17 research outputs found
Solution of the off-forward leading logarithmic evolution equation based on the Gegenbauer moments inversion
Using the conformal invariance the leading-log evolution of the off-forward
structure function is reduced to the forward evolution described by the
conventional DGLAP equation. The method relies on the fact that the anomalous
dimensions of the Gegenbauer moments of the off-forward distribution are
independent on the asymmetry, or skewedness, parameter and equal to the DGLAP
ones. The integral kernels relating the forward and off-forward functions with
the same Mellin and Gegenbauer moments are presented for arbitrary asymmetry
value.Comment: 11 pages, LaTeX, no figures, revised version, references adde
Helicity-Flip Off-Foward Parton Distributions of the Nucleon
We identify quark and gluon helicity-flip distributions defined between
nucleon states of unequal momenta. The evolution of these distributions with
change of renormalization scale is calculated in the leading-logarithmic
approximation. The helicity-flip gluon distributions do not mix with any quark
distribution and are thus a unique signature of gluons in the nucleon. Their
contribution to the generalized virtual Compton process is obtained both in the
form of a factorization theorem and an operator product expansion. In deeply
virtual Compton scattering, they can be probed through distinct angular
dependence of the cross section.Comment: a few corrections made, references change
The vertex with arbitrary gluon virtualities in the perturbative QCD hard scattering approach
We study the vertex for arbitrary gluon virtualities in
the time-like and space-like regions, using the perturbative QCD hard
scattering approach and an input wave-function of the -meson
consistent with the measured transition form
factor. The contribution of the gluonic content of the -meson is
taken into account, enhancing the form factor over the entire virtuality
considered. However, data on the electromagnetic transition form factor of the
-meson is not sufficient to quantify the gluonic enhancement. We
also study the effect of the transverse momenta of the partons in the
-meson on the vertex, using the modified
hard scattering approach based on Sudakov formalism. Analytic expressions for
the vertex are presented in limiting kinematic regions
and parametrizations are given satisfying the QCD anomaly, for real gluons, and
perturbative QCD behavior for large gluon virtualities, in both the time-like
and space-like regions. Our results have implications for the inclusive decay
and exclusive decays, such as , and in hadronic production processes .Comment: 23 pages, 19 figures (requires revtex4, amssymb, epsf); several typos
corrected, this version now identical to the one accepted for publication in
Phys. Rev.
Exclusive Radiative B-Decays in the Light-Cone QCD Sum Rule Approach
We carry out a detailed study of exclusive radiative rare -decays in the
framework of the QCD sum rules on the light cone, which combines the
traditional QCD sum rule technique with the description of final state vector
mesons in terms of the light-cone wave functions of increasing twist. The
decays considered are: and the corresponding decays of the mesons, and . Based on our estimate of the transition
form factor F_1^{B \to K^*\pg}(0) =0.32\pm0.05, we find for the branching
ratio , which is in
agreement with the observed value measured by the CLEO collaboration. We
present detailed estimates for the ratios of the radiative decay form factors,
which are then used to predict the rates for the exclusive radiative B-decays
listed above. This in principle allows the extraction of the CKM matrix element
from the penguin-dominated CKM-suppressed radiative decays when they
are measured. We give a detailed discussion of the dependence of the form
factors on the -quark mass and on the momentum transfer, as well as their
interrelation with the CKM-suppressed semileptonic decay form factors in , which we also calculate in our approach.Comment: 32 pages, 10 uuencoded figures, LaTeX, preprint CERN-TH 7118/9
Off-diagonal distributions fixed by diagonal partons at small and
We show that the off-diagonal (or skewed) parton distributions are completely
determined at small and by the (conventional) diagonal partons. We
present predictions which can be used to estimate the off-diagonal
distributions at small and at any scale.Comment: 16 pages, latex, 4 ps figure
Double distributions and evolution equations
Applications of perturbative QCD to deeply virtual Compton scattering and
hard exclusive meson electroproduction processes require a generalization of
usual parton distributions for the case when long-distance information is
accumulated in nonforward matrix elements of quark and gluon
light-cone operators. In our previous papers we used two types of
nonperturbative functions parametrizing such matrix elements: double
distributions F(x,y;t) and nonforward distribution functions F_\zeta(X;t). Here
we discuss in more detail the double distributions (DD's) and evolution
equations which they satisfy. We propose simple models for F(x,y;t=0) DD's with
correct spectral and symmetry properties which also satisfy the reduction
relations connecting them to the usual parton densities f(x). In this way, we
obtain self-consistent models for the \zeta-dependence of nonforward
distributions. We show that, for small \zeta, one can easily obtain nonforward
distributions (in the X > \zeta region) from the parton densities: F_\zeta
(X;t=0) \approx f(X-\zeta/2).Comment: 21 pages, Latex, 8 figures. To appear in Phys. Rev.
The Meson Light-Cone Distribution Amplitudes of Leading Twist Revisited
We give a complete re-analysis of the leading twist quark-antiquark
light-cone distribution amplitudes of longitudinal and transverse
mesons. We derive Wandzura-Wilczek type relations between different
distributions and update the coefficients in their conformal expansion using
QCD sum rules including next-to-leading order radiative corrections. We find
that the distribution amplitudes of quarks inside longitudinally and
transversely polarized mesons have a similar shape, which is in
contradiction to previous analyses.Comment: 21 pages, latex2e, requires a4wide.sty and epsf.sty, 6 PS figures
include
Angular distributions in hard exclusive production of pion pairs
Using the leading order amplitudes of hard exclusive electroproduction of
pion pairs we have analyzed the angular distribution of the two produced
particles. At leading twist a pion pair can be produced only in an isovector or
an isoscalar state. We show that certain components of the angular distribution
only get contributions from the interference of the I=1 and the (much smaller)
I=0 amplitude. Therefore our predictions prove to be a good probe of isospin
zero pion pair production. We predict effects of a measurable size that could
be observed at experiments like HERMES. We also discuss how hard exclusive pion
pair production can provide us with new information on the effective chiral
Lagrangian.Comment: 17 pages, version to appear in Phys. Rev.
Parton interactions in the Bjorken limit of QCD
We consider the Bjorken limit in the framework of the effective action
approach and discuss its similarities to the Regge limit. The proposed
effective action allows for a rather simple calculation of the known evolution
kernels. We represent the result in terms of two-parton interaction operators
involving gluon and quark operators depending on light-ray position and
helicity and analyze their symmetry properties.Comment: 32 pages LaTex, 4 eps-figures, comments added, minor correction
Scheme dependence of NLO corrections to exclusive processes
We apply the so-called conformal subtraction scheme to predict perturbatively
exclusive processes beyond leading order. Taking into account evolution
effects, we study the scheme dependence for the photon-to-pion transition form
factor and the electromagnetic pion form factor at next-to-leading order for
different pion distribution amplitudes. Relying on the conformally covariant
operator product expansion and using the known higher order results for
polarized deep inelastic scattering, we are able to predict perturbative
corrections to the hard-scattering amplitude of the photon-to-pion transition
form factor beyond next-to-leading order in the conformal scheme restricted to
the conformal limit of the theory.Comment: RevTeX, 25 pages, 2 figures, 5 tables, minor changes, to be published
in Phys. Rev.