366 research outputs found
On the heavy quark mass expansion for the operator Qbar gamma_5 Q and the charm content of eta, eta'
Recently in the context of studies of the intrinsic charm content of the
nucleon and of the eta' meson two groups have arrived at different results for
the 1/m^3 term of the heavy quark expansion for operator
differing by the factor of six. We show that the form of both results violates
certain general conditions. Using the expression for the axial anomaly with the
finite Pauli-Villars regularization we obtain a new expression for 1/m^3 term
of the heavy quark expansion for . With this new result we
obtain an estimate for the constant f_{\eta'}^{(c)}=-2 MeV.Comment: 4 page
Dual parameterization of generalized parton distributions and description of DVCS data
We discuss a new leading-order parameterization of generalized parton
distributions of the proton, which is based on the idea of duality. In its
minimal version, the parameterization is defined by the usual quark singlet
parton distributions and the form factors of the energy-momentum tensor. We
demonstrate that our parameterization describes very well the absolute value,
the Q^2-dependence and the W-dependence of the HERA data on the total DVCS
cross section and contains no free parameters in that kinematics. The
parameterization suits especially well the low-x_{Bj} region, which allows us
to advocate it as a better alternative to the frequently used double
distribution parameterization of the GPDs.Comment: 13 pages, 2 figures, LaTeX. Revised version: equation for the DVCS
cross section corrected; one reference added; numerical results did not
chang
DVCS amplitude in the parton model
We compute amplitude of deeply virtual Compton scattering in the parton
model. We found that the amplitude up to the accuracy O(1/Q) depends on new
skewed parton distributions (SPD's). These additional contributions make the
DVCS amplitude explicitly transverse.Comment: New spin sum rules for twist-3 SPD's are added. Discussion of 1/Q
behaviour of twist-3 contributions is correcte
Helicity skewed quark distributions of the nucleon and chiral symmetry
We compute the helicity skewed quark distributions and
in the chiral quark-soliton model of the nucleon. This model
emphasizes correctly the role of spontaneously broken chiral symmetry in
structure of nucleon. It is based on the large-N_c picture of the nucleon as a
soliton of the effective chiral lagrangian and allows to calculate the leading
twist quark- and antiquark distributions at a low normalization point. We
discuss the role of chiral symmetry in the helicity skewed quark distributions
and . We show that generalization of soft pion
theorems, based on chiral Ward identities, leads in the region of -\xi < x <
\xi to the pion pole contribution to which dominates at small
momentum transfer.Comment: 22 pages, 5 figure
Acoustic-electrical testing of defects in the cement-sand and cement-glass model samples
A complex method of acoustic-electrical testing of defects in dielectric samples made from cement-sand and cement-glass mixtures is discussed. The paper reports the results of studies of changes in the parameters of electromagnetic responses and their spectra under pulsed deterministic acoustic excitation of model samples with defects in the form of solid-state inclusions. The results of mathematical calculations of the time variation in the stress-strain state induced in a defective dielectric model sample by deterministic acoustic pulse are presented. The relationship is shown between the parameters of the acoustic excitation and the electromagnetic response to the impact in a magnetic field. The study revealed that the specific electrical resistance of the cement-sand and cement-glass mixtures differs significantly. Excitation of electrical double layers by acoustic pulses causes an electromagnetic signal, parameters of which depend on the parameters of the acoustic impact and acoustic and electrical properties of the material. As a result, a reduced specific electrical resistance of the mixture increases its conductivity. The numerical calculation of the propagation of the deterministic acoustic pulse showed that its parameters change when it passes through a defect with acoustic impedance different from that of the mixture used
Generalized parton distributions and strong forces inside nucleons and nuclei
We argue that generalized parton distributions (GPDs), accessible in hard
exclusive processes, carry information about the spatial distribution of forces
experienced by quarks and gluons inside hadrons. This way the measurements of
hard exclusive processes open a possibility for direct "measurements" of strong
forces in different parts of nucleons and nuclei. Also such studies open a
venue for addressing questions of the properties of the quark (gluon) matter
inside hadrons and nuclei. We give a simple example of relations between GPDs
and properties of "nuclear matter" in finite nuclei
DVCS on the nucleon : study of the twist-3 effects
We estimate the size of the twist-3 effects on deeply virtual Compton
scattering (DVCS) observables, in the Wandzura-Wilczek approximation. We
present results in the valence region for the DVCS cross sections, charge
asymmetries and single spin asymmetries, to twist-3 accuracy.Comment: 20 pages, 6 figure
Hard exclusive electroproduction of two pions and their resonances
We study the hard exclusive production of two pions in the virtual photon
fragmentation region with various invariant masses including the resonance
region. The amplitude is expressed in terms of two-pion light cone distribution
amplitudes (DA's). We derive dispersion relations for these amplitudes,
which enables us to fix them completely in terms of scattering phases
and a few low-energy subtraction constants determind by the effective chiral
lagrangian. Quantitative estimates of the resonance as well background
DA's at low normalization point are made. We also prove certain new soft pion
theorem relating two-pion DA's to the one-pion DA. Crossing relations between
DA's and parton distributions in a pion are discussed.
We demontrate that by studying the shape of the mass spectra (not
absolute cross section!) in a diffractive electroproduction one can extract the
deviation of the meson ( etc.) wave functions from their asymptotic
form and hence to get important information about the structure of
mesons. Also we discuss how the moments of quark distributions in a pion can be
measured in the hard diffraction. We suggest (alternative to S\"oding's)
parametrization of spectra which is suitable for large photon
virtuality.Comment: 30 pages, one figure. Discussion of crossing relations between meson
distribution amplitudes and quark densities is adde
Effective chiral lagrangian in the chiral limit from the instanton vacuum
We study the effective chiral Lagrangian in the chiral limit from the
instanton vacuum. Starting from the nonlocal effective chiral action, we derive
the effective chiral Lagrangian, using the derivative expansion to order
in the chiral limit. The low energy constants, , , and
are determined and compared with various models and the corresponding empirical
data. The results are in a good agreement with the data. We also discuss about
the upper limit of the sigma meson, based on the present results.Comment: 14 pages, 5 figures, submitted to Phys.Rev.
Exploring skewed parton distributions with two body models on the light front II: covariant Bethe-Salpeter approach
We explore skewed parton distributions for two-body, light-front wave
functions. In order to access all kinematical regimes, we adopt a covariant
Bethe-Salpeter approach, which makes use of the underlying equation of motion
(here the Weinberg equation) and its Green's function. Such an approach allows
for the consistent treatment of the non-wave function vertex (but rules out the
case of phenomenological wave functions derived from ad hoc potentials). Our
investigation centers around checking internal consistency by demonstrating
time-reversal invariance and continuity between valence and non-valence
regimes. We derive our expressions by assuming the effective qq potential is
independent of the mass squared, and verify the sum rule in a non-relativistic
approximation in which the potential is energy independent. We consider
bare-coupling as well as interacting skewed parton distributions and develop
approximations for the Green's function which preserve the general properties
of these distributions. Lastly we apply our approach to time-like form factors
and find similar expressions for the related generalized distribution
amplitudes.Comment: 25 pages, 12 figures, revised (minor changes but essential to
consistency
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