2,141 research outputs found
F^4 Terms in N=4 String Vacua
We discuss terms in torroidal compactifications of type-I and
heterotic SO(32) string theory. We give a simple argument why only short BPS
multiplets contribute to these terms at one loop, and verify heterotic-type-I
duality to this order.Comment: 10 pages, Latex. A statement about superinvariants was corrected.
Minor cosmetic changes were also made. To appear in Proceedings of Trieste
Spring School and Workshop, April 199
Modification of cosmic-ray energy spectra by stochastic acceleration
Context: Typical space plasmas contain spatially and temporally variable
turbulent electromagnetic fields. Understanding the transport of energetic
particles and the acceleration mechanisms for charged particles is an important
goal of today's astroparticle physics. Aims: To understand the acceleration
mechanisms at the particle source, subsequent effects have to be known.
Therefore, the modification of a particle energy distribution, due to
stochastic acceleration, needs to be investigated. Methods: The diffusion in
momentum space was investigated by using both a Monte-Carlo simulation code and
by analytically solving the momentum-diffusion equation. For simplicity, the
turbulence was assumed to consist of one-dimensional Alfven waves. Results:
Using both methods, it is shown that, on average, all particles with velocities
comparable to the Alfven speeds are accelerated. This influences the energy
distribution by significantly increasing the energy spectral index.
Conclusions: Because of electromagnetic turbulence, a particle energy spectrum
measured at Earth can drastically deviate from its initial spectrum. However,
for particles with velocities significantly above the Alfven speed, the effect
becomes negligible.Comment: 10 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
On Heterotic/Type I Duality in d=8
We discuss heterotic corrections to quartic internal U(1) gauge couplings and
check duality by calculating one-loop open string diagrams and identifying the
D-instanton sum in the dual type I picture. We also compute SO(8)^4 threshold
corrections and finally R^2 corrections in type I theory.Comment: 9 pages, Latex, To appear in the proceedings of "Quantum Aspects of
Gauge Theories, Supersymmetries and Unification", Corfu, September 199
Isolated unstable Weibel modes in unmagnetized plasmas with tunable asymmetry
In this letter, an initially unmagnetized pair plasma with asymmetric
velocity distributions is investigated where any unstable Weibel mode must be
isolated, with discrete values for the growth rates and the unstable
wavenumbers. For both a non-relativistic distribution with thermal spread and a
high-relativistic two-stream distribution it is shown that isolated modes are
excited and that, as the asymmetry tends to zero, the growth rate remains
finite, as long as the distribution function is not precisely symmetric.Comment: Comments: references adde
Aspects of quark mass generation on a torus
In this talk we report on recent results for the quark propagator on a
compact manifold. The corresponding Dyson-Schwinger equations on a torus are
solved on volumes similar to the ones used in lattice calculations. The
quark-gluon interaction is fixed such that the lattice results are reproduced.
We discuss both the effects in the infinite volume/continuum limit as well as
effects when the volume is small.Comment: 3 pages, 3 figures; talk given by CF at QNP06, Madrid, June 200
Superpotentials, A-infinity Relations and WDVV Equations for Open Topological Strings
We give a systematic derivation of the consistency conditions which constrain
open-closed disk amplitudes of topological strings. They include the A-infinity
relations (which generalize associativity of the boundary product of
topological field theory), as well as certain homotopy versions of
bulk-boundary crossing symmetry and Cardy constraint. We discuss integrability
of amplitudes with respect to bulk and boundary deformations, and write down
the analogs of WDVV equations for the space-time superpotential. We also study
the structure of these equations from a string field theory point of view. As
an application, we determine the effective superpotential for certain families
of D-branes in B-twisted topological minimal models, as a function of both
closed and open string moduli. This provides an exact description of tachyon
condensation in such models, which allows one to determine the truncation of
the open string spectrum in a simple manner.Comment: 53 page
The matrix factorisations of the D-model
The fundamental matrix factorisations of the D-model superpotential are found
and identified with the boundary states of the corresponding conformal field
theory. The analysis is performed for both GSO-projections. We also comment on
the relation of this analysis to the theory of surface singularities and their
orbifold description.Comment: 23 pages, LaTe
Et Par Ord om de foreslaaede flere Uldmarkeder.
Et Par Ord om de foreslaaede flere Uldmarkeder
Om Chevalierbyg og dets Dyrkning.
Om Chevalierbyg og dets Dyrkning
Heterotic/type I duality, D-instantons and an N=2 AdS/CFT correspondence
D-instanton effects are studied for the IIB orientifold T^2/I\Omega(-1)^{F_L}
of Sen using type I/heterotic duality. An exact one loop threshold calculation
of t_8 \tr F^4 and t_8(\tr F^2)^2 terms for the heterotic string on T^2 with
Wilson lines breaking SO(32) to SO(8)^4 is related to D-instanton induced terms
in the worldvolume of D7 branes in the orientifold. Introducing D3 branes and
using the AdS/CFT correspondence in this case, these terms are used to
calculate Yang-Mills instanton contributions to four point functions of the
large N_c limit of N=2 USp(2N_c) SYM with four fundamental and one
antisymmetric tensor hypermultiplets.Comment: 25 pages, harvmac(b), one figure, v2: minor changes, version to
appear in PR
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