346 research outputs found
Real Compton Scattering via Color Dipoles
We study photoabsorption reaction and real Compton scattering (RCS) within
the color dipole model. We rely on a photon wave function derived in the
instanton vacuum model, and on the energy dependent phenomenological elastic
dipole amplitude. Data for the photoabsorption cross section at high energies
agree with our parameter free calculations. We also provide predictions for the
differential RCS cross section. Although no data for small angle Compton
scattering are available so far, this process can be measured in
ultra-peripheral hadronic and nuclear collisions at the LHC.Comment: 9 pages, 4 figures. Some statements clarified, bibliographic
inaccuracy correcte
QCD factorizations in gamma* gamma* -> rho rho
We calculate the lowest order QCD amplitude, i.e. the quark exchange
contribution, to the forward production amplitude of a pair of longitudinally
polarized mesons in the scattering of two virtual photons . We show that the scattering amplitude
simultaneously factorizes in two quite different ways: the part with transverse
photons is described by the QCD factorization formula involving the generalized
distribution amplitude of two final mesons, whereas the part with
longitudinally polarized photons takes the QCD factorized form with the
transition distribution amplitude. Perturbative
expressions for these, in general, non-perturbative functions are obtained in
terms of the meson distribution amplitude
Generalized parton distributions of the pion in a Bethe-Salpeter approach
We calculate generalized parton distribution functions in a field theoretic
formalism using a covariant Bethe-Salpeter approach for the determination of
the bound-state wave function. We describe the procedure in an exact
calculation in scalar Electrodynamics proving that the relevant corrections
outside our scheme vanish. We extend the formalism to the Nambu--Jona-Lasinio
model, a realistic theory of the pion. We go in both cases beyond all previous
calculations and discover that all important features required by general
physical considerations, like symmetry properties, sum rules and the
polynomiality condition, are explicitly verified. We perform a numerical study
of their behavior in the weak and strong coupling limits.Comment: 19 pages, 21 eps figures, accepted for publication in EPJ
Rare decay pi0 -> e+e-: theory confronts KTeV data
Within the dispersive approach to the amplitude of the rare decay pi0 -> e+e-
the nontrivial dynamics is contained only in the subtraction constant. We
express this constant, in the leading order in (m_e/\Lambda)^2 perturbative
series, in terms of the inverse moment of the pion transition form factor given
in symmetric kinematics. By using the CELLO and CLEO data on the pion
transition form factor given in asymmetric kinematics the lower bound on the
decay branching ratio is found. The restrictions following from QCD allow us to
make a quantitative prediction for the branching B(pi0 -> e+e-) =(6.2\pm
0.1)*10^{-8} which is 3\sigma below the recent KTeV measurement. We confirm our
prediction by using the quark models and phenomenological approaches based on
the vector meson dominance. The decays \eta -> l^+l^- are also discussed.Comment: 7 pages, 1 figur
DVCS via color dipoles: Nonperturbative effects
We study the DVCS amplitude within the color dipole approach. The light-cone
wave function of a real photon is evaluated in the instanton vacuum model. Our
parameter free calculations are able to describe H1 data, both the absolute
values and the -dependences, at medium-high values of . The
dependence is found to be sensitive to the choice of the phenomenological cross
section fitted to DIS data.Comment: 11 pages; references added, bibliographic inaccuracy correcte
Light Cone Sum Rules for gamma* N -> Delta Transition Form Factors
A theoretical framework is suggested for the calculation of gamma* N -> Delta
transition form factors using the light-cone sum rule approach. Leading-order
sum rules are derived and compared with the existing experimental data. We find
that the transition form factors in a several GeV region are dominated by the
``soft'' contributions that can be thought of as overlap integrals of the
valence components of the hadron wave functions. The ``minus'' components of
the quark fields contribute significantly to the result, which can be
reinterpreted as large contributions of the quark orbital angular momentumComment: 38 pages, 10 figures; some typos fixed and references added, to
appear in Phys. Rev.
The gluon content of the and mesons and the , electromagnetic transition form factors
We compute power-suppressed corrections to the \eta\gamma and
\eta^{\prime}\gamma transition form factors
Q^2F_{\eta(\eta^{\prime})\gamma}(Q^2) arising from the end point regions x \to
0,1 by employing the infrared-renormalon approach. The contribution to the form
factors from the quark and gluon content of the \eta,\eta^{\prime} mesons is
taken into account using for the \eta-\eta^{\prime} mixing the SU_f(3) singlet
\eta_1 and octet \eta_8 basis. The theoretical predictions obtained this way
are compared with the corresponding CLEO data and restrictions on the input
parameters (Gegenbauer coefficients) B_2^q(\eta_1), B_2^g(\eta_1), and
B_2^q(\eta_8) in the distribution amplitudes for the \eta_1,\eta_8 states with
one nonasymptotic term are deduced. Comparison is made with the results from
QCD perturbation theory.Comment: 25 pages, RevTeX4 used. 9 figures as EPS files. Text significantly
changed to include variation of theoretical parameters. Figures modified.
Corrected typo in equation (34) and trivial mistake in -coefficient.
References added. Conclusions unchange
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