19 research outputs found
Resummation of Large Logarithms in
In the collinear factorization of the form factor for the transition
the hard part contains double log terms as with as the momentum fraction of partons from 0 to 1. A simple
exponentiation for resummation leads to divergent results. We study the
resummation of these terms. We show that the terms come
partly from the light-cone wave function(LCWF) and partly from the form factor.
We introduce a jet factor to factorize the term in the form factor.
To handel the terms from the LCWF we introduce a nonstandard
light-cone wave function(NLCWF) with the gauge links off the light-cone
direction. An interesting relation between two wave function is found. With the
introduced NLCWF and the jet factor we can re-factorize the form factor and
obtain a new hard part which does not contain terms with . Beside the
renormalization scale the introduce NLCWF and jet factor have extra
scales to characterize their -behaviors. Using the evolutions of the extra
scales and the relation we can do the resummation perturbatively in sense that
the LCWF is the only nonpertubative object in the resumed formula. Our results
with some models of LCWF show that there is a significant difference between
numerical predictions with the resummation and that without the resummation,
and the resummed predictions can describe the experimental data.Comment: one reference adde
Power corrections to the transition form factor and pion distribution amplitudes
Employing the standard hard-scattering approach and the running coupling
method we calculate a class of power-suppressed corrections to the electromagnetic transition form
factor (FF) arising from the end-point
integration regions. In the investigations we use a hard-scattering amplitude
of the subprocess , symmetrized under
exchange important for exclusive
processes containing two external photons. In the computations the pion model
distribution amplitudes (DA's) with one and two non-asymptotic terms are
employed. The obtained predictions are compared with the CLEO data and
constraints on the DA parameters and at the
normalization point are extracted. Further restrictions on
the pion DA's are deduced from the experimental data on the electromagnetic FF
.Comment: 23 pages, 6 figures; the version published in Phys. Rev. D69, 094010
(2004
Next-to-next-to-leading order prediction for the photon-to-pion transition form factor
We evaluate the next-to-next-to-leading order corrections to the
hard-scattering amplitude of the photon-to-pion transition form factor. Our
approach is based on the predictive power of the conformal operator product
expansion, which is valid for a vanishing -function in the so-called
conformal scheme. The Wilson--coefficients appearing in the non-forward
kinematics are then entirely determined from those of the polarized
deep-inelastic scattering known to next-to-next-to-leading accuracy. We propose
different schemes to include explicitly also the conformal symmetry breaking
term proportional to the -function, and discuss numerical predictions
calculated in different kinematical regions. It is demonstrated that the
photon-to-pion transition form factor can provide a fundamental testing ground
for our QCD understanding of exclusive reactions.Comment: 62 pages LaTeX, 2 figures, 9 tables; typos corrected, some references
added, to appear in Phys. Rev.
Perturbative Effects in the Form Factor \gamma\gamma^*\to \pi and Extraction of the Pion Wave Function from CLEO Data
We study the pion form factor F^{\pi \gamma\gamma^*}(Q^2) in the light-cone
sum rule approach, accounting for radiative corrections and higher twist
effects.
Comparing the results to the CLEO experimental data on F^{\pi
\gamma\gamma^*}(Q^2), we extract the the pion distribution amplitude of
twist-2. The deviation of the distribution amplitude from the asymptotic one is
small and is estimated to be a_2(\mu) = 0.12 \pm 0.03 at \mu=2.4 GeV, in the
model with one non-asymptotic term. The ansatz with two non-asymptotic terms
gives some region of a_2 and a_4, which is consistent with the asymptotic
distribution amplitude, but does not agree with some old models.Comment: 21 pages, LaTeX, 7 eps figures; (v2): Phys. Rev. D versio