21,146 research outputs found
Effective Potential for D-brane in Constant Electromagnetic Field
We discuss the one-loop effective potential and static (large ) potential
for toroidal D-brane described by DBI-action in constant magnetic and in
constant electric fields. Explicit calculation is done for membrane case
() for both types of external fields and in case of static potential for
an arbitrary . In the case of one-loop potential it is found that the
presence of magnetic background may stabilize D-brane giving the possibility
for non-pointlike ground state configuration. On the same time, constant
electrical field acts against stabilization and the correspondent one-loop
potential is unbounded from below. The properties of static potential which
also has stable minimum are described in detail. The back-reaction of quantum
gauge fields to one-loop potential is also evaluated.Comment: LaTeX file,17 pages, few misprints and mistakes are correcte
Scale Factor Duality: A Quantum Cosmological Approach
We consider the minisuperspace model arising from the lowest order string
effective action containing the graviton and the dilaton and study solutions of
the resulting Wheeler-Dewitt equation. The scale factor duality symmetry is
discussed in the context of our quantum cosmological model.Comment: 10 pages, plain tex, uses panda.tex (appended
Graviton propagator from background-independent quantum gravity
We study the graviton propagator in euclidean loop quantum gravity, using the
spinfoam formalism. We use boundary-amplitude and group-field-theory
techniques, and compute one component of the propagator to first order, under a
number of approximations, obtaining the correct spacetime dependence. In the
large distance limit, the only term of the vertex amplitude that contributes is
the exponential of the Regge action: the other terms, that have raised doubts
on the physical viability of the model, are suppressed by the phase of the
vacuum state, which is determined by the extrinsic geometry of the boundary.Comment: 6 pages. Substantially revised second version. Improved boundary
state ansat
A physiological basis for visual discomfort: Application in lighting design
Visual discomfort occurs when the statistics of the retinal image depart from those of natural scenes, particularly in respect of an excess energy at spatial frequencies close to 3 cycles/degree. Computer models suggest that uncomfortable stimuli are processed with a larger and less sparse neural response. Uncomfortable stimuli usually evoke a relatively large oxygenation of the visual cortex of the brain, consistent with inefficient neural encoding. The discomfort may be homeostatic. The neural computation that sustains sight is therefore likely to be more complex when the visual scene is spatially periodic, when the colour contrast is high or when saccadic suppression is impaired by flicker that is too rapid to be seen. </jats:p
Ring-shaped exact Hopf solitons
The existence of ring-like structures in exact hopfion solutions is shown.Comment: version accepted for publication in JMP, includes symmetry
transformation for finite paramete
The Noncommutative Supersymmetric Nonlinear Sigma Model
We show that the noncommutativity of space-time destroys the
renormalizability of the 1/N expansion of the O(N) Gross-Neveu model. A similar
statement holds for the noncommutative nonlinear sigma model. However, we show
that, up to the subleading order in 1/N expansion, the noncommutative
supersymmetric O(N) nonlinear sigma model becomes renormalizable in D=3. We
also show that dynamical mass generation is restored and there is no
catastrophic UV/IR mixing. Unlike the commutative case, we find that the
Lagrange multiplier fields, which enforce the supersymmetric constraints, are
also renormalized. For D=2 the divergence of the four point function of the
basic scalar field, which in D=3 is absent, cannot be eliminated by means of a
counterterm having the structure of a Moyal product.Comment: 15 pages, 7 figures, revtex, minor modifications in the text,
references adde
Toroidal Soliton Solutions in O(3)^N Nonlinear Sigma Model
A set of N three component unit scalar fields in (3+1) Minkowski space-time
is investigated. The highly nonlinear coupling between them is chosen to omit
the scaling instabilities. The multi-soliton static configurations with
arbitrary Hopf numbers are found. Moreover, the generalized version of the
Vakulenko-Kapitansky inequality is obtained. The possibility of attractive,
repulsing and noninteracting channels is discussed.Comment: to be published in Mod. Phys. Lett.
Fermionic Determinant of the Massive Schwinger Model
A representation for the fermionic determinant of the massive Schwinger
model, or , is obtained that makes a clean separation between the
Schwinger model and its massive counterpart. From this it is shown that the
index theorem for follows from gauge invariance, that the Schwinger
model's contribution to the determinant is canceled in the weak field limit,
and that the determinant vanishes when the field strength is sufficiently
strong to form a zero-energy bound state
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