272 research outputs found
Trajectory optimization by a direct descent process
The problem considered is that of trajectory optimization using step-by-step descent to minimum cost along the direction of the cost gradient with respect to the control. Using a hybrid computer, the gradient is computed di rectly as the response to nearly impulsive control perturba tions. A method is presented for computing the gradient when several terminal constraints are enforced. Examples of application of the method are presented. It is concluded that the direct gradient computation method has some significant advantages over other methods.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68738/2/10.1177_003754976801100308.pd
Divalent cations affect chain mobility and aggregate structure of lipopolysaccharide from Salmonella minnesota reflected in a decrease of its biological activity
AbstractThe physicochemical properties and biological activities of rough mutant lipopolysaccharides Re (LPS Re) as preformed divalent cation (Mg2+, Ca2+, Ba2+) salt form or as natural or triethylamine (Ten+)-salt form under the influence of externally added divalent cations were investigated using complementary methods: Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopic (FT-IR) measurements for the ÎČâα gel to liquid crystalline phase behaviour of the acyl chains of LPS, synchrotron radiation X-ray diffraction studies for their aggregate structures, electron density calculations of the LPS bilayer systems, and LPS-induced cytokine (interleukin-6) production in human mononuclear cells. The divalent cation salt forms of LPS exhibit considerable changes in physicochemical parameters such as acyl chain mobility and aggregate structures as compared to the natural or monovalent cation salt forms. Concomitantly, the biological activity was much lower in particular for the Ca2+- and Ba2+-salt forms. This decrease in activity results mainly from the conversion of the unilamellar/cubic aggregate structure of LPS into a multilamellar one. The reduced activity also clearly correlates with the higher order â lower mobility â of the lipid A acyl chains. Both effects can be understood by an impediment of the interactions of LPS with binding proteins such as lipopolysaccharide-binding protein (LBP) and CD14 due to the action of the divalent cations
Exactly solvable model of superstring in Ramond-Ramond plane wave background
We describe in detail the solution of type IIB superstring theory in the
maximally supersymmetric plane-wave background with constant null Ramond-Ramond
5-form field strength. The corresponding light-cone Green-Schwarz action found
in hep-th/0112044 is quadratic in both bosonic and fermionic coordinates. We
find the spectrum of the light-cone Hamiltonian and the string representation
of the supersymmetry algebra. The superstring Hamiltonian has a
``harmonic-oscillator'' form in both the string-oscillator and the zero-mode
parts and thus has discrete spectrum in all 8 transverse directions. We analyze
the structure of the zero-mode sector of the theory, establishing the precise
correspondence between the lowest-lying ``massless'' string states and the type
IIB supergravity fluctuation modes in the plane-wave background. The zero-mode
spectrum has certain similarity to the supergravity spectrum in AdS_5 x S^5 of
which the plane-wave background is a special limit. We also compare the
plane-wave string spectrum with expected form of the light-cone gauge spectrum
of superstring in AdS_5 x S^5.Comment: 33 pages, latex. v4: minor sign corrections in (1.5) and (3.62), to
appear in PR
Superparticle and superstring in AdS_3 x S^3 Ramond-Ramond background in light-cone gauge
We discuss superparticle and superstring dynamics in AdS_3 x S^3 supported by
R-R 3-form background using light-cone gauge approach. Starting with the
superalgebra psu(1,1|2) + psu(1,1|2) representing the basic symmetry of this
background we find the light-cone superparticle Hamiltonian. We determine the
harmonic decomposition of light-cone superfield describing fluctuations of type
IIB supergravity fields expanded near AdS_3 x S^3 background and compute the
corresponding Kaluza-Klein spectrum. We fix the fermionic and bosonic
light-cone gauges in the covariant Green-Schwarz AdS_3 x S^3 superstring action
and find the light-cone string Hamiltonian. We also obtain a realization of the
generators of psu(1,1|2) + psu(1,1|2) in terms of the superstring 2-d fields in
the light-cone gauge.Comment: 32 pages, late
Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24
Supergranulation is a component of solar convection that manifests itself on
the photosphere as a cellular network of around 35 Mm across, with a turnover
lifetime of 1-2 days. It is strongly linked to the structure of the magnetic
field. The horizontal, divergent flows within supergranule cells carry local
field lines to the cell boundaries, while the rotational properties of
supergranule upflows may contribute to the restoration of the poloidal field as
part of the dynamo mechanism that controls the solar cycle. The solar minimum
at the transition from cycle 23 to 24 was notable for its low level of activity
and its extended length. It is of interest to study whether the convective
phenomena that influences the solar magnetic field during this time differed in
character to periods of previous minima. This study investigates three
characteristics (velocity components, sizes and lifetimes) of solar
supergranulation. Comparisons of these characteristics are made between the
minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008,
respectively. It is found that whereas the lifetimes are equal during both
epochs (around 18 h), the sizes are larger in 1996 (35.9 +/- 0.3 Mm) than in
2008 (35.0 +/- 0.3 Mm), while the dominant horizontal velocity flows are weaker
(139 +/- 1 m/s in 1996; 141 +/- 1 m/s in 2008). Although numerical differences
are seen, they are not conclusive proof of the most recent minimum being
inherently unusual.Comment: 22 pages, 5 figures. Solar Physics, in pres
Introduction to M Theory and AdS/CFT Duality
An introductory survey of some of the developments that have taken place in
superstring theory in the past few years is presented. The main focus is on
three particular dualities. The first one is the appearance of an 11th
dimension in the strong coupling limit of the type IIA theory, which give rise
to M theory. The second one is the duality between the type IIB theory
compactified on a circle and M theory on a two-torus. The final topic is an
introduction to the recently proposed duality between superstring theory or M
theory on certain anti de Sitter space backgrounds and conformally invariant
quantum field theories.Comment: 26 pages; To be published in the Proceedings of a conference held in
Corfu, Greece in September 1998. v2: reference adde
Interconnections between type II superstrings, M theory and N=4 supersymmetric Yang--Mills
These lecture notes begin with a review of the first nonleading contributions
to the derivative expansion of the M theory effective action compactified on a
two-torus. The form of these higher-derivative interactions is shown to follow
from ten-dimensional type IIB supersymmetry as well as from one-loop quantum
corrections to classical eleven-dimensional supergravity. The detailed
information concerning D-instanton effects encoded in these terms is related to
the problem of evaluating the Witten index for D-particles in the type IIA
theory. Using the AdS/CFT conjecture, it also leads to very specific
predictions of multi-instanton contributions in supersymmetric SU(N)
Yang--Mills theory in the limit of strong 't Hooft coupling. [Extended version
of lectures given at 22nd Johns Hopkins Workshop (Gothenberg, August 20-22
1998); `Quantum Aspects of Gauge Theories, Supersymmetry and Unification', TMR
meeting (Corfu, September 20-26 1998); Andrjewski lectures (Berlin, November
1-6 1998).]Comment: 80 pages, LaTe
Lifetimes of High-Degree p Modes in the Quiet and Active Sun
We study variations of the lifetimes of high-degree solar p-modes in the
quiet and active Sun with the solar activity cycle. The lifetimes in the degree
range 300 - 600 and frequency 2.5 - 4.5 mHz were computed from SOHO/MDI data in
an area including active regions and quiet Sun using the time-distance
technique. We applied our analysis to the data in four different phases of
solar activity: in 1996 (at minimum), 1998 (rising phase), 2000 (at maximum)
and 2003 (declining phase). The results from the area with active regions show
that the lifetime decreases as activity increases. The maximal lifetime
variations are between solar minimum in 1996 and maximum in 2000; the relative
variation averaged over all mode degree values and frequencies is a decrease of
about 13%. The lifetime reductions relative to 1996 are about 7% in 1998 and
about 10% in 2003. The lifetime computed in the quiet region still decreases
with solar activity although the decrease is smaller. On average, relative to
1996, the lifetime decrease is about 4% in 1998, 10% in 2000 and 8% in 2003.
Thus, measured lifetime increases when regions of high magnetic activity are
avoided. Moreover, the lifetime computed in quiet regions also shows variations
with activity cycle.Comment: 13 pages, 5 figures; Accepted for publication in Solar Physic
Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS
We present the results of a search for new, heavy particles that decay at a
significant distance from their production point into a final state containing
charged hadrons in association with a high-momentum muon. The search is
conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV
and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS
detector operating at the Large Hadron Collider. Production of such particles
is expected in various scenarios of physics beyond the standard model. We
observe no signal and place limits on the production cross-section of
supersymmetric particles in an R-parity-violating scenario as a function of the
neutralino lifetime. Limits are presented for different squark and neutralino
masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final
version to appear in Physics Letters
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
- âŠ