717 research outputs found
On the perturbative corrections around D-string instantons
We study -threshold corrections in an eight dimensional S-dual
pair of string theories, as a prototype of dual string vacua with sixteen
supercharges. We show that the orbifold CFT description of D-string instantons
gives rise to a perturbative expansion similar to the one appearing on the
fundamental string side. By an explicit calculation, using the Nambu-Goto
action in the static gauge, we show that the first subleading term agrees
precisely on the two sides. We then give a general argument to show that the
agreement extends to all orders.Comment: 12 page
Superspace WZW Models and Black Holes
We show how to write an off-shell action for the
supersymmetric WZW model in terms of chiral and twisted chiral
multiplets. We discuss the supersymmetry of this model and exhibit the
superconformal current algebra. Finally, we show that the off-shell
formulation makes it possible to perform a duality transformation, which leads
to a supersymmetric sigma model on a manifold with a black hole type
singularity.Comment: 12 page
AdS/CFT correspondence and D1/D5 systems in theories with 16 supercharges
We discuss spectra of supergravities, arising in the near horizon
geometry of D1/D5 systems in orbifolds/orientifolds of type IIB theory with 16
supercharges. These include models studied in a recent paper (hep-th/0012118),
where the group action involves also a shift along a transversal circle, as
well as IIB/, which is dual to IIB on . After appropriate
assignements of the orbifold group eigenvalues and degrees to the supergravity
single particle spectrum, we compute the supergravity elliptic genus and find
agreement, in the expected regime of validity, with the elliptic genus obtained
using U-duality map from (4,4) CFTs of U-dual backgrounds. Since this U-duality
involves the exchange of KK momentum and D1 charge , it allows us to
test the (4,4) CFTs in the and regimes by two different
supergravity duals.Comment: 28 pages, no figure
Duality Symmetries in N=2 Heterotic Superstring
We review the derivation and the basic properties of the perturbative
prepotential in N=2 compactifications of the heterotic superstring. We discuss
the structure of the perturbative monodromy group and the embedding of rigidly
supersymmetric monodromies associated with enhanced gauge groups, at both
perturbative and non-perturbative level.Comment: Based on talks presented at several conferences. 12 pages, LaTe
A dynamical collective calculation of supernova neutrino signals
We present the first calculations with three flavors of collective and shock
wave effects for neutrino propagation in core-collapse supernovae using
hydroynamical density profiles and the S matrix formalism. We explore the
interplay between the neutrino-neutrino interaction and the effects of multiple
resonances upon the time signal of positrons in supernova observatories. A
specific signature is found for the inverted hierarchy and a large third
neutrino mixing angle and we predict, in this case, a dearth of lower energy
positrons in Cherenkov detectors midway through the neutrino signal and the
simultaneous revelation of valuable information about the original fluxes. We
show that this feature is also observable with current generation neutrino
detectors at the level of several sigmas.Comment: 4 pages, 5 figure
Instantons in Superconformal Gauge Theories and the AdS/CFT Correspondence
We study, using ADHM construction, instanton effects in an {\CN}=2
superconformal gauge theory, arising as effective field theory on a
system of D-3-branes near an orientifold 7-plane and 8 D-7-branes in type
I' string theory. We work out the measure for the collective coordinates of
multi-instantons in the gauge theory and compare with the measure for the
collective coordinates of -branes in the presence of 3- and 7-branes in
type I' theory. We analyse the large-N limit of the measure and find that it
admits two classes of saddle points: In the first class the space of collective
coordinates has the geometry of which on the string theory
side has the interpretation of the D-instantons being stuck on the 7-branes and
therefore the resulting moduli space being , In the second
class the geometry is and on the string theory side it
means that the D-instantons are free to move in the 10-dimensional bulk. We
discuss in detail a correlator of four O(8) flavour currents on the Yang-Mills
side, which receives contributions from the first type of saddle points only,
and show that it matches with the correlator obtained from coupling on
the string theory side, which receives contribution from D-instantons, in
perfect accord with the AdS/CFT correspondence. In particular we observe that
the sectors with odd number of instantons give contribution to an O(8)-odd
invariant coupling, thereby breaking O(8) down to SO(8) in type I' string
theory. We finally discuss correlators related to , which receive
contributions from both saddle points.Comment: 28 pages, no figures, typos corrected, a reference adde
Produção de mudas de palma-forrageira utilizando fragmentos de cladódios.
A palma (Opuntia ficus-indica L.) é uma das principais alternativas para alimentação de caprinos, ovinos e bovinos no Semiárido brasileiro. Contudo, o ataque da cochonilhado-carmim (Dactylopius opuntiae Cockerel) erradicou grande parte da área plantada, já que a maioria das variedades cultivadas são suscetíveis à praga. Três novas variedades resistentes à praga foram identificadas: Orelha-de-elefante mexicana, Miúda ou Doce e a variedade Mão-de-moça (clones IPA-Sertânea e PALMEPA-PB1). Contudo, a disponibilidade de material propagativo (cladódios ou raquetes) ainda é baixa, aumentando o preço da palma-semente. Assim, desenvolveu-se uma metodologia utilizando-se fragmentos (pedaços) de raquetes para a obtenção de novas mudas
On the Bound States of p- and (p+2)-Branes
We study bound states of D-p-branes and D-(p+2)-branes. By switching on a
large magnetic field F on the (p+2) brane, the problem is shown to admit a
perturbative analysis in an expansion in inverse powers of F. It is found that,
to the leading order in 1/F, the quartic potential of the tachyonic state from
the open string stretched between the p- and (p+2)-brane gives a vacuum energy
which agrees with the prediction of the BPS mass formula for the bound state.
We generalize the discussion to the case of m p-branes plus 1 (p+2)-brane with
magnetic field. The T dual picture of this, namely several (p+2)-branes
carrying some p-brane charges through magnetic flux is also discussed, where
the perturbative treatment is available in the small F limit. We show that once
again, in the same approximation, the tachyon condensates give rise to the
correct BPS mass formula. The role of 't Hooft's toron configurations in the
extension of the above results beyond the quartic approximation as well as the
issue of the unbroken gauge symmetries are discussed. We comment on the
connection between the present bound state problem and Kondo-like problems in
the context of relevant boundary perturbations of boundary conformal field
theories.Comment: 34 pages, Late
Moduli Corrections to Gauge and Gravitational Couplings in four dimensional Superstrings
We study one-loop, moduli-dependent corrections to gauge and gravitational
couplings in supersymmetric vacua of the heterotic string. By exploiting their
relation to the integrability condition for the associated CP-odd couplings, we
derive general expressions for them, both for and models, in
terms of tree level four-point functions in the internal superconformal
theory. The case, in particular symmetric orbifolds, is discussed in
detail.Comment: 19 page
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