Hard exclusive production of two pions: Dipion mass distributions in gamma* N -->pi pi N and gamma* gamma --> pi pi

The leading twist parametrization of dipion mass distribution in hard exclusive reactions is proposed. Its parameters are related to quark distributions (usual and skewed) in the pion and to distributions amplitudes of mesons ($\pi$, $\rho$, etc.). We show that measurements of the shape of dipion mass distribution in hard exclusie reactions can give important information about partonic structure of the pion. The expression for the amplitude of the reaction $\gamma^*\gamma\to \pi\pi$ near the threshold in terms of singlet quark distribution in the pion is presented.Comment: Talk given at 7th International Workshop on Deep Inelastic Scattering and QCD (DIS 99), Zeuthen, Germany, 19-23 Apr 1999. Minor typos are correcte

Breakdown of chiral expansion for parton distributions

In the framework of the chiral perturbation theory we computed two- and three-loop corrections to the pion parton distributions which posses $\delta$-function singularities at x->0. This calculation explicitly demonstrates that in the region of small x ~ mpi^2/(4 pi F)^2 standard ChPT breaks down and one needs resummation of all orders. We give an example of such resummation.Comment: 10 pages, 7 figure

Tomography for amplitudes of hard exclusive processes

We discuss which part of information about hadron structure encoded in the Generalized Parton Distributions (GPDs) [part of total GPD image] can be restored from the known amplitude of a hard exclusive process. The physics content of this partial image is analyzed. Among other things, we show that this partial image contains direct information about how the target hadron responses to the (string) quark-antiquark operator of {\it arbitrary} spin J. Explicit equations relating physics content of the partial image of GPDs directly to the data are derived. Also some new results concerning the dual parametrization of GPDs are presented.Comment: 13 pages, no figures, misprints are correcte

Generalized parton distributions and strong forces inside nucleons and nuclei

We argue that generalized parton distributions (GPDs), accessible in hard exclusive processes, carry information about the spatial distribution of forces experienced by quarks and gluons inside hadrons. This way the measurements of hard exclusive processes open a possibility for direct "measurements" of strong forces in different parts of nucleons and nuclei. Also such studies open a venue for addressing questions of the properties of the quark (gluon) matter inside hadrons and nuclei. We give a simple example of relations between GPDs and properties of "nuclear matter" in finite nuclei

Wandzura-Wilczek approximation for the twist-3 DVCS amplitude

We present a derivation of Wandzura-Wilczek (WW) like relations for skewed parton distributions. It is demonstrated for photon-pion scattering that the skewed twist-3 parton distributions contributing to the DVCS amplitude have discontinuities at the points $x=\pm\xi$ in the WW approximation. This may lead to a violation of factorisation for the twist-3 DVCS amplitude with transverse polarization of the virtual photon. We show, however, that the contribution of the divergencies to the scattering of a transversely polarized virtual photon affects DVCS observables only at order $1/Q^2$ and can be neglected at twist-3 accuracy. For the scattering of a longitudinally polarized photon the twist-3 amplitude is free of such divergencies.Comment: 9 pages, no figures (To appear in Phys. Lett. B

Confinement and short distance physics

We consider non-perturbative effects at short distances in theories with confinement. The analysis is straightforward within the Abelian models in which the confinement arises on classical level. In all cases considered (compact U(1) in 3D and 4D, dual Abelian Higgs model) there are non-perturbative contributions associated with short distances which are due to topological defects. In QCD case, both classical and quantum effects determine the role of the topological defects and the theoretical analysis has not been completed so far. Generically, the topological defects would result in 1/Q^2 corrections going beyond the standard Operator Product Expansion. We review existing data on the power corrections and find that the data favor existence of the novel corrections, at least at the mass scale of (1-2) GeV. We indicate crucial experiments which could further clarify the situation on the phenomenological side.Comment: 11 pp., Latex2e, 4 figures, epsfig.sty. Accepted for publication in Physics Letters

Dual parameterization of generalized parton distributions and description of DVCS data

We discuss a new leading-order parameterization of generalized parton distributions of the proton, which is based on the idea of duality. In its minimal version, the parameterization is defined by the usual quark singlet parton distributions and the form factors of the energy-momentum tensor. We demonstrate that our parameterization describes very well the absolute value, the Q^2-dependence and the W-dependence of the HERA data on the total DVCS cross section and contains no free parameters in that kinematics. The parameterization suits especially well the low-x_{Bj} region, which allows us to advocate it as a better alternative to the frequently used double distribution parameterization of the GPDs.Comment: 13 pages, 2 figures, LaTeX. Revised version: equation for the DVCS cross section corrected; one reference added; numerical results did not chang

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 amplitude in the parton model

We compute amplitude of deeply virtual Compton scattering in the parton model. We found that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions (SPD's). These additional contributions make the DVCS amplitude explicitly transverse.Comment: New spin sum rules for twist-3 SPD's are added. Discussion of 1/Q behaviour of twist-3 contributions is correcte

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