Electroproduction Of Vector Mesons Research Articles

UNIVERSALITY IN THE ELECTROPRODUCTION OF VECTOR MESONS

We study universality in the data of the electroproduction of vector mesons, using as guideline the results of a unified nonperturbative approach which reproduces well the available experimental data. After the extraction of factors that are specific of each vector meson, we arrive at a reduced integrated elastic cross-section which is universal. Our analysis suggests a finite infrared behavior for the strong coupling constant.

Exclusive meson production at HERMES

The exclusive electroproduction of pseudoscalar and vector mesons was studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The results are compared to calculations based on generalized parton distributions, some of which are sensitive to the contribution of the total angular momentum of the quarks to the proton spin.

Small x resummation in collinear factorisation

The summation of the small x-corrections to hard-scattering QCD amplitudes by collinear factorisation method is reconsidered and the K-factor is derived in leading ln x approximation with a result differing from the corresponding expression by Catani and Hautmann (Nucl. Phys. B 427, 475, 1994). The significance of the difference is demonstrated in the examples of structure function F_L and of exclusive vector meson electroproduction. The formulation covers the channels of non-vanishing conformal spin n paving the way for new applications.

Vector meson J/Ψ,ϕ electro-production in QCD

We study the vector meson electro-production off the proton in a QCD inspired model. A calculation of the differential cross section is performed for the J/Ψ,ϕ meson off the proton. The theoretical results are consistent with the experimental data, and remind us to consider the contribution from the tensor glueball and Odderon to the differential cross section. Since gluons interact among themselves via self-interaction, the gluons can form a glueball with quantum numbers IG, JPC = 0+, 2++, with a decay width Γt = 100 MeV and mass of mG = 2.23 GeV. The three gluons can form a three gluon color bound state with charge conjugation quantum number C = −1. This study is quite important to verify the validity of QCD and to search for new particles (tensor glueball and Odderon) as well as quest for new physics.

An attempt to understand exclusive π + electroproduction

Hard exclusive pi+ electroproduction is investigated within the handbag approach. The prominent role of the pion-pole contribution is demonstrated. It is also shown that the experimental data require a twist-3 effect which ensues from the helicity-flip generalized parton distribution H_T and the twist-3 pion wave function. The results calculated from this handbag approach are compared in detail with the experimental data on cross sections and spin asymmetries measured with a polarized target. It is also commented on consequences of this approach for exclusive \pi^0 and vector-meson electroproduction.

The target asymmetry in hard vector-meson electroproduction and parton angular momenta

The target asymmetry for electroproduction of vector mesons is investigated within the handbag approach. While the generalized parton distribution (GPD) H is taken from a previous analysis of the electroproduction cross section, we here construct the GPD E from double distributions and constrain it by the Pauli form factors of the nucleon, positivity bounds and sum rules. Predictions for the target asymmetry are given for various vector mesons and it is discussed how experimental data on the asymmetry will further constrain E and what we may learn about the angular momenta the partons carry.

Nonlinearity of vacuum Reggeons and exclusive diffractive production of vector mesons at HERA

The processes of exclusive photo- and electroproduction of vector mesons ${\ensuremath{\rho}}^{0}(770)$, $\ensuremath{\phi}(1020)$, and $J/\ensuremath{\psi}(3096)$ at collision energies $30\text{ }\text{ }\mathrm{GeV}<W<300\text{ }\text{ }\mathrm{GeV}$ and transferred momenta squared $0<\ensuremath{-}t<2\text{ }\text{ }{\mathrm{GeV}}^{2}$ are considered in the framework of a phenomenological Regge-eikonal scheme with nonlinear Regge trajectories in which their QCD asymptotic behavior is taken into account explicitly. By comparison of available experimental data from ZEUS and H1 Collaborations with the model predictions it is demonstrated that corresponding angular distributions and integrated cross sections in the above-mentioned kinematical range can be quantitatively described with use of two $C$-even vacuum Regge trajectories. These are the ``soft'' Pomeron dominating the high-energy reactions without a hard scale and the ``hard'' Pomeron giving an essential contribution to photo- and electroproduction of heavy vector mesons and deeply virtual electroproduction of light vector mesons.

Diffractive vector meson electroproduction at small Bjorken x within GPD approach

We study light vector meson electroproduction at small x within the generalized parton distributions (GPDs) model. The modified perturbative approach is used, where the quark transverse degrees of freedom in the vector meson wave function and hard subprocess are considered. Our results on the cross section and spin observables are in good agreement with experiment.

The role of the quark and gluon GPDs in hard vector-meson electroproduction

Electroproduction of light vector mesons is analyzed on the basis of handbag factorization. The required generalized parton distributions are constructed from the CTEQ6 parton distributions with the help of double distributions. The partonic subprocesses are calculated within the modified perturbative approach. The present work extends our previous analysis of the longitudinal cross section to the transverse one and other observables related to both the corresponding amplitudes. Our results are compared to recent experimental findings in detail.

Color transparency in electroproduction of the ρ meson at low energies

The study of color transparency (CT) in elastic electroproduction of vector mesons off nuclei encounters the problem of the onset of coherence length (CL) effects. The problem of CT-CL separation arises especially at medium energies, corresponding to the HERMES experiment, when the coherence length is of the order of the nuclear radius RA . Only at asymptotic high energies, corresponding to large CL, l c >> R A , can the CT-CL mixing be eliminated. On the other hand, the net CT effects can be studied in the kinematic range accessible by the experiment at the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility, since in that case the CL is much smaller than the nuclear radius. Using light-cone QCD dipole formalism, we investigate manifestations of CT effects in the electroproduction of vector mesons. Motivated by expected data from the CLAS experiment, we predict the A and Q 2 dependence of nuclear transparency for p° mesons produced incoherently off nuclei. We demonstrate that in the CLAS kinematic region, the CL effects are weak enough to keep the photon energy at such values as to obtain maximal photon virtualities keeping optimal statistics of the data. This has a clear advantage over a standard investigation of net CT effects fixing CL.

Roles of wave functions in the electroproduction of vector mesons

Elastic vector meson electroproduction is calculated through integrals of the overlap product of photon and vector meson wave functions, multiplied by the amplitude for the scattering of $q \bar{q}$ dipole pairs off the proton. In this nonperturbative QCD calculation, for sizes of the overlap functions that are smaller than the typical ranges of the interaction of the dipoles with the proton, the amplitudes factorize, with overlap strengths (integration extended over the light front coordinates describing the $q \bar{q}$ dipoles) containing all Q² dependence of the observables. This factorization is important in the description of the experimental data for all S-wave vector mesons.

Nonperturbative and perturbative aspects of photo- and electroproduction of vector mesons

We discuss various aspects of vector meson production, first analysing the interplay between perturbative and nonperturbative aspects of the QCD calculation. Using a general method adapted to incorporate both perturbative and nonperturbative aspects, we show that nonperturbative effects are important for all experimentally available values of the photon virtuality Q2. We compare the huge amount of experimental information now available with our theoretical results obtained using a specific nonperturbative model without free parameters, showing that quite simple features are able to explain the data.

The longitudinal cross section of vector meson electroproduction

We analyze electroproduction of light vector mesons (V=ρ,φ and ω) at small Bjorken-x in the handbag approach, in which the process factorizes into general parton distributions and partonic subprocesses. The latter are calculated in the modified perturbative approach in which the transverse momenta of the quark and antiquark forming the vector meson are retained and Sudakov suppression is taken into account. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal cross sections at large photon virtualities. The results are in fair agreement with the findings of recent experiments performed at HERA and HERMES.

Elastic photo and electroproduction of vector mesons

A unified treatment of elastic ρ , ω , ϕ , ψ and ϒ photo and electroproduction, in a non-perturbative QCD framework, without free specific parameters is presented. Some general predictions of the theoretical calculations are given, and comparison with experimental data is made in several cases.

Erratum to The total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson production

Based on the two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low x ~= Q^2/W^2 <<1, we stress the intimate connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply-virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA.

Vector-meson electroproduction at small Bjorken-x and generalized parton distributions

We analyze electroproduction of light vector mesons ( $V = \rho, \phi$ ) at small Bjorken-x in an approach that includes the gluonic generalized parton distributions and a partonic subprocess, $\gamma g \to (q\bar{q}) g$ , $q\bar{q}\to V$ . The subprocess is calculated to lowest order of perturbative QCD taking into account the transverse momenta of the quark and antiquark as well as Sudakov suppressions. Our approach allows us to investigate the transition amplitudes for all kinds of polarized virtual photons and polarized vector mesons. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal and transverse cross sections at large photon virtualities as well as the spin-density matrix elements for the vector mesons. Our results are in fair agreement with the findings of recent experiments performed at HERA.

Exclusive electro-production of pions and vector mesons at HERMES

A review is given of measurements of hard exclusive π+ production on a hydrogen target, as well as of the production of exclusive π+π− pairs and light vector mesons on hydrogen and deuterium targets at HERMES using the 27.6 GeV HERA positron beam. First preliminary results for the unpolarized total π+ production cross section in the kinematic range 1.5<Q2<10.5 GeV2 and for 0.02<x<0.8 are presented and compared to GPD calculations. Furthermore, the first Legendre moment of the angular distributions of π+ mesons in the CM frame of the exclusively produced pion pairs is presented. Its dependence on the π+π− invariant mass can be understood as being due to the interference between relative P-wave (isovector) and S, D-wave (isoscalar) states of the two pions. From the exclusive light vector meson production preliminary results on the ratio of the longitudinal to transverse components of the ρ and ϕ production cross section, and the longitudinal component of the corresponding cross section are reported.

Factorization of the [formula omitted] dependence in electroproduction of vector mesons

In a framework for non-perturbative QCD calculation of high energy processes, the amplitudes for photo- and electroproduction of vector mesons are written as integrals of the overlap product of photon and vector meson wave functions, multiplied by the amplitude for the scattering of q q ¯ dipole pairs off the proton. For sizes of the overlap functions that are smaller than the typical ranges of the interaction of the dipoles with the proton, the amplitudes factorize in the form of integrals, here called overlap strengths, formed only with the overlap of the wave functions times the square of the dipole separation. The integration is extended over the light front coordinates describing the q q ¯ dipoles. With this result, the overlap strengths contain all Q 2 dependence of the observables. We show the importance of this factorization in the description of the experimental data for all S-wave vector mesons.

Space-time structure of hard-scattering processes

Recent studies of exclusive electroproduction of vector mesons at JLab make it possible for the first time to play with two independent hard scales: the virtuality Q^2 of the photon, which sets the observation scale, and the momentum transfer t to the hadronic system, which sets the interaction scale. They reinforce the description of hard scattering processes in terms of few effective degrees of freedom relevant to the Jlab-Hermes energy range.

Vector meson electroproduction at next-to-leading order

The process of a light neutral vector meson electroproduction is studied in the framework of QCD factorization in which the amplitude factorizes in a convolution of the nonperturbative meson distribution amplitude and the generalized parton densities with the perturbatively calculable hard-scattering amplitudes. We derive a complete set of hard-scattering amplitudes at next-to-leading order for the production of vector mesons, $V=\rho^0, \omega, \phi$.