2,452 research outputs found
On the transport of charged particles in turbulent fields: comparison of an exact solution with the quasilinear approximation
The problem of charged-particle transport in a magnetic field which is solely a function of time is solved. The solution is obtained exactly, to all orders in the field, in the limit of large wavelengths normal to the magnetic field. It is shown that the usual quasilinear, Fokker-Planck approximation is equal to the exact solution in the limit of times large compared with the correlation time of the fluctuating field. This is just the regime where the approximation has been used in the past, and this special case thus gives some support to the standard approximation techniques
A new kind of McKay correspondence from non-Abelian gauge theories
The boundary chiral ring of a 2d gauged linear sigma model on a K\"ahler
manifold classifies the topological D-brane sectors and the massless open
strings between them. While it is determined at small volume by simple group
theory, its continuation to generic volume provides highly non-trivial
information about the -branes on , related to the derived category
. We use this correspondence to elaborate on an extended notion of
McKay correspondence that captures more general than orbifold singularities. As
an illustration, we work out this new notion of McKay correspondence for a
class of non-compact Calabi-Yau singularities related to Grassmannians.Comment: 29 pages, harvmac(b), 2 fig
High-energy pulses and phase-resolved spectra by inverse Compton emission in the pulsar striped wind - Application to Geminga
(abridged) Although discovered 40 years ago, the emission mechanism
responsible for the observed pulsar radiation remains unclear. However, the
high-energy pulsed emission is usually explained in the framework of either the
polar cap or the outer gap model. The purpose of this work is to study the
pulsed component, that is the light-curves as well as the spectra of the
high-energy emission, above 10 MeV, emanating from the striped wind model.
Gamma rays are produced by scattering off the soft cosmic microwave background
photons on the ultrarelativistic leptons flowing in the current sheets. We
compute the time-dependent inverse Compton emissivity of the wind, in the
Thomson regime, by performing three-dimensional numerical integration in space
over the whole striped wind. The phase-dependent spectral variability is then
calculated as well as the change in pulse shape when going from the lowest to
the highest energies. Several light curves and spectra of inverse Compton
radiation with phase resolved dependence are presented. We apply our model to
the well-known gamma-ray pulsar Geminga. We are able to fit the EGRET spectra
between 10 MeV and 10 GeV as well as the light curve above 100 MeV with good
accuracy.Comment: Accepted by A&
Picard-Fuchs Equations and Special Geometry
We investigate the system of holomorphic differential identities implied by
special K\"ahlerian geometry of four-dimensional N=2 supergravity. For
superstring compactifications on \cy threefolds these identities are equivalent
to the Picard-Fuchs equations of algebraic geometry that are obeyed by the
periods of the holomorphic three-form. For one variable they reduce to linear
fourth-order equations which are characterized by classical -generators; we
find that the instanton corrections to the Yukawa couplings are directly
related to the non-vanishing of . We also show that the symplectic
structure of special geometry can be related to the fact that the Yukawa
couplings can be written as triple derivatives of some holomorphic function
. Moreover, we give the precise relationship of the Yukawa couplings of
special geometry with three-point functions in topological field theory.Comment: 43 page
Boundary Rings and N=2 Coset Models
We investigate boundary states of N=2 coset models based on Grassmannians
Gr(n,n+k), and find that the underlying intersection geometry is given by the
fusion ring of U(n). This is isomorphic to the quantum cohomology ring of
Gr(n,n+k+1), and thus can be encoded in a ``boundary'' superpotential whose
critical points correspond to the boundary states. In this way the intersection
properties can be represented in terms of a soliton graph that forms a
generalized, Z_{n+k+1} symmetric McKay quiver. We investigate the spectrum of
bound states and find that the rational boundary CFT produces only a small
subset of the possible quiver representations.Comment: 40p, 5 figs, refs added, typos and minor errors correcte
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
Diffusive electron acceleration at SNR shock fronts and the observed SNR radio spectral indices
The radio synchrotron emission from relativistic electrons in shell supernova remnants (SNRs) provides a unique opportunity to probe the energy distribution of energetic electrons at their acceleration site (SNR shock fronts). This information provides insight into the acceleration mechanism(s). The implications of these observations for the diffusive (first-order Fermi) acceleration of electrons at the SNR shock fronts are discussed
Calculations for Mirror Symmetry with D-branes
We study normal functions capturing D-brane superpotentials on several one-
and two-parameter Calabi-Yau hypersurfaces and complete intersections in
weighted projective space. We calculate in the B-model and interpret the
results using mirror symmetry in the large volume regime, albeit without
identifying the precise A-model geometry in all cases. We identify new classes
of extensions of Picard-Fuchs equations, as well as a novel type of topology
changing phase transition involving quantum D-branes. A 4-d domain wall which
is obtained in one region of closed string moduli space from wrapping a
four-chain interpolating between two Lagrangian submanifolds is, for other
values of the parameters, represented by a disk ending on a single Lagrangian.Comment: 42 page
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