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

QCD Calculation of the B→π,KB \rightarrow \pi,K Form Factors

We calculate the form factors for the heavy-to-light transitions $B\rightarrow \pi,K$ by means of QCD sum rules using $\pi$ and $K$ 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

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

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

D∗DπD^*D\pi and B∗BπB^*B\pi couplings in QCD

We calculate the $D^*D\pi$ and $B^*B\pi$ 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 $g_{D^*D\pi}= 12.5\pm 1$ and $g_{B^*B\pi}= 29\pm 3$. 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 $g_{B^*B\pi}$ and $g_{D^*D\pi}$ 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 $\Delta^- = \Delta^0 - \Delta^3$ of order $\sim 1$ 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.