9,329 research outputs found
BF Theories and Group-Level Duality
It is known that the partition function and correlators of the
two-dimensional topological field theory on the Riemann
surface is given by Verlinde numbers, dim() and that
the large limit of dim() gives Vol(), the volume of
the moduli space of flat connections of gauge group on ,
up to a power of . Given this relationship, we complete the computation of
Vol() using only algebraic results from conformal field theory. The
group-level duality of is used to show that if is a classical
group, then is a BF
theory with gauge group . Therefore this limit computes Vol(), the volume of the moduli space of flat connections of gauge
group
Penrose Limits and Spacetime Singularities
We give a covariant characterisation of the Penrose plane wave limit: the
plane wave profile matrix is the restriction of the null geodesic
deviation matrix (curvature tensor) of the original spacetime metric to the
null geodesic, evaluated in a comoving frame. We also consider the Penrose
limits of spacetime singularities and show that for a large class of black
hole, cosmological and null singularities (of Szekeres-Iyer ``power-law
type''), including those of the FRW and Schwarzschild metrics, the result is a
singular homogeneous plane wave with profile , the scale
invariance of the latter reflecting the power-law behaviour of the
singularities.Comment: 9 pages, LaTeX2e; v2: additional references and cosmetic correction
Fretting wear of Ti(CxNy) PVD coatings under variable environmental conditions
Fretting wear as a specific type of degradation is defined as an oscillatory motion at small amplitude between two nominally stationary solid bodies in mutual contact. Under external stresses the interface is being damaged by debris generation and its successive ejections outside the contact area. A potential protection against fretting damage by means of hard coatings is being offered by different surface engineering techniques. For this study TiC, TiN and TiCN hard coatings manufactured by a PVD method have been selected and tested against smooth polycrystalline alumina ball. A fretting test programme has been carried out at the frequency of 5Hz, 100N normal load, 100µm displacement amplitude and at three values of a relative humidity: 10, 50 and 90% at 295-298K temperature. It turned out that the intensity of wear process was depending not only on loading conditions but on environmental ones as well. A significant impact of RH on wear rate and friction behaviour of the coatings under investigation has been observed. Two different damage mechanisms have been identified and related to the phenomena of debris oxidation and debris adhesion to the counterbody surface. In the latter case the debris deposited onto the surface of the alumina ball lead to a change of stress distribution at the interface and as a result to accelerated wear. In this work experiments with variable relative humidity increasing from 10% to 90% within 1 a single fretting test have been completed. It follows from these experiments that there exists an intermediate value of the RH at which the friction coefficient changes rapidly. Finally a dissipated energy approach has been applied in the work in order to quantify and compare fretting wear rates of different hard coatings
Исследование процесса шлифования стальных кулачков инструментом из кубического нитрида бора без охлаждения
Экспериментальными исследованиями, имитирующими шлифование кулачков из закаленной легированной стали, установлено, что их обработка с минимальным нагревом обрабатываемой поверхности с охлаждением масляным туманом или без охлаждения возможна высокопористым кругом из кубического нитрида бора. Выявлено, что для минимизации термодинамической напряженности сухого шлифования целесообразно назначать режим обработки с максимальной возможной глубиной врезания круга и минимальной скоростью вращения детали.Експериментальними дослідженнями, які імітують шліфування кулачків із загартованої легованої сталі, встановлено, що їх обробка з мінімальним нагрівом оброблюваної поверхні з охолодженням масляним туманом або без охолодження можлива високопористим кругом з кубічного нітриду бору. Виявлено, що для мінімізації термодинамічної напруженості сухого шліфування доцільно призначати режим обробки з максимальною можливою глибиною врізання круга і мінімальною швидкістю обертання деталі.Experimental studies simulating grinding of Cams of hardened alloy steel, determined that their treatment with the lowest heating of the surface cooled oil mist or without cooling are possible by using highly porous grinding wheels from cubic boron nitride. Also identified that to minimize the thermodynamic tension dry sanding it is advisable to appoint a mode of processing with the maximum possible depth plunging grinding wheel and a minimum speed of rotation of the part
Penrose Limits of the Baryonic D5-brane
The Penrose limits of a D5-brane wrapped on the sphere of AdS_5 x S^5 and
connected to the boundary by M fundamental strings, which is dual to the baryon
vertex of the N=4 SU(M) super Yang-Mills theory, are investigated. It is shown
that, for null geodesics that lead to the maximally supersymmetric Hpp-wave
background, the resulting D5-brane is a 1/2-supersymmetric null brane. For an
appropriate choice of radial geodesic, however, the limiting configuration is
1/4-supersymmetric and closely related to the Penrose limit of a flat space
BIon.Comment: LaTeX, 1+18 pages, 1 figure; v2: obvious misquotation of the number
of preserved supersymmetries correcte
Supersymmetry and homogeneity of M-theory backgrounds
We describe the construction of a Lie superalgebra associated to an arbitrary
supersymmetric M-theory background, and discuss some examples. We prove that
for backgrounds with more than 24 supercharges, the bosonic subalgebra acts
locally transitively. In particular, we prove that backgrounds with more than
24 supersymmetries are necessarily (locally) homogeneous.Comment: 19 pages (Erroneous Section 6.3 removed from the paper.
Geometry of Schroedinger Space-Times II: Particle and Field Probes of the Causal Structure
We continue our study of the global properties of the z=2 Schroedinger
space-time. In particular, we provide a codimension 2 isometric embedding which
naturally gives rise to the previously introduced global coordinates.
Furthermore, we study the causal structure by probing the space-time with point
particles as well as with scalar fields. We show that, even though there is no
global time function in the technical sense (Schroedinger space-time being
non-distinguishing), the time coordinate of the global Schroedinger coordinate
system is, in a precise way, the closest one can get to having such a time
function. In spite of this and the corresponding strongly Galilean and almost
pathological causal structure of this space-time, it is nevertheless possible
to define a Hilbert space of normalisable scalar modes with a well-defined
time-evolution. We also discuss how the Galilean causal structure is reflected
and encoded in the scalar Wightman functions and the bulk-to-bulk propagator.Comment: 32 page
On solvable models of type IIB superstring in NS-NS and R-R plane wave backgrounds
We consider type IIB string in the two plane-wave backgrounds which may be
interpreted as special limits of the AdS_3 x S^3 metric supported by either the
NS-NS or R-R 3-form field. The NS-NS plane-wave string model is equivalent to a
direct generalization of the Nappi-Witten model, with its spectrum being
similar to that of strings in constant magnetic field. The R-R model can be
solved in the light-cone gauge, where the Green-Schwarz action describes 4
massive and 4 massless copies of free bosons and fermions. We find the spectra
of the two string models and study the asymptotic density of states. We also
discuss a more general class of exactly solvable plane-wave models with reduced
supersymmetry which is obtained by adding twists in two spatial 2-planes.Comment: 36 pages, harvmac. v2: discussion of equivalence of the supergravity
parts of the spectra of the NS-NS and R-R models added in sect.5.3; v3: added
remark on periodicity of the NS-NS spectrum; v4: minor correction in sect.6.
pp-waves in 11-dimensions with extra supersymmetry
The Killing spinor equations for pp-wave solutions of eleven dimensional
supergravity are analysed and it is shown that there are solutions that
preserve 18,20,22 and 24 supersymmetries, in addition to the generic solution
preserving 16 supersymmetries and the Kowalski-Glikman solution preserving 32
supersymmetries.Comment: 13 pages. Reference added, typos corrected, new examples of
7-parameter case presente
Uniqueness of M-theory PP-Wave Background with Extra Supersymmetries
We examine Killing spinor equations of the general eleven-dimensional pp-wave
backgrounds, which contain a scalar H(x^m,x^-) in the metric and a three-form
\xi(x^m,x^-) in the flux. Considering non-harmonic extra Killing spinors, we
show that if the backgrounds admit at least one extra Killing spinor in
addition to the standard 16 Killing spinors, they can be reduced to the form
with H=A_{mn}(x^-)x^mx^n and \xi(x^-) modulo coordinate transformations. We
further examine the cases in which the extra Killing spinor is characterized by
a set of Cartan matrices. The super-isometry algebras of the resulting
backgrounds are also derived.Comment: 25 pages, LaTeX2e, comments added, version to appear in PR
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