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L. Mankiewicz

Publications and source records attributed to L. Mankiewicz.

At least 37 records · Page 2Linked to original sources
arXiv1999

NLO corrections and contribution of a tensor gluon operator to the process $γ^*γ\to ππ$

We have calculated the NLO corrections for the leading twist amplitude of the reaction $γ^*γ\toππ$ at large virtuality of the photon and at $W^2\ll Q^2$. With a simple model for the two-pion quark and gluon distribution amplitudes we have estimated that the NLO effects may reduce the leading-order amplitude by as much as 30%. We have found that besides the usual twist-2 quark and gluon operators the NLO amplitude receives contribution from a distribution associated with twist-2 tensor gluon operator. The new gluon distribution can be investigated by studying the azimuthal asymmetry of the cross section for $e+γ\to e'+ππ$.

hep-ph↗
arXiv1999

On scale dependence of QCD string operators

We have obtained a general solution of evolution equations for QCD twist-2 string operators in form of expansion over complete set of orthogonal eigenfunctions of evolution kernels in coordinate-space representation. In the leading logarithmic approximation the eigenfunctions can be determined using constraints imposed by conformal symmetry. Explicit formulae for the LO scale-dependence of quark and gluon twist-2 string operators are given.

hep-ph↗
arXiv1999

Comments on Exclusive Electroproduction of Transversely Polarized Vector Mesons

We discuss the electroproduction of light vector mesons from transversely polarized photons. Here QCD factorization cannot be applied as shown explicitly in a leading order calculation of corresponding Feynman diagrams. It is emphasized that present infrared singular contributions cannot be regularized through phenomenological meson distribution amplitudes with suppressed endpoint configurations. We point out that infrared divergencies arise also from integrals over skewed parton distributions of the nucleons. In a phenomenological analysis of transverse vector meson production model dependent regularizations have to be applied. If this procedure preserves the analytic structure suggested by a leading order calculation of Feynman diagrams, one obtains contributions from nucleon parton distributions and their derivatives. In particular polarized gluons enter only through their derivative.

hep-ph↗
arXiv1999

Conformal string operators and evolution of skewed parton distributions

We have investigated skewed parton distributions in coordinate space. We found that their evolution can be described in a simple manner in terms of non-local, conformal operators introduced by Balitsky and Braun. The resulting formula is given by a Neumann series expansion. Its structure resembles, for all values of the asymmetry parameter, the well-known solution of the ERBL equation in the momentum space. Performing Fourier transformation we have reproduced known results for evolution of momentum-space distributions.

hep-ph↗
arXiv1998

Hard Exclusive Electroproduction of Pions

We investigate the exclusive electroproduction of pi^+ mesons from nucleons. To leading twist, leading order alpha_s accuracy the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at x < 0.1. On the other hand, for 0.1 < x < 0.4 contributions from one pion exchange dominate.

hep-ph↗
arXiv1998

Exclusive $J/ψ$ photoproduction and gluon polarization

In exclusive $J/ψ$ production by polarized photons incident on polarized protons, a finite polarization asymmetry arises because of $c\bar c$ Fermi-motion and binding-energy effects. The asymmetry depends on the polarized nonforward gluon distribution of the proton and thus gives information on gluon polarization in the proton. The analyzing power, however, is rather small.

hep-ph↗
arXiv1998

Nuclear Quark and Gluon Distributions in Coordinate Space

In coordinate space, quark and gluon distributions of the nucleon are defined as correlation functions involving two field operators separated by a light-cone distance $y^+ = 2l$. We study the nuclear modifications of these distributions. The largest effect is a strong depletion of parton distributions (shadowing) at large longitudinal distances, which starts for all parton species at $l=2$ fm, i.e. at the average nucleon-nucleon separation in nuclei. On the other hand, the nuclear radius does not play a significant role. At $l \lsim 1$ fm, nuclear modifications of parton distributions are small. The intrinsic structure of individual nucleons is evidently not very much affected by nuclear binding. In particular, there is no evidence for a significant increase of the quark or gluon correlation length in bound nucleons.

hep-ph↗
arXiv1998

Hard exclusive $J/ψ$ leptoproduction on polarized targets

We consider the exclusive production of $J/ψ$ mesons in polarized virtual-photon--proton collisions. We derive helicity amplitudes for the dominant subprocess $γ^* g \to J/ψg$ and show that polarization asymmetries vanish in the case of collinear scattering. Thus, contrary to what has been suggested earlier, this process is not a good probe of the polarized gluon distribution $Δg(x,Q^2)$ of the proton.

hep-ph↗
arXiv1997

Hard Exclusive Meson Production and Nonforward Parton Distributions

We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of mesons in terms of double/nonforward parton distribution functions. After reviewing some general features of nonforward nucleon matrix elements of twist-2 QCD string operators, we propose a phenomenological model for quark and gluon nonforward distribution functions. The corresponding QCD evolution equations are solved in the leading logarithmic approximation for flavor nonsinglet distributions. We derive explicit expressions for hard exclusive pi^0, eta, and neutral vector meson production amplitudes and discuss general features of the corresponding cross sections.

hep-ph↗
arXiv1997

Hard Leptoproduction of Charged Vector Mesons

We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of charged vector mesons. These processes are determined by nonforward parton distribution functions which are nondiagonal in quark flavor. We derive relations between flavor diagonal and nondiagonal distribution functions based on isospin symmetry. Furthermore, we discuss general features of rho^+ and rho^- production cross sections, and present estimates based on a simple model for nonforward distribution functions.

hep-ph↗
arXiv1997

Gluon Polarization in Nucleons

In QCD the gauge-invariant gluon polarization Delta G in a nucleon can be defined either in a non-local way as the integral over the Ioffe-time distribution of polarized gluons, or in light-cone gauge as the forward matrix element of the local topological current. We have investigated both possibilities within the framework of QCD sum rules. Although the topological current is built from local fields, we have found that its matrix element retains sensitivity to large longitudinal distances. Because QCD sum rules produce artificial oscillations of the Ioffe-time distribution of polarized glue at moderate and large light-like distances, the calculation of the matrix element of the topological current results in a small value of Delta G(mu^2 = 1 GeV^2) = 0.6 \pm 0.2. In a more consistent approach QCD sum rules are used to describe the polarized gluon distribution only at small light-like distances. Assuming that significant contributions to Delta G arise only from longitudinal length scales not larger than the nucleon size leads to Delta G(mu^2 = 1 GeV^2) = 2 \pm 1.

hep-ph↗
arXiv1997

A lattice determination of the second moment of the polarised valence quark distribution

We perform a Monte Carlo calculation of the second moment of the polarised valence quark distributions in the nucleon, using quenched Wilson fermions. The special feature of this moment is that it is directly accessible experimentally. At a scale of $μ\approx 2$GeV we find $Δ^{(1)}u = 0.198(8)$, $Δ^{(1)}d = -0.0477(33)$. We compare these numbers with recent experimental results of the SMC collaboration.

hep-ph↗
arXiv1997

Analytic Approximation to the GLAP evolution of $F_2(x,Q^2)$ in the small-$x$ region

We propose a systematic approximation scheme for solving GLAP evolution equations at small Bjorken-$x$. The approximate solutions interpolate smoothly between hard (singular as $x^{-(1+λ)}$, $λ> 0$) and soft (singular at most as $x^{-1}$) initial conditions and may be applied in a wide range of $Q^2$. The small-$x$ behavior of $F_2^p(x,Q^2)$ which is extracted from a fit to HERA data agrees with results from a global fit.

hep-ph↗
arXiv1997

Perturbative Part of the Non-Singlet Structure Function F_2 in the Large-N_F Limit

We have calculated $\bar{MS}$ Wilson coefficients and anomalous dimensions for the non-singlet part of the structure function F_2 in the large-N_F limit. Our result agrees with exact two and three loop calculations and gives the leading N_F dependence of the perturbative non-singlet Wilson coefficients to all orders in $α_S$.

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