77 research outputs found
Vinyl cations. Comparison of gas-phase thermodynamic and solvolysis data with ab initio MO calculations
N=1 Dual String Pairs and their Massless Spectra
We construct two chains of fourdimensional F-theory/heterotic dual string
pairs with N=1 supersymmetry. On the F-theory side as well as on the heterotic
side the geometry of the involved manifolds relies on del Pezzo surfaces. We
match the massless spectra by using, for one chain of models, an index formula
to count the heterotic bundle moduli and determine the dual F-theory spectra
from the Hodge numbers of the fourfolds and of the type IIB base spaces.Comment: 22 pages, latex, some references correcte
Phenolic compounds in juices of apple cultivars and their relation to antioxidant activity
Apples and their juices are important sources of phenolic compounds in human diet. Using the same juice processing method, we compared phenolic composition as well as antioxidant activity of juices from various table apple cultivars, grown at the same location. Antioxidant activity of apple juices was estimated by application of two assays, ABTS cation radical decolorization and by scavenging of reactive oxygen species (ROS) generated by xanthine/xanthine oxidase (O2·-, H2O2, OH·; superoxide assisted Fenton reaction), as measured by inhibition of ethylene release from KMB (α-Keto-γ-(methylthio)butyric acid). The apple juices differed in phenolic content and antioxidant activity, where cultivar differences were more relevant than environmental factors. Furthermore, to improve antioxidant performance in the XOD-test, a juice low in phenol content was supplemented with the appropriate amounts of phenols to the level of the best juice indicating that these phenolics contribute to the antioxidant activity of the apple juice. In accordance to literature, the phenolic compounds representing the main antioxidant activity in the apple juices comprise flavan-3-ols and chlorogenic acid.
Moduli Dependent Non-Holomorphic Contributions of Massive States to Gravitational Couplings and -Terms in -Orbifold Compactifications
It is pointed out that massive states in D=4, N=1 supergravity-matter
theories can, in general, at the 1-loop level contribute non-holomorphic terms
to quadratic gravitational couplings. It is then shown in the context of
-symmetric -orbifold theories that, for constant moduli
backgrounds, the inclusion of such contributions can result in the cancellation
of naked -terms. -terms can also arise but, being ghost free,
need not cancel.Comment: 38 pages, HUB-IEP-94/20, UPR-634T (references added
Threshold Corrections and Symmetry Enhancement in String Compactifications
We present the computation of threshold functions for Abelian orbifold
compactifications. Specifically, starting from the massive, moduli-dependent
string spectrum after compactification, we derive the threshold functions as
target space duality invariant free energies (sum over massive string states).
In particular we work out the dependence on the continuous Wilson line moduli
fields. In addition we concentrate on the physically interesting effect that at
certain critical points in the orbifold moduli spaces additional massless
states appear in the string spectrum leading to logarithmic singularities in
the threshold functions. We discuss this effect for the gauge coupling
threshold corrections; here the appearance of additional massless states is
directly related to the Higgs effect in string theory. In addition the
singularities in the threshold functions are relevant for the loop corrections
to the gravitational coupling constants.Comment: 68 pages (equations 8.16 and 8.17 corrected; added footnotes 4,5,11
Curved BPS domain wall solutions in four-dimensional N=2 supergravity
We construct four-dimensional domain wall solutions of N=2 gauged
supergravity coupled to vector and to hypermultiplets. The gauged supergravity
theories that we consider are obtained by performing two types of Abelian
gauging. In both cases we find that the behaviour of the scalar fields
belonging to the vector multiplets is governed by the so-called attractor
equations known from the study of BPS black hole solutions in ungauged N=2
supergravity theories. The scalar fields belonging to the hypermultiplets, on
the other hand, are either constant or exhibit a run-away behaviour. These
domain wall solutions preserve 1/2 of supersymmetry and they are, in general,
curved. We briefly comment on the amount of supersymmetry preserved by domain
wall solutions in gauged supergravity theories obtained by more general
gaugings.Comment: 18 pages, latex, no figures, final version for Nuclear Physics B
(typos corrected plus minor changes
Moduli Spaces and Target Space Duality Symmetries in Orbifold Theories with Continuous Wilson Lines
We present the coset structure of the untwisted moduli space of heterotic
orbifold compactifications with continuous Wilson lines. For the
cases where the internal 6-torus is given by the direct sum , we explicitly construct the K\"{a}hler potentials associated with the
underlying 2-torus . We then discuss the transformation properties of
these K\"{a}hler potentials under target space modular symmetries. For the case
where the twist possesses eigenvalues of , we find that holomorphic
terms occur in the K\"{a}hler potential describing the mixing of complex Wilson
moduli. As a consequence, the associated and moduli are also shown to
mix under target space modular transformations.Comment: 43 pages, HUB-IEP-94/
On the Vacuum Structure of Type II String Compactifications on Calabi-Yau Spaces with H-Fluxes
We discuss the vacuum structure of type IIA/B Calabi-Yau string
compactifications to four dimensions in the presence of n-form H-fluxes. These
will lift the vacuum degeneracy in the Calabi-Yau moduli space, and for generic
points in the moduli space, N=2 supersymmetry will be broken. However, for
certain `aligned' choices of the H-flux vector, supersymmetric ground states
are possible at the degeneration points of the Calabi-Yau geometry. We will
investigate in detail the H-flux induced superpotential and the corresponding
scalar potential at several degeneration points, such as the Calabi-Yau large
volume limit, the conifold loci, the Seiberg-Witten points, the strong coupling
point and the conformal points. Some emphasis is given to the question whether
partial supersymmetry breaking can be realized at those points. We also relate
the H-flux induced superpotential to the formalism of gauged N=2 supergravity.
Finally we point out the analogies between the Calabi-Yau vacuum structure due
to H-fluxes and the attractor formalism of N=2 black holes.Comment: 47 pages, 4 eps figures, references adde
Perturbative Couplings and Modular Forms in N=2 String Models with a Wilson Line
We consider a class of four parameter D=4, N=2 string models, namely
heterotic strings compactified on K3 times T2 together with their dual type II
partners on Calabi-Yau three-folds. With the help of generalized modular forms
(such as Siegel and Jacobi forms), we compute the perturbative prepotential and
the perturbative Wilsonian gravitational coupling F1 for each of the models in
this class. We check heterotic/type II duality for one of the models by
relating the modular forms in the heterotic description to the known instanton
numbers in the type II description. We comment on the relation of our results
to recent proposals for closely related models.Comment: 42 pages, LaTeX, revised version contains additional reference
On-Mass-Shell Gaugino Condensation in Z_{N} Orbifold Compactifications
We discuss non-perturbative aspects of string effective field theories with
N=1 supersymmetry in four dimensions. By the use of a scalar potential, which
is on-shell invariant under the supersymmetric duality of the dilaton, we study
gaugino condensation in (2,2) symmetric Z_{N} orbifold compactifications. The
duality under consideration relates a two-form antisymmetric tensor to a
pseudoscalar. We show, that our approach is independent of the
superfield-representation of the dilaton and preserves the U(1)_{PQ}
Peccei-Quinn symmetry exactly.Comment: Standard-Late
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