6,917 research outputs found
Calculation of transition probabilities and ac Stark shifts in two-photon laser transitions of antiprotonic helium
Numerical ab initio variational calculations of the transition probabilities
and ac Stark shifts in two-photon transitions of antiprotonic helium atoms
driven by two counter-propagating laser beams are presented. We found that
sub-Doppler spectroscopy is in principle possible by exciting transitions of
the type (n,L)->(n-2,L-2) between antiprotonic states of principal and angular
momentum quantum numbers n~L-1~35, first by using highly monochromatic,
nanosecond laser beams of intensities 10^4-10^5 W/cm^2, and then by tuning the
virtual intermediate state close (e.g., within 10-20 GHz) to the real state
(n-1,L-1) to enhance the nonlinear transition probability. We expect that ac
Stark shifts of a few MHz or more will become an important source of systematic
error at fractional precisions of better than a few parts in 10^9. These shifts
can in principle be minimized and even canceled by selecting an optimum
combination of laser intensities and frequencies. We simulated the resonance
profiles of some two-photon transitions in the regions n=30-40 of the
\bar{p}^4He^+ and \bar{p} ^3He^+ isotopes to find the best conditions that
would allow this.Comment: 18 pages 2 tables 12 figures, submitted to Phys. Rev.
Direct yaw-moment control of an in-wheel-motored electric vehicle based on body slip angle fuzzy observer
A stabilizing observer-based control algorithm for an in-wheel-motored vehicle is proposed, which generates direct yaw moment to compensate for the state deviations. The control scheme is based on a fuzzy rule-based body slip angle (beta) observer. In the design strategy of the fuzzy observer, the vehicle dynamics is represented by Takagi-Sugeno-like fuzzy models. Initially, local equivalent vehicle models are built using the linear approximations of vehicle dynamics for low and high lateral acceleration operating regimes, respectively. The optimal beta observer is then designed for each local model using Kalman filter theory. Finally, local observers are combined to form the overall control system by using fuzzy rules. These fuzzy rules represent the qualitative relationships among the variables associated with the nonlinear and uncertain nature of vehicle dynamics, such as tire force saturation and the influence of road adherence. An adaptation mechanism for the fuzzy membership functions has been incorporated to improve the accuracy and performance of the system. The effectiveness of this design approach has been demonstrated in simulations and in a real-time experimental settin
D-branes in Topological Minimal Models: the Landau-Ginzburg Approach
We study D-branes in topologically twisted N=2 minimal models using the
Landau-Ginzburg realization. In the cases of A and D-type minimal models we
provide what we believe is an exhaustive list of topological branes and compute
the corresponding boundary OPE algebras as well as all disk correlators. We
also construct examples of topological branes in E-type minimal models. We
compare our results with the boundary state formalism, where possible, and find
agreement.Comment: 29 pages, late
N=2 Supersymmetric Sigma Models and D-branes
We study D-branes of N=2 supersymmetric sigma models. Supersymmetric
nonlinear sigma models with 2-dimensional target space have D0,D1,D2-branes,
which are realized as A-,B-type supersymmetric boundary conditions on the
worldsheet. When we embed the models in the string theory, the Kahler potential
is restricted and leads to a 2-dim black hole metric with a dilaton background.
The D-branes in this model are susy cycles and consistent with the analysis of
conjugacy classes. The generalized metrics with U(n) isometry is proposed and
dynamics on them are realized by linear sigma models. We investigate D-branes
of the linear sigma models and compare the results with those in the nonlinear
sigma models.Comment: 23 pages, 5 figure
Field dynamics and tunneling in a flux landscape
We investigate field dynamics and tunneling between metastable minima in a
landscape of Type IIB flux compactifications, utilizing monodromies of the
complex structure moduli space to continuously connect flux vacua. After
describing the generic features of a flux-induced potential for the complex
structure and Type IIB axio-dilaton, we specialize to the Mirror Quintic
Calabi--Yau to obtain an example landscape. Studying the cosmological dynamics
of the complex structure moduli, we find that the potential generically does
not support slow-roll inflation and that in general the landscape separates
neatly into basins of attraction of the various minima. We then discuss
tunneling, with the inclusion of gravitational effects, in many-dimensional
field spaces. A set of constraints on the form of the Euclidean paths through
field space are presented, and then applied to construct approximate instantons
mediating the transition between de Sitter vacua in the flux landscape. We find
that these instantons are generically thick-wall and that the tunneling rate is
suppressed in the large-volume limit. We also consider examples where
supersymmetry is not broken by fluxes, in which case near-BPS thin-wall bubbles
can be constructed. We calculate the bubble wall tension, finding that it
scales like a D- or NS-brane bubble, and comment on the implications of this
correspondence. Finally, we present a brief discussion of eternal inflation in
the flux-landscape.Comment: 23 PRD-style pages with 11 embedded figures. Added refs, corrected
typos, and clarified Sec. V. Replaced to match published versio
Non-compact Mirror Bundles and (0,2) Liouville Theories
We study (0,2) deformations of N=2 Liouville field theory and its mirror
duality. A gauged linear sigma model construction of the ultraviolet theory
connects (0,2) deformations of Liouville field theory and (0,2) deformations of
N=2 SL(2,R)/U(1) coset model as a mirror duality. Our duality proposal from the
gauged linear sigma model completely agrees with the exact CFT analysis. In the
context of heterotic string compactifications, the deformation corresponds to
the introduction of a non-trivial gauge bundle. This non-compact
Landau-Ginzburg construction yields a novel way to study the gauge bundle
moduli for non-compact Calabi-Yau manifolds.Comment: 34 page
D-brane Categories for Orientifolds -- The Landau-Ginzburg Case
We construct and classify categories of D-branes in orientifolds based on
Landau-Ginzburg models and their orbifolds. Consistency of the worldsheet
parity action on the matrix factorizations plays the key role. This provides
all the requisite data for an orientifold construction after embedding in
string theory. One of our main results is a computation of topological field
theory correlators on unoriented worldsheets, generalizing the formulas of Vafa
and Kapustin-Li for oriented worldsheets, as well as the extension of these
results to orbifolds. We also find a doubling of Knoerrer periodicity in the
orientifold context.Comment: 45 pages, 6 figure
N=2 Liouville Theory with Boundary
We study N=2 Liouville theory with arbitrary central charge in the presence
of boundaries. After reviewing the theory on the sphere and deriving some
important structure constants, we investigate the boundary states of the theory
from two approaches, one using the modular transformation property of annulus
amplitudes and the other using the bootstrap of disc two-point functions
containing degenerate bulk operators. The boundary interactions describing the
boundary states are also proposed, based on which the precise correspondence
between boundary states and boundary interactions is obtained. The open string
spectrum between D-branes is studied from the modular bootstrap approach and
also from the reflection relation of boundary operators, providing a
consistency check for the proposal.Comment: 1+48 pages, no figure. typos corrected and references added. the
version to appear in JHE
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