698 research outputs found
Singular Liouville fields and spiky strings in \rr^{1,2} and SL(2,\rr)
The closed string dynamics in \rr^{1,2} and SL(2,\rr) is studied within
the scheme of Pohlmeyer reduction. In both spaces two different classes of
string surfaces are specified by the structure of the fundamental quadratic
forms. The first class in \rr^{1,2} is associated with the standard lightcone
gauge strings and the second class describes spiky strings and their conformal
deformations on the Virasoro coadjoint orbits. These orbits correspond to
singular Liouville fields with the monodromy matrixes . The first class
in SL(2,\rr) is parameterized by the Liouville fields with vanishing chiral
energy functional. Similarly to \rr^{1,2}, the second class in SL(2,\rr)
describes spiky strings, related to the vacuum configurations of the
SL(2,\rr)/U(1) coset model.Comment: 37 p. 6 fi
Kahler Potentials of Chiral Matter Fields for Calabi-Yau String Compactifications
The Kahler potential is the least understood part of effective N=1
supersymmetric theories derived from string compactifications. Even at
tree-level, the Kahler potential for the physical matter fields, as a function
of the moduli fields, is unknown for generic Calabi-Yau compactifications and
has only been computed for simple toroidal orientifolds. In this paper we
describe how the modular dependence of matter metrics may be extracted in a
perturbative expansion in the Kahler moduli. Scaling arguments, locality and
knowledge of the structure of the physical Yukawa couplings are sufficient to
find the relevant Kahler potential. Using these techniques we compute the
`modular weights' for bifundamental matter on wrapped D7 branes for
large-volume IIB Calabi-Yau flux compactifications. We also apply our
techniques to the case of toroidal compactifications, obtaining results
consistent with those present in the literature. Our techniques do not provide
the complex structure moduli dependence of the Kahler potential, but are
sufficient to extract relevant information about the canonically normalised
matter fields and the soft supersymmetry breaking terms in gravity mediated
scenarios.Comment: JHEP style, 24 pages, 4 figures. v2: New section and reference adde
Further results for the two-loop Lcc vertex in the Landau gauge
In the previous paper hep-th/0604112 we calculated the first of the five
planar two-loop diagrams for the Lcc vertex of the general non-Abelian
Yang-Mills theory, the vertex which allows us in principle to obtain all other
vertices via the Slavnov-Taylor identity. The integrand of this first diagram
has a simple Lorentz structure. In this letter we present the result for the
second diagram, whose integrand has a complicated Lorentz structure. The
calculation is performed in the D-dimensional Euclidean position space. We
initially perform one of the two integrations in the position space and then
reduce the Lorentz structure to D-dimensional scalar single integrals. Some of
the latter are then calculated by the uniqueness method, others by the
Gegenbauer polynomial technique. The result is independent of the ultraviolet
and the infrared scale. It is expressed in terms of the squares of spacetime
intervals between points of the effective fields in the position space -- it
includes simple powers of these intervals, as well as logarithms and
polylogarithms thereof, with some of the latter appearing within the Davydychev
integral J(1,1,1). Concerning the rest of diagrams, we present the result for
the contributions correponding to third, fourth and fifth diagrams without
giving the details of calculation. The full result for the Lcc correlator of
the effective action at the planar two-loop level is written explicitly for
maximally supersymmetric Yang-Mills theory.Comment: 29 pages, 1 figure, minor changes; three references added, one new
paragraph in Introduction added, Note Added is extended; to appear in JHE
Neutrino masses in with adjoint flavons
We present a supersymmetric model for neutrino masses
and mixings that implements the seesaw mechanism by means of the heavy SU(2)
singlets and triplets states contained in three adjoints of SU(5). We discuss
how Abelian symmetries can naturally yield non-hierarchical light
neutrinos even when the heavy states are strongly hierarchical, and how it can
also ensure that --parity arises as an exact accidental symmetry. By
assigning two flavons that break to the adjoint representation of
SU(5) and assuming universality for all the fundamental couplings, the
coefficients of the effective Yukawa and Majorana mass operators become
calculable in terms of group theoretical quantities. There is a single free
parameter in the model, however, at leading order the structure of the light
neutrinos mass matrix is determined in a parameter independent way.Comment: 16 pages, 9 figures. Included contributions to neutrino masses from
the triplet states contained in the three adjoints of SU(5
Proof of the MHV vertex expansion for all tree amplitudes in N=4 SYM theory
We prove the MHV vertex expansion for all tree amplitudes of N=4 SYM theory.
The proof uses a shift acting on all external momenta, and we show that every
N^kMHV tree amplitude falls off as 1/z^k, or faster, for large z under this
shift. The MHV vertex expansion allows us to derive compact and efficient
generating functions for all N^kMHV tree amplitudes of the theory. We also
derive an improved form of the anti-NMHV generating function. The proof leads
to a curious set of sum rules for the diagrams of the MHV vertex expansion.Comment: 40 pages, 7 figure
D-branes in PP-wave light cone string field theory
The cubic interaction vertex and the dynamical supercharges are constructed
for open strings ending on D7-branes, in light-cone superstring field theory in
PP-wave background. In this context, we write down the symmetry generators in
terms of the relevant group structure: SU(2) x SU(2) x SO(2) x SO(2),
originating from the eight transverse directions in the PP-wave background and
use the expressions to explicitly construct the vertex at the level of stringy
zero modes. The results are further generalized to include all the stringy
excitations as well.Comment: 30 pages, correction in eqn. (4.28), few equations (appendix),
Comments (p.17-18) and a reference (no. 58) added, typo is corrected in eqn.
(4.5
Tomography of Galactic star-forming regions and spiral arms with the Square Kilometre Array
Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike LicenceVery Long Baseline Interferometry (VLBI) at radio wavelengths can provide astrometry accurate to 10 micro-arcseconds or better (i.e. better than the target GAIA accuracy) without being limited by dust obscuration. This means that unlike GAIA, VLBI can be applied to star-forming regions independently of their internal and line-of-sight extinction. Low-mass young stellar objects (particularly T Tauri stars) are often non-thermal compact radio emitters, ideal for astrometric VLBI radio continuum experiments. Existing observations for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI astrometry of such active T Tauri stars enables the reconstruction of both the regions' 3D structure (through parallax measurements) and their internal kinematics (through proper motions, combined with radial velocities). The extraordinary sensitivity of the SKA telescope will enable similar "tomographic mappings" to be extended to regions located several kpc from Earth, in particular to nearby spiral arm segments. This will have important implications for Galactic science, galactic dynamics and spiral structure theories.Final Published versio
A Note on D-brane - Anti-D-brane Interactions in Plane Wave Backgrounds
We study aspects of the interaction between a D-brane and an anti-D-brane in
the maximally supersymmetric plane wave background of type IIB superstring
theory, which is equipped with a mass parameter mu. An early such study in flat
spacetime (mu=0) served to sharpen intuition about D-brane interactions,
showing in particular the key role of the ``stringy halo'' that surrounds a
D-brane. The halo marks the edge of the region within which tachyon
condensation occurs, opening a gateway to new non-trivial vacua of the theory.
It seems pertinent to study the fate of the halo for non--zero mu. We focus on
the simplest cases of a Lorentzian brane with p=1 and an Euclidean brane with
p=-1, the D--instanton. For the Lorentzian brane, we observe that the halo is
unaffected by the presence of non--zero mu. This most likely extends to other
(Lorentzian) p. For the Euclidean brane, we find that the halo is affected by
non-zero mu. As this is related to subtleties in defining the exchange
amplitude between Euclidean branes in the open string sector, we expect this to
extend to all Euclidean branes in this background.Comment: 14 pages, LaTeX, 2 eps figures. v2: a reference and some clarifying
remarks added; v3: Considerably revised version; halo unaffected by plane
wave background for Lorentzian branes, but Euclidean branes' halo is modifie
Intersecting D-branes in Type IIB Plane Wave Background
We study intersecting D-branes in a type IIB plane wave background using
Green-Schwarz worldsheet formulation. We consider all possible -branes
intersecting at angles in the plane wave background and identify their residual
supersymmetries. We find, in particular, that brane
intersections preserve no supersymmetry. We also present the explicit
worldsheet expressions of conserved supercharges and their supersymmetry
algebras.Comment: 32 pages, 2 tables; Corrected typos, to appear in Phys. Rev.
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