A novel way to probe distribution amplitudes of neutral mesons in e^+e^- annihilation

We derive the amplitude for the process $e^+e^-\to \pi^0\pi^0$ at large invariant energy. The process goes through the two-photon exchange and its amplitude is expressed in terms of the convolution integral which depends on the shape of the pion distribution amplitude (DA) and the centre of mass scattering angle. Remarkable feature of the integral is that it is very sensitive to the end-point behaviour of the pion DA -- it starts to diverge if pion DA nullifies at the end-point as $\sqrt x$ or slower. That makes the $e^+e^-\to \pi^0\pi^0$ process unique probe of the shape of the meson DAs. The estimated cross section is rather small, for $\sqrt s = 3$ GeV it ranges from a fraction of femtobarn (for the asymptotic DA) to couple of femtobarn (for the Chernyak-Zhitnitsky DA). The observation of the process $e^+e^-\to\pi^0\pi^0$ with the cross section higher as estimated here would imply very unusual form of the pion DA, e.g. the flat one. The derived amplitude can be easily generalized to other processes like $e^+e^-\to \sigma\sigma, K_SK_S, \eta\eta, \eta^\prime\eta, \pi^0 f_2$, etc.Comment: 5 pages, 3 figure

DVCS on the nucleon : study of the twist-3 effects

We estimate the size of the twist-3 effects on deeply virtual Compton scattering (DVCS) observables, in the Wandzura-Wilczek approximation. We present results in the valence region for the DVCS cross sections, charge asymmetries and single spin asymmetries, to twist-3 accuracy.Comment: 20 pages, 6 figure

Twist-three analysis of photon electroproduction with pion

We study twist-three effects in spin, charge, and azimuthal asymmetries in deeply virtual Compton scattering on a spin-zero target. Contributions which are power suppressed in 1/Q generate a new azimuthal angle dependence of the cross section which is not present in the leading twist results. On the other hand the leading twist terms are not modified by the twist three contributions. They may get corrected at twist four level. In the Wandzura-Wilczek approximation these new terms in the Fourier expansion with respect to the azimuthal angle are entirely determined by the twist-two skewed parton distributions. We also discuss more general issues like the general form of the angular dependence of the differential cross section, validity of factorization at twist-three level, and a relation of skewed parton distributions to spectral functions.Comment: 21 pages, LaTeX, 2 figures, text clarifications, an equation, a note and references adde

Topological current algebras in two dimensions

Two-dimensional topological field theories possessing a non-abelian current symmetry are constructed. The topological conformal algebra of these models is analysed. It differs from the one obtained by twisting the $N=2$ superconformal models and contains generators of dimensions $1$, $2$ and $3$ that close a linear algebra. Our construction can be carried out with one and two bosonic currents and the resulting theories can be interpreted as topological sigma models for group manifoldsComment: 16 page

The dual parametrization for gluon GPDs

We consider the application of the dual parametrization for the case of gluon GPDs in the nucleon. This provides opportunities for the more flexible modeling unpolarized gluon GPDs in a nucleon which in particular contain the invaluable information on the fraction of nucleon spin carried by gluons. We perform the generalization of Abel transform tomography approach for the case of gluons. We also discuss the skewness effect in the framework of the dual parametrization. We strongly suggest to employ the fitting strategies based on the dual parametrization to extract the information on GPDs from the experimental data.Comment: 37 pages, 2 figure

Anomalous Negative Magnetoresistance Caused by Non-Markovian Effects

A theory of recently discovered anomalous low-field magnetoresistance is developed for the system of two-dimensional electrons scattered by hard disks of radius $a,$ randomly distributed with concentration $n.$ For small magnetic fields the magentoresistance is found to be parabolic and inversely proportional to the gas parameter, $\delta \rho_{xx}/\rho \sim - (\omega_c \tau)^2 / n a^2.$ With increasing field the magnetoresistance becomes linear $\delta \rho_{xx}/\rho \sim - \omega_c \tau$ in a good agreement with the experiment and numerical simulations.Comment: 4 pages RevTeX, 5 figure

On the azimuthal asymmetries in DIS

Using the recent experimental data on the left right asymmetry in fragmentation of transversely polarized quarks and the theoretical calculation of the proton transversity distribution in the effective chiral quark soliton model we explain the azimuthal asymmetries in semi-inclusive hadron production on longitudinal (HERMES) and transversely (SMC) polarized targets with no free parameters. On this basis we state that the proton transversity distribution could be successfully measured in future DIS experiments with longitudinally polarized target.Comment: 8 pages, latex, 5 eps figures, uses epsfig and wrapfi

A Relation Between Approaches to Integrability in Superconformal Yang-Mills Theory

We make contact between the infinite-dimensional non-local symmetry of the typeIIB superstring on AdS5xS5 worldsheet theory and a non-abelian infinite-dimensional symmetry algebra for the weakly coupled superconformal gauge theory. We explain why the planar limit of the one-loop dilatation operator is the Hamiltonian of a spin chain, and show that it commutes with the g*2 N = 0 limit of the non-abelian charges.Comment: 19 pages, harvma

Modelling generalized parton distributions to describe deeply virtual Compton scattering data

We present a new model for generalized parton distributions (GPDs), based on the aligned jet model, which successfully describes the deeply virtual Compton scattering (DVCS) data from H1, ZEUS, HERMES and CLAS. We also present an easily implementable and flexible algorithm for their construction. This new model is necessary since the most widely used models for GPDs, which are based on factorized double distributions, cannot, in their current form, describe the DVCS data when employed in a full QCD analysis. We demonstrate explicitly the reason for the shortcoming in the data description. We also highlight several non-perturbative input parameters which could be used to tune the GPDs, and the $t$-dependence, to the DVCS data using a fitting procedure.Comment: 12 pages, 12 figures, revtex4, shortened version accepted for publication in PRD, figures improved and references adde

Helicity skewed quark distributions of the nucleon and chiral symmetry

We compute the helicity skewed quark distributions $\widetilde{H}$ and $\widetilde{E}$ in the chiral quark-soliton model of the nucleon. This model emphasizes correctly the role of spontaneously broken chiral symmetry in structure of nucleon. It is based on the large-N_c picture of the nucleon as a soliton of the effective chiral lagrangian and allows to calculate the leading twist quark- and antiquark distributions at a low normalization point. We discuss the role of chiral symmetry in the helicity skewed quark distributions $\widetilde{H}$ and $\widetilde{E}$. We show that generalization of soft pion theorems, based on chiral Ward identities, leads in the region of -\xi < x < \xi to the pion pole contribution to $\widetilde{E}$ which dominates at small momentum transfer.Comment: 22 pages, 5 figure