712 research outputs found
Study of exclusive processes e^+ e^- \to VP
This paper is devoted to consideration of the hard exclusive processes
, where . Experimental measurement of
the cross section of the process at BaBar collaboration
at large center mass energy GeV and some low energy experimental
data GeV give us the possibility to study the cross section
in the broad energy region. As the result, we have determined the asymptotic
behavior of the cross section of in the limit , which is in agreement with perturbative QCD prediction. Assuming that
the same asymptotic behavior is valid for the other processes under
consideration and using low energy experimental data we have predicted the
cross sections of these processes at energies GeV. In
addition, we have calculated the cross sections of these processes at the same
energies within perturbative QCD. Our results are in agreement with available
experimental data.Comment: 10 pages, 2 tables, 6 figures; minor changes; article accepted to
publication is Phys. Rev.
Two-photon exchange in elastic electron-nucleon scattering
A detailed study of two-photon exchange in unpolarized and polarized elastic
electron--nucleon scattering is presented, taking particular account of nucleon
finite size effects. Contributions from nucleon elastic intermediate states are
found to have a strong angular dependence, which leads to a partial resolution
of the discrepancy between the Rosenbluth and polarization transfer
measurements of the proton electric to magnetic form factor ratio, G_E/G_M. The
two-photon exchange contribution to the longitudinal polarization transfer P_L
is small, whereas the contribution to the transverse polarization transfer P_T
is enhanced at backward angles by several percent, increasing with Q^2. This
gives rise to a small, ~3% suppression of G_E/G_M obtained from the
polarization transfer ratio P_T/P_L at large Q^2. We also compare the
two-photon exchange effects with data on the ratio of e^+ p to e^- p cross
sections, which is predicted to be enhanced at backward angles. Finally, we
evaluate the corrections to the form factors of the neutron, and estimate the
elastic intermediate state contribution to the ^3He form factors
Parton content of the nucleon from distribution amplitudes and transition distribution amplitudes
The nucleon distribution amplitudes and the nucleon-to-pion transition
distribution amplitudes are investigated at leading twist within the frame of a
light-cone quark model. The distribution amplitudes probe the three-quark
component of the nucleon light-cone wave function, while higher order
components in the Fock-space expansion of the nucleon state are essential to
describe the nucleon-to-pion transition distribution amplitudes. Adopting a
meson-cloud model of the nucleon the nucleon-to-pion transition distribution
amplitudes are calculated for the first time.Comment: comments and references added; version to appear in Phys. Rev.
Pion distribution amplitude from holographic QCD and the electromagnetic form factor F_pi(Q2)
The holographic QCD prediction for the pion distribution amplitude (DA)
is used to compute the pion spacelike electromagnetic form
factor within the QCD light-cone sum rule method. In
calculations the pion's renormalon-based model twist-4 DA, as well as the
asymptotic twist-4 DA are employed. Obtained theoretical predictions are
compared with experimental data and with results of the holographic QCD
Accuracy of the pion elastic form factor extracted from a local-duality sum rule
We analyze the accuracy of the pion elastic form factor predicted by a
local-duality (LD) version of dispersive sum rules. To probe the precision of
this theoretical approach, we adopt potential models with interactions that
involve both Coulomb and confining terms. In this case, the exact form factor
may be obtained from the solution of the Schroedinger equation and confronted
with the LD sum rule results. We use parameter values appropriate for hadron
physics and observe that, independently of the details of the confining
interaction, the deviation of the LD form factor from the exact form factor
culminates in the region Q^2~4-6 GeV^2. For larger Q^2, the accuracy of the LD
description increases rather fast with Q^2. A similar picture is expected for
QCD. For the pion form factor, existing data suggest that the LD limit may be
reached already at the relatively low values Q^2=4-10 GeV^2. Thus, large
deviations of the pion form factor from the behaviour predicted by LD QCD sum
rules for higher values of Q^2, as found by some recent analyses, appear to us
quite improbable. New accurate data on the pion form factor at Q^2=4-10 GeV^2
expected soon from JLab will have important implications for the behaviour of
the pion form factor in a broad Q^2 range up to asymptotically large values of
Q^2.Comment: 12 pages, extended version, conclusions remain unchange
A consistent model for \pi N transition distribution amplitudes and backward pion electroproduction
The extension of the concept of generalized parton distributions leads to the
introduction of baryon to meson transition distribution amplitudes (TDAs),
non-diagonal matrix elements of the nonlocal three quark operator between a
nucleon and a meson state. We present a general framework for modelling nucleon
to pion () TDAs. Our main tool is the spectral representation for \pi N
TDAs in terms of quadruple distributions. We propose a factorized Ansatz for
quadruple distributions with input from the soft-pion theorem for \pi N TDAs.
The spectral representation is complemented with a D-term like contribution
from the nucleon exchange in the cross channel. We then study backward pion
electroproduction in the QCD collinear factorization approach in which the
non-perturbative part of the amplitude involves \pi N TDAs. Within our two
component model for \pi N TDAs we update previous leading-twist estimates of
the unpolarized cross section. Finally, we compute the transverse target single
spin asymmetry as a function of skewness. We find it to be sizable in the
valence region and sensitive to the phenomenological input of our \pi N TDA
model.Comment: 39 pages, 9 figure
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
Next-to-leading-order corrections to exclusive processes in factorization
We calculate next-to-leading-order (NLO) corrections to exclusive processes
in factorization theorem, taking as an example.
Partons off-shell by are considered in both the quark diagrams from
full QCD and the effective diagrams for the pion wave function. The gauge
dependences in the above two sets of diagrams cancel, when deriving the
-dependent hard kernel as their difference. The gauge invariance of the
hard kernel is then proven to all orders by induction. The light-cone
singularities in the -dependent pion wave function are regularized by
rotating the Wilson lines away from the light cone. This regularization
introduces a factorization-scheme dependence into the hard kernel, which can be
minimized in the standard way. Both the large double logarithms and
, being a parton momentum fraction, arise from the loop correction
to the virtual photon vertex, the former being absorbed into the pion wave
function and organized by the resummation, and the latter absorbed into a
jet function and organized by the threshold resummation. The NLO corrections
are found to be only few-percent for , if setting the
factorization scale to the momentum transfer from the virtual photon.Comment: 13 pages; version to appear in Physical Review
Periodic and Quasi-Periodic Compensation Strategies of Extreme Outages caused by Polarization Mode Dispersion and Amplifier Noise
Effect of birefringent disorder on the Bit Error Rate (BER) in an optical
fiber telecommunication system subject to amplifier noise may lead to extreme
outages, related to anomalously large values of BER. We analyze the Probability
Distribution Function (PDF) of BER for various strategies of Polarization Mode
Dispersion (PMD) compensation. A compensation method is proposed that is
capable of more efficient extreme outages suppression, which leads to
substantial improvement of the fiber system performance.Comment: 3 pages, 1 figure, Submitted to IEEE Photonics Letter
Excited charmonium mesons production in e^+e^- annihilation at \sqrt{s}=10.6 GeV
In this paper the production of excited vector and pseudoscalar charmonium
mesons in \epem annihilation is analyzed in the framework of light cone. In
particular the cross sections \epem \to \psi(2S)\eta_c(1S),
\psi(1S)\eta_c(2S),\psi(2S)\eta_c(2S) have been calculated. It is shown, that
contrary to NRQCD the cross sections calculated in the framework of light cone
agree with experimental data.Comment: 7 pages; 2 figures, some refrences adde
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