non-circular motions associated with the inner m=1
perturbations agree qualitatively with the pattern expected for a trailing wave
developed outside corotation ('fast' wave). In contrast, the streaming motions
in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical
consequence is that the inner m=1 perturbations would not favour AGN feeding.
An independent confirmation that the AGN is not being generously fueled at
present is found in the low values of the gravitational torques exerted by the
stellar potential for R<530pc. The distribution of star formation in the disk
of NGC4826 is also strongly asymmetrical. Massive star formation is still
vigorous, fed by the significant molecular gas reservoir at R<700pc. There is
supporting evidence for a recent large mass inflow episode in NGC4826.
These observations have been made in the context of the NUclei of GAlaxies
(NUGA) project, aimed at the study of the different mechanisms for gas fueling
of AGN.Comment: A&A, 2003, Paper accepted (04/06/03). For a full-resolution version
of this paper see http://www.oan.es/preprint
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Phylogenetic distribution and experimental characterization of corrinoid production and dependence in soil bacterial isolates
Soil microbial communities impact carbon sequestration and release, biogeochemical cycling, and agricultural yields. These global effects rely on metabolic interactions that modulate community composition and function. However, the physicochemical and taxonomic complexity of soil and the scarcity of available isolates for phenotypic testing are significant barriers to studying soil microbial interactions. Corrinoids-the vitamin B12 family of cofactors-are critical for microbial metabolism, yet they are synthesized by only a subset of microbiome members. Here, we evaluated corrinoid production and dependence in soil bacteria as a model to investigate the ecological roles of microorganisms involved in metabolic interactions. We isolated and characterized a taxonomically diverse collection of 161 soil bacteria from a single study site. Most corrinoid-dependent bacteria in the collection prefer B12 over other corrinoids, while all tested producers synthesize B12, indicating metabolic compatibility between producers and dependents in the collection. Furthermore, a subset of producers release B12 at levels sufficient to support dependent isolates in laboratory culture at estimated ratios of up to 1000 dependents per producer. Within our isolate collection, we did not find strong phylogenetic patterns in corrinoid production or dependence. Upon investigating trends in the phylogenetic dispersion of corrinoid metabolism categories across sequenced bacteria from various environments, we found that these traits are conserved in 47 out of 85 genera. Together, these phenotypic and genomic results provide evidence for corrinoid-based metabolic interactions among bacteria and provide a framework for the study of nutrient-sharing ecological interactions in microbial communities
Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC
Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H âÎł Îł, H â Z Zâ â4l and H âW Wâ âlνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of âs = 7 TeV and âs = 8 TeV, corresponding to an integrated luminosity of about 25 fbâ1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined ďŹts probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson
Standalone vertex ďŹnding in the ATLAS muon spectrometer
A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at âs = 7 TeV collected with the ATLAS detector at the LHC during 2011
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