70 research outputs found
Intrinsic Transverse Size Effect
Two recently proposed concepts to improve the perturbative calculation of
exclusive amplitudes, gluonic radiative corrections (Sudakov factor) and
confinement size effects (intrinsic transverse momentum) are combined to study
the neutron magnetic form factor in the space-like region. We find that nucleon
distribution amplitudes modelled on the basis of current QCD sum rules indicate
overlap with the existing data at the highest measured values of momentum
transfer. However, sizeable higher-order perturbative corrections (K-factor)
and/or higher-twist contributions cannot be excluded, although they may be
weaker than in the proton case.Comment: 12 pages LATEX, 4 figures as compressed uu-encoded PS-file, preprint
University of Wuppertal WU-B-94-16, University of Bochum RUB-TPII-04/94 (some
typos eliminated
Large-order trend of the anomalous-dimensions spectrum of trilinear twist-3 quark operators
The anomalous dimensions of trilinear-quark operators are calculated at
leading twist by diagonalizing the one-gluon exchange kernel of the
renormalization-group type evolution equation for the nucleon distribution
amplitude. This is done within a symmetrized basis of Appell polynomials of
maximum degree for (up to order 400) by combining analytical and
numerical algorithms. The calculated anomalous dimensions form a degenerate
system, whose upper envelope shows asymptotically logarithmic behavior.Comment: 12 pages; 1 table; 4 figures as PS files; RevTex styl
Erratum to: “QCD-based pion distribution amplitudes confronting experimental data” [Phys. Lett. B 508 (2001) 279]
New vistas of the meson structure in QCD from low to high energies
This talk presents issues pertaining to the quark structure of the pion
within QCD, both from the theoretical and from the experimental point of view.
We review and discuss the pion-photon transition form factor and the pion's
electromagnetic form factor vs. corresponding experimental data from the CLEO
Collaboration and the JLab. We also examine the extent to which recent
high-precision lattice computations of the second moment of the pion's
distribution amplitude conform with theoretical models. Finally, we include
predictions for the azimuthal asymmetry of the distribution in the
polarized -pair-induced DY production employing various pion distribution
amplitudes.Comment: 6 pages, 7 figures, 2 tables. Talk presented at International
Workshop on e^+e^- collisions from Phi to Psi, Frascati, Italy, 7-10 April
2008. V2 is an exclusive HEP version which includes predictions from AdS/QCD.
V3: typo in title correcte
Pion form factor analysis using NLO analytic perturbation theory
I present results for the pion's electromagnetic form factor in the spacelike
region, which implement the most advanced perturbative information currently
available for this observable in conjunction with a pion distribution amplitude
that agrees with the CLEO data on the pion-photon transition form factor at the
level. I show that using for the running strong coupling and its
powers their analytic versions in the sense of Shirkov and Solovtsov, the
obtained predictions become insensitive to the renormalization scheme and scale
setting adopted. Joining the hard contribution with the soft part on account of
local duality and respecting the Ward identity at , the agreement with
the available experimental data, including expectations from planned
experiments at JLab, is remarkable both in trend and magnitude. I also comment
on Sudakov resummation within the analytic approach.Comment: 5 pages, 3 figures embedded; uses espcrc2.tex. Invited talk presented
at QCD04, 5-9 July 2004, Montpellier, Franc
Analyticity and power corrections in hard-scattering hadronic functions
Demanding the analyticity of hadronic observables (calculated in terms of
power series of the running coupling) as a {\it whole}, we show that they are
free of the Landau singularity. Employing resummation and dispersion-relation
techniques, we compute in a unifying way power corrections to two different
hard-scattering functions in perturbative QCD: the electromagnetic pion form
factor to leading order and the inclusive cross section of the Drell-Yan
process. In the second case, the leading nonperturbative power correction in
gives rise to a Sudakov-like exponential factor in the
impact parameter space which provides enhancement rather than suppression.Comment: V1: 11 pages in RevTeX; 1 figure as EPS file; V2: typos in Eqs. (5)
and (24) corrected; V3: Phys. Lett. B. 504 (2001) 225; V4: The following
equations have been corrected: (2), (5), (14), (15), (21), (22), (27), (32),
(33), (34); one equation added: new (28). Figure corrected. Conclusions
unchange
Two-loop contribution to the pion transition form factor vs. experimental data
We present predictions for the pion-photon transition form factor, derived
with the help of light-cone sum rules and including the main part of the NNLO
radiative corrections. We show that, when the Bakulev-Mikhailov-Stefanis (BMS)
pion distribution amplitude is used, the obtained predictions agree well with
the CELLO and the CLEO data. We found that no model distribution amplitude can
reproduce the observed growth of the new BaBar data, though the BMS model
complies with several BaBar data points.Comment: 5 pages, 2 figures, 3 tables. Presented by the first author at 3rd
joint International HADRON STRUCTURE'09 Conference at Tatranska Strba,
Slovakia, August 30th - September 3rd, 2009. v2 Eqs. (9), (10) corrected;
wrong reference tag removed. v3 Fig. 2 and Ref. [13] correcte
Perturbative power Q^2-corrections to the structure function g(1)
We prove that regulating infrared divergencies generates power (~1/(Q^2)^k)
corrections to the spin structure function g_1 at small x and large Q^2. At the
same time it leads to the corrections ~(Q^2)^k at small Q^2. We present the
explicit series of such terms as well as the formulae for their resummation.
These contributions are not included in the standard analysis of the
experimental data. We argue that accounting for such terms can sizably change
the impact of the other power corrections conventionally attributed to the
higher twists.Comment: Theoretical grounds for our approach are considered in much more
detailed way than in the previous version; 10 pages, 2 figure
Pion-Photon Transition Form Factor and Pion Distribution Amplitude in QCD: Facing the Enigmatic Behavior of the BaBar Data
We present an extended analysis of the data for the pion-photon transition
form factor from different experiments, CELLO, CLEO, and BaBar, and discuss
various theoretical approaches which try to reason from them. We focus on the
divergent behavior of the BaBar data for the pion and those for the
pseudoscalar mesons and comment on recently proposed explanations
for this discrepancy. We argue that it is not possible at present to
accommodate these data within the standard QCD framework self-consistently.Comment: 5 pages, 2 figures, talk presented by the second author at the
International Workshop on e^+e^- collisions from phi to psi (PHIPSI11),
Novosibirsk (Russia), Sept. 19--22, 201
Worldline approach to Sudakov-type form factors in non-Abelian gauge theories
We calculate Sudakov-type form factors for isolated spin-1/2 particles
(fermions) entering non-Abelian gauge-field systems. We consider both the on-
and the off-mass-shell case using a methodology which rests on a worldline
casting of field theories. The simplicity and utility of our approach derives
from the fact that we are in a position to make, a priori, a more transparent
separation (factorization), with respect to a given scale, between short- and
long-distance physics than diagramatic methods.Comment: 18 pages. RevTex is used. No figure
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