257 research outputs found
Ramond-Ramond Fields, Fractional Branes and Orbifold Differential K-Theory
We study D-branes and Ramond-Ramond fields on global orbifolds of Type II
string theory with vanishing H-flux using methods of equivariant K-theory and
K-homology. We illustrate how Bredon equivariant cohomology naturally realizes
stringy orbifold cohomology. We emphasize its role as the correct cohomological
tool which captures known features of the low-energy effective field theory,
and which provides new consistency conditions for fractional D-branes and
Ramond-Ramond fields on orbifolds. We use an equivariant Chern character from
equivariant K-theory to Bredon cohomology to define new Ramond-Ramond couplings
of D-branes which generalize previous examples. We propose a definition for
groups of differential characters associated to equivariant K-theory. We derive
a Dirac quantization rule for Ramond-Ramond fluxes, and study flat
Ramond-Ramond potentials on orbifolds.Comment: 46 pages; v2: typos correcte
Detection of Atmospheric Cherenkov Radiation Using Solar Heliostat Mirrors
The gamma-ray energy region between 20 and 250 GeV is largely unexplored.
Ground-based atmospheric Cherenkov detectors offer a possible way to explore
this region, but large Cherenkov photon collection areas are needed to achieve
low energy thresholds. This paper discusses the development of a Cherenkov
detector using the heliostat mirrors of a solar power plant as the primary
collector. As part of this development, we built a prototype detector
consisting of four heliostat mirrors and used it to record atmospheric
Cherenkov radiation produced in extensive air showers created by cosmic ray
particles.Comment: 16 latex pages, 8 postscript figures, uses psfig.sty, to be published
in Astroparticle Physic
Absence of a metallic phase in random-bond Ising models in two dimensions: applications to disordered superconductors and paired quantum Hall states
When the two-dimensional random-bond Ising model is represented as a
noninteracting fermion problem, it has the same symmetries as an ensemble of
random matrices known as class D. A nonlinear sigma model analysis of the
latter in two dimensions has previously led to the prediction of a metallic
phase, in which the fermion eigenstates at zero energy are extended. In this
paper we argue that such behavior cannot occur in the random-bond Ising model,
by showing that the Ising spin correlations in the metallic phase violate the
bound on such correlations that results from the reality of the Ising
couplings. Some types of disorder in spinless or spin-polarized p-wave
superconductors and paired fractional quantum Hall states allow a mapping onto
an Ising model with real but correlated bonds, and hence a metallic phase is
not possible there either. It is further argued that vortex disorder, which is
generic in the fractional quantum Hall applications, destroys the ordered or
weak-pairing phase, in which nonabelian statistics is obtained in the pure
case.Comment: 13 pages; largely independent of cond-mat/0007254; V. 2: as publishe
Prime ideals in nilpotent Iwasawa algebras
Let G be a nilpotent complete p-valued group of finite rank and let k be a
field of characteristic p. We prove that every faithful prime ideal of the
Iwasawa algebra kG is controlled by the centre of G, and use this to show that
the prime spectrum of kG is a disjoint union of commutative strata. We also
show that every prime ideal of kG is completely prime. The key ingredient in
the proof is the construction of a non-commutative valuation on certain
filtered simple Artinian rings
Assessment of weeds of cassava and farmers management practices in Nigeria
Open Access JournalLa concurrence des mauvaises herbes est un obstacle à la rentabilité de la production du manioc. La connaissance des espèces de mauvaises herbes qui affectent négativement la productivité du manioc est essentielle pour une gestion efficace. Une étude a été conduite entre mai et juin 2014 et 2015 pour évaluer les mauvaises herbes dans 200 exploitations agricoles de manioc, dans trois zones agroécologiques au Nigeria. L’Analyse des Correspondances Détendancées (ACD) a permis d’identifier quatre groupes distincts illustrant la variation des espèces de mauvaises herbes parmi les zones agroécologiques. Le pH du sol et la teneur en limon, la durée des jachères, la méthode de culture et la méthode de gestion des mauvaises herbes ont contribué à la variation de la composition des espèces. L’évaluation par les agriculteurs et sur le terrain ont identifié Euphorbia heterophylla, Imperata cylindrica, Aspilia africana, Panicum maximum, Chromolaena odorata, Commelina benghalensis, Digitaria horizontalis, et Rottboellia cochinchinensis comme principales mauvaises herbes du manioc. La gestion de ces mauvaises herbes par les paysans varie à travers les zones, suggérant ainsi que les stratégies de gestion de ces mauvaises herbes devraient être axées sur les zones écologiques. Dans la zone forestière humide, le désherbage à la houe (51,2%) et à la machette (43,0%) étaient les principales méthodes de contrôle. L’utilisation d’herbicides était élevée dans le sud de la savane guinéenne et modérée dans la savane dérivée. L’éducation afin d’accroitre la connaissance des agriculteurs sur la problématique des mauvaises herbes et l’amélioration de leur choix à la fois sur les herbicides appropriés et leur utilisation sans risque est essentielle pour une gestion efficace des mauvaises herbes dans la culture du manioc.
Competition from weeds is an obstacle to profitable cassava production. Knowledge of weed species negatively affecting productivity is essential for effective management. A field evaluation of weeds and management practices was conducted between May and June in 2014 and 2015 in 200 cassava farms in three agroecologies in Nigeria. Detrended Correspondence Analysis identified four distinct clusters depicting variation in weed species composition among the agroecologies. Soil pH and silt content, fallow length, cultivation method, and weed management method contributed to the variation in species composition. Farmers and field evaluations identified Euphorbia heterophylla, Imperata cylindrica, Aspilia africana, Panicum maximum, Chromolaena odorata, Commelina benghalensis, Digitaria horizontalis, and Rottboellia cochinchinensis as major problem weeds in cassava. Farmers’ management of these weeds varied across zones, suggesting that weed management strategies in cassava should be focused on ecological zones. In the Humid forest, hoe-weeding (51.2%) and slashing (43.0%) with machetes were the predominant methods of control. Herbicide use was high in the Southern Guinea Savanna and medium to high in the Derived Savanna (26.3-42.2%). Education to increase farmers’ knowledge of the problematic of weeds and to improve both their choice of appropriate herbicides and their safe use is critical to effective and efficient weed management in cassava
Very high energy observations of the BL Lac objects 3C 66A and OJ 287
Using the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), we
have observed the BL Lac objects 3C 66A and OJ 287. These are members of the
class of low-frequency-peaked BL Lac objects (LBLs) and are two of the three
LBLs predicted by Costamante and Ghisellini to be potential sources of very
high energy (>100 GeV) gamma-ray emission. The third candidate, BL Lacertae,
has recently been detected by the MAGIC collaboration. Our observations have
not produced detections; we calculate a 99% CL upper limit of flux from 3C 66A
of 0.15 Crab flux units and from OJ 287 our limit is 0.52 Crab. These limits
assume a Crab-like energy spectrum with an effective energy threshold of 185
GeV.Comment: 24 pages, 15 figures, Accepted for publication in Astroparticle
Physic
Building a Better Racetrack
We find IIb compactifications on Calabi-Yau orientifolds in which all Kahler
moduli are stabilized, along lines suggested by Kachru, Kallosh, Linde and
Trivedi.Comment: 47 pages, 1 figure, harvmac (v2: added references, minor comments,
v3: improved discussion of metastability and explicit flux vacua
L-infinity algebra connections and applications to String- and Chern-Simons n-transport
We give a generalization of the notion of a Cartan-Ehresmann connection from
Lie algebras to L-infinity algebras and use it to study the obstruction theory
of lifts through higher String-like extensions of Lie algebras. We find
(generalized) Chern-Simons and BF-theory functionals this way and describe
aspects of their parallel transport and quantization.
It is known that over a D-brane the Kalb-Ramond background field of the
string restricts to a 2-bundle with connection (a gerbe) which can be seen as
the obstruction to lifting the PU(H)-bundle on the D-brane to a U(H)-bundle. We
discuss how this phenomenon generalizes from the ordinary central extension
U(1) -> U(H) -> PU(H) to higher categorical central extensions, like the
String-extension BU(1) -> String(G) -> G. Here the obstruction to the lift is a
3-bundle with connection (a 2-gerbe): the Chern-Simons 3-bundle classified by
the first Pontrjagin class. For G = Spin(n) this obstructs the existence of a
String-structure. We discuss how to describe this obstruction problem in terms
of Lie n-algebras and their corresponding categorified Cartan-Ehresmann
connections. Generalizations even beyond String-extensions are then
straightforward. For G = Spin(n) the next step is "Fivebrane structures" whose
existence is obstructed by certain generalized Chern-Simons 7-bundles
classified by the second Pontrjagin class.Comment: 100 pages, references and clarifications added; correction to section
5.1 and further example to 9.3.1 adde
Classical approach in quantum physics
The application of a classical approach to various quantum problems - the
secular perturbation approach to quantization of a hydrogen atom in external
fields and a helium atom, the adiabatic switching method for calculation of a
semiclassical spectrum of hydrogen atom in crossed electric and magnetic
fields, a spontaneous decay of excited states of a hydrogen atom, Gutzwiller's
approach to Stark problem, long-lived excited states of a helium atom recently
discovered with the help of Poincar section, inelastic
transitions in slow and fast electron-atom and ion-atom collisions - is
reviewed. Further, a classical representation in quantum theory is discussed.
In this representation the quantum states are treating as an ensemble of
classical states. This approach opens the way to an accurate description of the
initial and final states in classical trajectory Monte Carlo (CTMC) method and
a purely classical explanation of tunneling phenomenon. The general aspects of
the structure of the semiclassical series such as renormgroup symmetry,
criterion of accuracy and so on are reviewed as well. In conclusion, the
relation between quantum theory, classical physics and measurement is
discussed.Comment: This review paper was rejected from J.Phys.A with referee's comment
"The author has made many worthwhile contributions to semiclassical physics,
but this article does not meet the standard for a topical review"
Diapause incidence in the two-spotted spider mite increases due to predator presence, not due to selective predation
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