24 research outputs found
Spontaneous Creation of the Brane World and Direction of the Time Arrow
In this note we consider the spontaneous creation of the brane world in
five-dimensional space with nondynamical external four-form field via
spherically asymmetric bounce solution. We argue that spherically asymmetric
bounce suggests several inequivalent directions of the time arrow upon the
analytic continuation to the space-time with Lorentzian signature. It it shown
that S-branes in the imaginary time emerge naturally upon the particular
continuation.Comment: 12 pages, 3 figure
Schwinger type processes via branes and their gravity duals
We consider Schwinger type processes involving the creation of the charge and
monopole pairs in the external fields and propose interpretation of these
processes via corresponding brane configurations in Type IIB string theory. We
suggest simple description of some new interesting nonperturbative processes
like monopole/dyon transitions in the electric field and W-boson decay in the
magnetic field using the brane language. Nonperturbative pair production in the
strong coupling regime using the AdS/CFT correspondence is studied. The
treatment of the similar processes in the noncommutative theories when
noncommutativity is traded for the background fields is presented and the
possible role of the critical magnetic field which is S-dual to the critical
electric field is discussed.Comment: 29pp, LaTeX; v3. reference adde
Flux-Tube Formation and Holographic Tunneling
We consider correlator of two concentric Wilson loops, a small and large ones
related to the problem of flux-tube formation. There are three mechanisms which
can contribute to the connected correlator and yield different dependences on
the radius of the small loop. The first one is quite standard and concerns
exchange by supergravity modes. We also consider a novel mechanism when the
flux-tube formation is described by a barrier transition in the string
language, dual to the field-theoretic formulation of Yang-Mills theories. The
most interesting possibility within this approach is resonant tunneling which
would enhance the correlator of the Wilson loops for particular geometries. The
third possibility involves exchange by a dyonic string supplied with the string
junction. We introduce also t'Hooft and composite dyonic loops as probes of the
flux tube. Implications for lattice measurements are briefly discussed.Comment: 15 pages, 3 figure
The Partonic Nature of Instantons
In both Yang-Mills theories and sigma models, instantons are endowed with
degrees of freedom associated to their scale size and orientation. It has long
been conjectured that these degrees of freedom have a dual interpretation as
the positions of partonic constituents of the instanton. These conjectures are
usually framed in d=3+1 and d=1+1 dimensions respectively where the partons are
supposed to be responsible for confinement and other strong coupling phenomena.
We revisit this partonic interpretation of instantons in the context of d=4+1
and d=2+1 dimensions. Here the instantons are particle-like solitons and the
theories are non-renormalizable. We present an explicit and calculable model in
d=2+1 dimensions where the single soliton in the CP^N sigma-model can be shown
to be a multi-particle state whose partons are identified with the ultra-violet
degrees of freedom which render the theory well-defined at high energies. We
introduce a number of methods which reveal the partons inside the soliton,
including deforming the sigma model and a dual version of the Bogomolnyi
equations. We conjecture that partons inside Yang-Mills instantons hold the key
to understanding the ultra-violet completion of five-dimensional gauge
theories.Comment: 28 pages. v3: extra references and comments. Mathematica notebooks
for the figures can be downloaded from
http://www.damtp.cam.ac.uk/user/dt281/parton.htm
QCD Calculation of the Form Factors
We calculate the form factors for the heavy-to-light transitions
by means of QCD sum rules using and light-cone
wave functions. Higher twist contributions as well as gluonic corrections are
taken into account. The sensitivity to the shape of the leading-twist wave
functions and effects of SU(3)-breaking are discussed. The results are compared
with quark model predictions and with the results from QCD sum rules for
three-point correlators.Comment: 13 pages +5 figures available upon request , LaTeX , CERN-TH.6880/93,
MPI-Ph/93-32, LMU-07/9
On noncommutative vacua and noncommutative solitons
We consider noncommutative theory of a compact scalar field. The recently
discovered projector solitons are interpreted as classical vacua in the model
considered. Localized solutions to the projector equation are pointed out and
their brane interpretation is discussed. An example of the noncommutative
soliton interpolating between such vacua is given. No strong noncommutativity
limit is assumed.Comment: 9 pages, latex, references adde
and couplings in QCD
We calculate the and couplings using QCD sum rules on the
light-cone. In this approach, the large-distance dynamics is incorporated in a
set of pion wave functions. We take into account two-particle and
three-particle wave functions of twist 2, 3 and 4. The resulting values of the
coupling constants are and .
From this we predict the partial width \Gamma (D^{*+} \ra D^0 \pi^+ )=32 \pm
5~ keV . We also discuss the soft-pion limit of the sum rules which is
equivalent to the external axial field approach employed in earlier
calculations. Furthermore, using and the pole
dominance model for the B \ra \pi and D\ra \pi semileptonic form factors
is compared with the direct calculation of these form factors in the same
framework of light-cone sum rules.Comment: 27 pages (LATEX) +3 figures enclosed as .uu file MPI-PhT/94-62 ,
CEBAF-TH-94-22, LMU 15/9
Logarithmic scaling in gauge/string correspondence
We study anomalous dimensions of (super)conformal Wilson operators at weak
and strong coupling making use of the integrability symmetry on both sides of
the gauge/string correspondence and elucidate the origin of their
single-logarithmic behavior for long operators/strings in the limit of large
Lorentz spin. On the gauge theory side, we apply the method of the Baxter
Q-operator to identify different scaling regimes in the anomalous dimensions in
integrable sectors of (supersymmetric) Yang-Mills theory to one-loop order and
determine the values of the Lorentz spin at which the logarithmic scaling sets
in. We demonstrate that the conventional semiclassical approach based on the
analysis of the distribution of Bethe roots breaks down in this domain. We work
out an asymptotic expression for the anomalous dimensions which is valid
throughout the entire region of variation of the Lorentz spin. On the string
theory side, the logarithmic scaling occurs when two most distant points of the
folded spinning string approach the boundary of the AdS space. In terms of the
spectral curve for the classical string sigma model, the same configuration is
described by an elliptic curve with two branching points approaching values
determined by the square root of the 't Hooft coupling constant. As a result,
the anomalous dimensions cease to obey the BMN scaling and scale
logarithmically with the Lorentz spin.Comment: 37 pages, 4 figure
Gauge/string duality for QCD conformal operators
Renormalization group evolution of QCD composite light-cone operators, built
from two and more quark and gluon fields, is responsible for the logarithmic
scaling violations in diverse physical observables. We analyze spectra of
anomalous dimensions of these operators at large conformal spins at weak and
strong coupling with the emphasis on the emergence of a dual string picture.
The multi-particle spectrum at weak coupling has a hidden symmetry due to
integrability of the underlying dilatation operator which drives the evolution.
In perturbative regime, we demonstrate the equivalence of the one-loop cusp
anomaly to the disk partition function in two-dimensional Yang-Mills theory
which admits a string representation. The strong coupling regime for anomalous
dimensions is discussed within the two pictures addressed recently, -- minimal
surfaces of open strings and rotating long closed strings in AdS background. In
the latter case we find that the integrability implies the presence of extra
degrees of freedom -- the string junctions. We demonstrate how the analysis of
their equations of motion naturally agrees with the spectrum found at weak
coupling.Comment: Latex, 59 pages, 6 figure
Ioffe-time distributions instead of parton momentum distributions in description of deep inelastic scattering
We argue that parton distributions in coordinate space provide a more natural
object for nonperturbative methods compared to the usual momentum distributions
in which the physics of different longitudinal distances is being mixed. To
illustrate the advantages of the coordinate space formulation, we calculate the
coordinate space distributions for valence quarks in the proton using the QCD
sum rule approach. A remarkable agreement is found between the calculated and
the experimentally measured u-quark distribution up to light-cone distances
of order fm in the proton rest frame.
The calculation for valence d quarks gives much worse results; the reasons for
this discrepancy are discussed.Comment: 24 pages plus 13 pages with figures, requires epsf.sty, revised
version to appear in Phys.Rev.