1,917 research outputs found
Algorithm Diversity for Resilient Systems
Diversity can significantly increase the resilience of systems, by reducing
the prevalence of shared vulnerabilities and making vulnerabilities harder to
exploit. Work on software diversity for security typically creates variants of
a program using low-level code transformations. This paper is the first to
study algorithm diversity for resilience. We first describe how a method based
on high-level invariants and systematic incrementalization can be used to
create algorithm variants. Executing multiple variants in parallel and
comparing their outputs provides greater resilience than executing one variant.
To prevent different parallel schedules from causing variants' behaviors to
diverge, we present a synchronized execution algorithm for DistAlgo, an
extension of Python for high-level, precise, executable specifications of
distributed algorithms. We propose static and dynamic metrics for measuring
diversity. An experimental evaluation of algorithm diversity combined with
implementation-level diversity for several sequential algorithms and
distributed algorithms shows the benefits of algorithm diversity
A simple toy model for effective restoration of chiral symmetry in excited hadrons
A simple solvable toy model exhibiting effective restoration of chiral
symmetry in excited hadrons is constructed. A salient feature is that while
physics of the low-lying states is crucially determined by the spontaneous
breaking of chiral symmetry, in the high-lying states the effects of chiral
symmetry breaking represent only a small correction. Asymptotically the states
approach the regime where their properties are determined by the underlying
unbroken chiral symmetry.Comment: This is the published version of this paper. Note that the title has
changed from earlier versions as has the abstract. The emphasis is slightly
different from previous versions but the essential physical content is the
sam
Why the high lying glueball does not mix with the neighbouring
Chiral symmetry restoration in high-lying hadron spectra implies that hadrons
which belong to different irreducible representations of the parity-chiral
group cannot mix. This explains why the , which was suggested
to be a glueball, and hence must belong to the scalar (0,0) representation of
the chiral group, cannot mix with the neighbouring , which
was interpreted as a state, and that belongs to the
representation of the chiral group. If confirmed, then we have an access to a
"true" glueball of QCD.Comment: 4 pages, LaTeX, final version, Eur. Phys. J. A 19 (2004) 15
Chiral multiplets versus parity doublets in highly excited baryons
It has recently been suggested that the parity doublet structure seen in the
spectrum of highly excited baryons may be due to effective chiral restoration
for these states. We argue how the idea of chiral symmetry restoration high in
the spectrum is consistent with the concept of quark-hadron duality. If chiral
symmetry is effectively restored for highly-lying states, then the baryons
should fall into representations of that are compatible
with the given parity of the states - the parity-chiral multiplets. We classify
all possible parity-chiral multiplets: (i) that contain
parity doublet for nucleon spectrum;(ii) consists of
the parity doublet for delta spectrum; (iii) contains
one parity doublet in the nucleon spectrum and one parity doublet in the delta
spectrum of the same spin that are degenerate in mass. Here we show that the
available spectroscopic data for nonstrange baryons in the 2 GeV range
is consistent with all possibilities, but the approximate degeneracy of parity
doublets in nucleon and delta spectra support the latter possibility with
excited baryons approximately falling into
representation of SU(2)_L\timesSU(2)_R with approximate degeneracy between
positive and negative parity and resonances of the same spin.Comment: RevTeX, 6 pages. The paper has been expanded in order to make the
idea of chiral symmetry restoration as it follows from the concept of
quark-hadron duality more transparent. To appear in Phys. Rev.
Drosophila Melted Modulates FOXO and TOR Activity
SummaryThe insulin/PI3K signaling pathway controls both tissue growth and metabolism. Here, we identify Melted as a new modulator of this pathway in Drosophila. Melted interacts with both Tsc1 and FOXO and can recruit these proteins to the cell membrane. We provide evidence that in the melted mutant, TOR activity is reduced and FOXO is activated. The melted mutant condition mimics the effects of nutrient deprivation in a normal animal, producing an animal with 40% less fat than normal
The space of m-ary differential operators as a module over the Lie algebra of vector fields
The space of m-ary differential operators acting on weighted densities is a
(m+1)-parameter family of modules over the Lie algebra of vector fields. For
almost all the parameters, we construct a canonical isomorphism between this
space and the corresponding space of symbols as sl(2)-modules. This yields to
the notion of the sl(2)-equivariant symbol calculus for m-ary differential
operators. We show, however, that these two modules cannot be isomorphic as
sl(2)-modules for some particular values of the parameters. Furthermore, we use
the symbol map to show that all modules of second-order m-ary differential
operators are isomorphic to each other, except for few modules called singular.Comment: 20 pages; LaTeX2e; minor correction
Dressed matter waves
We suggest to view ultracold atoms in a time-periodically shifted optical
lattice as a "dressed matter wave", analogous to a dressed atom in an
electromagnetic field. A possible effect lending support to this concept is a
transition of ultracold bosonic atoms from a superfluid to a Mott-insulating
state in response to appropriate "dressing" achieved through time-periodic
lattice modulation. In order to observe this effect in a laboratory experiment,
one has to identify conditions allowing for effectively adiabatic motion of a
many-body Floquet state.Comment: 9 pages, 4 figures, to be published in: J. Phys.: Conference Serie
Lyapunov Exponents from Kinetic Theory for a Dilute, Field-driven Lorentz Gas
Positive and negative Lyapunov exponents for a dilute, random,
two-dimensional Lorentz gas in an applied field, , in a steady state
at constant energy are computed to order . The results are:
where
are the exponents for the field-free Lorentz gas,
, is the mean free time between collisions,
is the charge, the mass and is the speed of the particle. The
calculation is based on an extended Boltzmann equation in which a radius of
curvature, characterizing the separation of two nearby trajectories, is one of
the variables in the distribution function. The analytical results are in
excellent agreement with computer simulations. These simulations provide
additional evidence for logarithmic terms in the density expansion of the
diffusion coefficient.Comment: 7 pages, revtex, 3 postscript figure
- …