355 research outputs found
A virtual manipulator model for space robotic systems
Future robotic manipulators carried by a spacecraft will be required to perform complex tasks in space, like repairing satellites. Such applications of robotic manipulators will encounter a number of kinematic, dynamic and control problems due to the dynamic coupling between the manipulators and the spacecraft. A new analytical modeling method for studying the kinematics and dynamics of manipulators in space is presented. The problem is treated by introducing the concept of a Virtual Manipulator (VM). The kinematic and dynamic motions of the manipulator, vehicle and payload, can be described relatively easily in terms of the Virtual Manipulator movements, which have a fixed base in inertial space at a point called a Virtual Ground. It is anticipated that the approach described here will aid in the design and development of future space manipulator systems
The Orbifolds of N=2 Superconformal Theories with c=3
We construct Z_M, M= 2, 3, 4, 6 orbifold models of the N=2 superconformal
field theories with central charge c=3. Then we check the description of the
Z_3, Z_4 and Z_6 orbifolds by the N=2 superconformal Landau-Ginzburg models
with c=3, by comparing the spectrum of chiral fields, in particular the Witten
index Tr(-1)^F.Comment: 20 pages; typos corrected, references adde
Vertex operator algebras and operads
Vertex operator algebras are mathematically rigorous objects corresponding to
chiral algebras in conformal field theory. Operads are mathematical devices to
describe operations, that is, -ary operations for all greater than or
equal to , not just binary products. In this paper, a reformulation of the
notion of vertex operator algebra in terms of operads is presented. This
reformulation shows that the rich geometric structure revealed in the study of
conformal field theory and the rich algebraic structure of the theory of vertex
operator algebras share a precise common foundation in basic operations
associated with a certain kind of (two-dimensional) ``complex'' geometric
object, in the sense in which classical algebraic structures (groups, algebras,
Lie algebras and the like) are always implicitly based on (one-dimensional)
``real'' geometric objects. In effect, the standard analogy between
point-particle theory and string theory is being shown to manifest itself at a
more fundamental mathematical level.Comment: 16 pages. Only the definitions of "partial operad" and of "rescaling
group" have been improve
Toward the M(F)--Theory Embedding of Realistic Free-Fermion Models
We construct a Landau-Ginzburg model with the same data and symmetries as a
orbifold that corresponds to a class of realistic free-fermion
models. Within the class of interest, we show that this orbifolding connects
between different orbifold models and commutes with the mirror
symmetry. Our work suggests that duality symmetries previously discussed in the
context of specific and theory compactifications may be extended to the
special orbifold that characterizes realistic free-fermion
models.Comment: 15 pages. Standard Late
On Large N Gauge Theories from Orientifolds
We consider four dimensional supersymmetric gauge theories
obtained via orientifolds of Type IIB on Abelian C^3/G orbifolds. We construct
all such theories that have well defined world-sheet expansion. The number of
such orientifolds is rather limited. We explain this fact in the context of
recent developments in four dimensional Type IIB orientifolds. In particular,
we elaborate these issues in some examples of theories where world-sheet
description is inadequate due to non-perturbative (from the orientifold
viewpoint) states arising from D-branes wrapping (collapsed) 2-cycles in the
orbifold. We find complete agreement with the corresponding statements recently
discussed in the context of Type I compactifications on toroidal orbifolds.
This provides a non-trivial check for correctness of the corresponding
conclusions. We also find non-trivial agreement with various field theory
expectations, and point out their origin in string language. The orientifold
gauge theories that do possess well defined world-sheet description have the
property that in the large N limit computation of any M-point correlation
function in these theories reduces to the corresponding computation in the
parent oriented theory.Comment: 21 pages, revtex, minor errors and misprints corrected (to appear in
Phys. Rev. D
Anomaly Free Non-Supersymmetric Large Gauge Theories from Orientifolds
We construct anomaly free non-supersymmetric large N gauge theories from
orientifolds of Type IIB on C^3/G orbifolds. In particular, massless as well as
tachyonic one-loop tadpoles are cancelled in these models. This is achieved by
starting with supersymmetric orientifolds with well defined
world-sheet description and including discrete torsion (which breaks
supersymmetry) in the orbifold action. In this way we obtain non-trivial
non-chiral as well as anomaly free chiral large N gauge theories. We point out
certain subtleties arising in the chiral cases. Subject to certain assumptions,
these theories are shown to have the property that computation of any M-point
correlation function in these theories reduces to the corresponding computation
in the parent oriented theory. This generalizes the analogous
results recently obtained in supersymmetric large N gauge theories from
orientifolds, as well as in (non)supersymmetric large N gauge theories without
orientifold planes.Comment: 18 pages, revtex, minor misprints corrected, a clarifying footnote
added (to appear in Phys. Rev. D
NC Calabi-Yau Orbifolds in Toric Varieties with Discrete Torsion
Using the algebraic geometric approach of Berenstein et {\it al}
(hep-th/005087 and hep-th/009209) and methods of toric geometry, we study non
commutative (NC) orbifolds of Calabi-Yau hypersurfaces in toric varieties with
discrete torsion. We first develop a new way of getting complex mirror
Calabi-Yau hypersurfaces in toric manifolds with a action and analyze the general group of the
discrete isometries of . Then we build a general class of
complex dimension NC mirror Calabi-Yau orbifolds where the non
commutativity parameters are solved in terms of discrete
torsion and toric geometry data of in which the original
Calabi-Yau hypersurfaces is embedded. Next we work out a generalization of the
NC algebra for generic dimensions NC Calabi-Yau manifolds and give various
representations depending on different choices of the Calabi-Yau toric geometry
data. We also study fractional D-branes at orbifold points. We refine and
extend the result for NC to higher dimensional torii orbifolds
in terms of Clifford algebra.Comment: 38 pages, Late
Quantum Clifford-Hopf Algebras for Even Dimensions
In this paper we study the quantum Clifford-Hopf algebras
for even dimensions and obtain their intertwiner matrices, which are
elliptic solutions to the Yang- Baxter equation. In the trigonometric limit of
these new algebras we find the possibility to connect with extended
supersymmetry. We also analyze the corresponding spin chain hamiltonian, which
leads to Suzuki's generalized model.Comment: 12 pages, LaTeX, IMAFF-12/93 (final version to be published, 2
uuencoded figures added
Rigidity and defect actions in Landau-Ginzburg models
Studying two-dimensional field theories in the presence of defect lines
naturally gives rise to monoidal categories: their objects are the different
(topological) defect conditions, their morphisms are junction fields, and their
tensor product describes the fusion of defects. These categories should be
equipped with a duality operation corresponding to reversing the orientation of
the defect line, providing a rigid and pivotal structure. We make this
structure explicit in topological Landau-Ginzburg models with potential x^d,
where defects are described by matrix factorisations of x^d-y^d. The duality
allows to compute an action of defects on bulk fields, which we compare to the
corresponding N=2 conformal field theories. We find that the two actions differ
by phases.Comment: 53 pages; v2: clarified exposition of pivotal structures, corrected
proof of theorem 2.13, added remark 3.9; version to appear in CM
Type IIB Orientifolds with NS-NS Antisymmetric Tensor Backgrounds
We consider six dimensional N=1 space-time supersymmetric Type IIB
orientifolds with non-zero untwisted NS-NS sector B-field. The B-field is
quantized due to the requirement that the Type IIB spectrum be left-right
symmetric. The presence of the B-field results in rank reduction of both 99 and
55 open string sector gauge groups. We point out that in some of the models
with non-zero B-field there are extra tensor multiplets in the Z_2 twisted
closed string sector, and we explain their origin in a simple example. Also,
the 59 open string sector states come with a multiplicity that depends on the
B-field. These two facts are in accord with anomaly cancellation requirements.
We point out relations between various orientifolds with and without the
B-field, and also discuss the F-theory duals of these models.Comment: 13 pages, revtex, minor misprints correcte
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