155 research outputs found

### 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

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