420 research outputs found
Theory and phenomenology of helicity amplitudes for high energy exclusive leptoproduction of the rho-meson
We review here two approaches to describe hard leptoproduction of
transversally polarized rho-meson, based on recent calculation of the gamma*T
-> rhoT impact factor up to twist 3 accuracy in the collinear factorization
frame, including 2- and 3- particles Fock-states. The first approach uses a
model of the unintegrated gluon density (the proton impact factor) which allows
a comparison of our predictions with H1 and ZEUS data for the ratios of
helicity amplitudes T(gamma*T -> rhoT)/T(gamma*L -> rhoL) and T(gamma*T ->
rhoL)/T(gamma*L -> rhoL). In the second approach, we transform the gamma*T ->
rhoT impact factor into the impact parameter space. We show that the
transformed amplitude factorizes according to conventional dipole picture. We
shortly discuss a way to implement the nucleon saturation effects in our
approach.Comment: 5 pages, 2 figures, to appear in the proceedings of 6th International
Conference on Quarks and Nuclear Physics (QNP 2012), Palaiseau, France, April
16-20, 201
High Energy exclusive Leptoproduction of the rho-meson: Theory and Phenomenology
We describe the hard leptoproduction of transversally polarized rho-meson, up
to twist 3 accuracy, including 2- and 3- particles Fock-states, in the HERA
kinematics of high center-of-mass energy. We first build a model based on a
simple approach to the unintegrated gluon density (the parton impact factor)
that we compare with H1 and ZEUS data for the ratios of helicity amplitudes
T(gamma*T -> rhoT)/T(gamma*L -> rhoL) and T(gamma*T -> rhoL)/T(gamma*L -> rhoL)
and get a good description of the data. We also show how saturation effects can
be included in this model by extending the dipole representation of the
scattering amplitude in coordinate space up to twist 3.Comment: 4 pages, 3 figures, to appear in the proceedings of 20th
International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS
2012), Bonn, Germany, 26-30 March 201
A phenomenological study of helicity amplitudes of high energy exclusive leptoproduction of the rho meson
We apply a previously developed scheme to consistently include the twist-3
distribution amplitudes for transversely polarized rho meson in order to
evaluate, in the framework of kT factorization, the helicity amplitudes for
exclusive leptoproduction of a light vector meson, at leading order in alphaS.
We compare our results with high energy experimental data for the ratios of
helicity amplitudes T11/T00 and T01/T00 and get a good description of the data.Comment: 20 pages, 11 figures, published version in PR
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
Tagging the pion quark structure in QCD
We combine the constraints on the pion quark structure available from
perturbative QCD, nonperturbative QCD (nonlocal QCD sum rules and light cone
sum rules) with the analysis of current data on F_{\pi\gamma\gamma^*}(Q^2),
including recent high-precision lattice calculations of the second moment of
the pion's distribution amplitude. We supplement these constraints with those
extracted from the renormalon approach by means of the twist-four contributions
to the pion distribution amplitude in order to further increase stability with
respect to related theoretical uncertainties. We show which regions in the
space of the first two non-trivial Gegenbauer coefficients a_2 and a_4 of all
these constraints overlap, tagging this way the pion structure to the highest
degree possible at present.Comment: V1: 6 pages, 2 figures, 1 table. V2: Two references added with
corresponding insertions in the text. Matches version published in PR
Methods For Evaluation Of The Education Quality In Higher Education Institutions
The article aims to disclose the essence of the “education quality” concept, to study the international approaches and concepts of the higher education quality evaluation, to analyze specific methods and tools within one of the approaches. Establishing education quality assessment necessitates choosing the teaching personnel of higher educational institution, as well as a target group of students and independent experts in the field of organization of the educational process, on a distance basis particularly. Monitoring requires using such methods as interviews, surveys, questionnaires, and similar. The results were processed according to the authors’ algorithm allowing to calculate the quality of education. The authors’ interpretation of the concept of “education quality” is proposed. Given the insufficient study of the topic of distance education in the domestic scientific discourse, authors’ attention to this problem is an important contribution to study of the issue of distance education. The methods used to evaluate the education quality in higher education institutions at the international level are an integral element in the development of the educational space, although they cause certain controversy and criticism. The authors advise to select the methods for evaluating the educational process quality with account of the specific evaluation stage and purpose
Nonlinear Dynamics of Asynchronous Electric Drive: Engineering Interpretation and Correction Techniques
The results of theoretical and practical research studies most widely used in the industry of variable frequency drives (VFD) are presented in this manuscript. Such objects are characterized by dynamic nonlinearities that are difficult to take into account in the mathematical description for the development of control algorithms. Accounting for these nonlinearities leads to equations that are very problematic to solve. Therefore, the equations of the mathematical model on which the vector control system is based are compiled with the assumption of the sinusoidality of the processes occurring in the control object. Comparative results of the analysis of dynamic of VFD with two types of sensorless control, vector and scalar, show the problems that these assumptions lead to.. For identification of nonlinearities, dynamic formulas of transfer functions of torque generator in VFD are proposed, taking into account slip and stator voltage frequency The nonlinear transfer functions obtained in this work made it possible to substantiate structural solutions that linearize the VFD and substantially increase their efficiency. The use of dynamic feedback on the stator current allowed to significantly increase the dynamics and efficiency of a more stable scalar control
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
Singlet VA \tilde V correlator within the instanton vacuum model
The correlator of singlet axial-vector and vector currents in the external
electromagnetic field is studied within the instanton liquid model of QCD
vacuum. In the chiral limit we calculate the longitudinal w_L^0 and transversal
w_T^0 with respect to axial-vector index invariant amplitudes at arbitrary
momentum transfer q. It is demonstrated how the anomalous longitudinal part of
the correlator is renormalized at low momenta due to the presence of the U_A(1)
anomaly.Comment: 9 pages, 4 figure
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