22 research outputs found
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
D-branes and the Conifold Singularity
We analyze in detail the description of type IIB theory on a Calabi-Yau
three-fold near a conifold singularity in terms of intersecting D-branes. In
particular we study the singularity structure of higher derivative -terms of
the form where is the gravitational superfield. This
singularity is expected to be due to a one -loop contribution from a charged
soliton hypermultiplet becoming massless at the conifold point. In the
intersecting D-brane description this soliton is described by an open string
stretched between the two D-branes. After identifying the graviphoton vertex as
a closed string operator we show that 's have the expected singularity
structure in the limit of vanishing soliton mass.Comment: Latex file, 14 page
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
Models for Light-Cone Meson Distribution Amplitudes
Leading-twist distribution amplitudes (DAs) of light mesons like pi,rho etc.
describe the leading nonperturbative hadronic contributions to exclusive QCD
reactions at large energy transfer, for instance electromagnetic form factors.
They also enter B decay amplitudes described in QCD factorisation, in
particular nonleptonic two-body decays. Being nonperturbative quantities, DAs
cannot be calculated from first principles, but have to be described by models.
Most models for DAs rely on a fixed order conformal expansion, which is
strictly valid for large factorisation scales, but not always sufficient in
phenomenological applications. We derive models for DAs that are valid to all
orders in the conformal expansion and characterised by a small number of
parameters which are related to experimental observables.Comment: 19 pages, 10 figure
Exclusive processes in position space and the pion distribution amplitude
We suggest to carry out lattice calculations of current correlators in
position space, sandwiched between the vacuum and a hadron state (e.g. pion),
in order to access hadronic light-cone distribution amplitudes (DAs). In this
way the renormalization problem for composite lattice operators is avoided
altogether, and the connection to the DA is done using perturbation theory in
the continuum. As an example, the correlation function of two electromagnetic
currents is calculated to the next-to-next-to-leading order accuracy in
perturbation theory and including the twist-4 corrections. We argue that this
strategy is fully competitive with direct lattice measurements of the moments
of the DA, defined as matrix elements of local operators, and offers new
insight in the space-time picture of hard exclusive reactions.Comment: 15 pages, 10 figure
A multiloop improvement of non-singlet QCD evolution equations
An approach is elaborated for calculation of "all loop" contributions to the
non-singlet evolution kernels from the diagrams with renormalon chain
insertions. Closed expressions are obtained for sums of contributions to
kernels for the DGLAP equation and for the "nonforward" ER-BL
equation from these diagrams that dominate for a large value of , the
first -function coefficient. Calculations are performed in the covariant
-gauge in a MS-like scheme. It is established that a special choice of the
gauge parameter generalizes the standard "naive nonabelianization"
approximation. The solutions are obtained to the ER-BL evolution equation
(taken at the "all loop" improved kernel), which are in form similar to
one-loop solutions. A consequence for QCD descriptions of hard processes and
the benefits and incompleteness of the approach are briefly discussed.Comment: 13 pages, revtex, 2 figures are enclosed as eps-file, the text style
and figures are corrected following version, accepted for publication to
Phys. Rev.
Next-to-next-to-leading order prediction for the photon-to-pion transition form factor
We evaluate the next-to-next-to-leading order corrections to the
hard-scattering amplitude of the photon-to-pion transition form factor. Our
approach is based on the predictive power of the conformal operator product
expansion, which is valid for a vanishing -function in the so-called
conformal scheme. The Wilson--coefficients appearing in the non-forward
kinematics are then entirely determined from those of the polarized
deep-inelastic scattering known to next-to-next-to-leading accuracy. We propose
different schemes to include explicitly also the conformal symmetry breaking
term proportional to the -function, and discuss numerical predictions
calculated in different kinematical regions. It is demonstrated that the
photon-to-pion transition form factor can provide a fundamental testing ground
for our QCD understanding of exclusive reactions.Comment: 62 pages LaTeX, 2 figures, 9 tables; typos corrected, some references
added, to appear in Phys. Rev.
Perturbative Effects in the Form Factor \gamma\gamma^*\to \pi and Extraction of the Pion Wave Function from CLEO Data
We study the pion form factor F^{\pi \gamma\gamma^*}(Q^2) in the light-cone
sum rule approach, accounting for radiative corrections and higher twist
effects.
Comparing the results to the CLEO experimental data on F^{\pi
\gamma\gamma^*}(Q^2), we extract the the pion distribution amplitude of
twist-2. The deviation of the distribution amplitude from the asymptotic one is
small and is estimated to be a_2(\mu) = 0.12 \pm 0.03 at \mu=2.4 GeV, in the
model with one non-asymptotic term. The ansatz with two non-asymptotic terms
gives some region of a_2 and a_4, which is consistent with the asymptotic
distribution amplitude, but does not agree with some old models.Comment: 21 pages, LaTeX, 7 eps figures; (v2): Phys. Rev. D versio
A study of correlates of academic achievement in third grade guidance school students by TIMSS-R and path analysis of its effects
The purpose of this study was to identify academic achievement correlates of science in third grade guidance school students and path analysis of the effects of these correlates. Therefore 5301 students were selected and TISS-R was administered to them. Fifteen variables were found. Thirteen of these had significant correlations with science achievement (