655 research outputs found
Spinor and Isospinor Structure of Relativistic Particle Propagators
Representations by means of path integrals are used to find spinor and
isospinor structure of relativistic particle propagators in external fields.
For Dirac propagator in an external electromagnetic field all grassmannian
integrations are performed and a general result is presented via a bosonic path
integral. The spinor structure of the integrand is given explicitly by its
decomposition in the independent -matrix structures. Similar technique
is used to get the isospinor structure of the scalar particle propagator in an
external non-Abelian field.Comment: 9 pages, Preprint IC/93/197 Triest
Highly Scalable Algorithms for Robust String Barcoding
String barcoding is a recently introduced technique for genomic-based
identification of microorganisms. In this paper we describe the engineering of
highly scalable algorithms for robust string barcoding. Our methods enable
distinguisher selection based on whole genomic sequences of hundreds of
microorganisms of up to bacterial size on a well-equipped workstation, and can
be easily parallelized to further extend the applicability range to thousands
of bacterial size genomes. Experimental results on both randomly generated and
NCBI genomic data show that whole-genome based selection results in a number of
distinguishers nearly matching the information theoretic lower bounds for the
problem
Nonadiabatic charged spherical evolution in the postquasistatic approximation
We apply the postquasistatic approximation, an iterative method for the
evolution of self-gravitating spheres of matter, to study the evolution of
dissipative and electrically charged distributions in General Relativity. We
evolve nonadiabatic distributions assuming an equation of state that accounts
for the anisotropy induced by the electric charge. Dissipation is described by
streaming out or diffusion approximations. We match the interior solution, in
noncomoving coordinates, with the Vaidya-Reissner-Nordstr\"om exterior
solution. Two models are considered: i) a Schwarzschild-like shell in the
diffusion limit; ii) a Schwarzschild-like interior in the free streaming limit.
These toy models tell us something about the nature of the dissipative and
electrically charged collapse. Diffusion stabilizes the gravitational collapse
producing a spherical shell whose contraction is halted in a short
characteristic hydrodynamic time. The streaming out radiation provides a more
efficient mechanism for emission of energy, redistributing the electric charge
on the whole sphere, while the distribution collapses indefinitely with a
longer hydrodynamic time scale.Comment: 11 pages, 16 Figures. Accepted for publication in Phys Rev
Applications of vortex gas models to tornadogenesis and maintenance
Processes related to the production of vorticity in the forward and rear
flank downdrafts and their interaction with the boundary layer are thought to
play a role in tornadogenesis. We argue that an inverse energy cascade is a
plausible mechanism for tornadogenesis and tornado maintenance and provide
supporting evidence which is both numerical and observational. We apply a
three-dimensional vortex gas model to supercritical vortices produced at the
surface boundary layer possibly due to interactions of vortices brought to the
surface by the rear flank downdraft and also to those related to the forward
flank downdraft. Two-dimensional and three-dimensional vortex gas models are
discussed, and the three-dimensional vortex gas model of Chorin, developed
further by Flandoli and Gubinelli, is proposed as a model for intense small-
scale subvortices found in tornadoes and in recent numerical studies by Orf et
al. In this paper, the smaller scales are represented by intense, supercritical
vortices, which transfer energy to the larger-scale tornadic flows (inverse
energy cascade). We address the formation of these vortices as a result of the
interaction of the flow with the surface and a boundary layer.Comment: 20 pages, 6 figure
Time and length scales of autocrine signals in three dimensions
A model of autocrine signaling in cultures of suspended cells is developed on
the basis of the effective medium approximation. The fraction of autocrine
ligands, the mean and distribution of distances traveled by paracrine ligands
before binding, as well as the mean and distribution of the ligand lifetime are
derived. Interferon signaling by dendritic immune cells is considered as an
illustration.Comment: 15 page
Dilution of zero point energies in the cosmological expansion
The vacuum fluctuations of all quantum fields filling the universe are
supposed to leave enormous energy and pressure contributions which are
incompatible with observations. It has been recently suggested that, when the
effective nature of quantum field theories is properly taken into account,
vacuum fluctuations behave as a relativistic gas rather than as a cosmological
constant. Accordingly, zero-point energies are tremendously diluted by the
universe expansion but provide an extra contribution to radiation energy.
Ongoing and future cosmological observations could offer the opportunity to
scrutinize this scenario. The presence of such additional contribution to
radiation energy can be tested by using primordial nucleosynthesis bounds or
measured on Cosmic Background Radiation anisotropy.Comment: 8 pages, no figures. Submitted the 17th of March to Modern Physics
Letters
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