Leptoproduction Of Vector Mesons Research Articles

Overview of recent HERMES results

An overview of more recent and important results from the HERMES experiment is presented in this paper. HERMES collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This allows for a series of unique measurements of observables sensitive to the multidimensional structure of the nucleon. Amongst them are the semi-inclusive deep-inelastic scattering measurements of azimuthal modulations sensitive to the leading-twist transverse momentum distributions and the distributions sensitive to convolutions of the twist-2 and twist-3 distribution and fragmentation functions. They all provide information on the three-momentum-dependent quark distributions. Knowledge about the quark distribution as a function of longitudinal momentum and transverse position in impact-parameter space can be accessed, e.g., through exclusive vector–meson leptoproduction. Particularly, the measurement of spin-density-matrix elements on an unpolarized hydrogen target and the measurement of azimuthal modulations on transversely polarized hydrogen target for the ω–meson production are described in this paper. The results of the measurement of ratios of helicity amplitudes for the ρ0 –meson leptoproduction on transversely polarized hydrogen target are also presented.

Role of pion pole in hard exlusive meson leptoproduction

We consider the pion pole contribution and transversity effects determined by the HT and ĒT Generalized Parton Distributions (GPDs) which are essential in hard pseudoscalar and vector meson leptoproduction. We investigate spin effects in the ω and ρ0 reactions. It is shown that the pion pole contribution is very important in the w production. Such effects in the ρ0 channel are much smaller. Our results on spin asymmetries and spin density matrix elements in these reactions were found to be in good agreement with HERMES data.

Transversity in exclusive vector-meson leptoproduction

The role of transversity or helicity-flip generalized parton distributions (GPDs) in leptoproduction of vector mesons is investigated within the framework of the handbag approach. The transversity GPDs in combination with twist-3 meson wave functions, occur in the amplitudes for transitions from a transversely polarized virtual photon to a longitudinal polarized vector meson. The importance of the transversity GPDs can be examined in some of the spin density matrix elements and in transverse target spin asymmetries. Using suitable parametrizations of both helicity-flip and non-flip GPDs, which are essentially taken from our previous papers, we estimate these observables and compare the results with available data.

Spin effects in diffractive Q $$\bar Q$$ production and compass experimentproduction and compass experiment

We calculate the double spin asymmetry of the diffractive Q $$\bar Q$$ and vector meson leptoproduction at COMPASS energies. We analyze dependences of the asymmetry on the structure of the Pomeron-proton coupling. It is shown that it is difficult to study the spin structure of the Pomeron coupling with the proton from the A ll asymmetry. The A lT asymmetry might be an appropriate object for this investigation.

Exclusive production of vector mesons ϱ, ϱ′ and ϱ″ by real and virtual photons

We present a non-perturbative QCD calculation of high-energy diffractive photo- and leptoproduction of vector mesons ρ, ρ′ and ρ″ on a nucleon. The initial photon splits up in a q q ̄ - dipole and transforms into a vector meson by scattering on the quark-diquark nucleon. The dipole-dipole scattering amplitude is provided by the non-perturbative model of the stochastic QCD vacuum, the wave functions result from considerations on the light-cone. We assume the physical ϱ′- and ϱ″-states to be mixed states of an active 2 S-excitation and a rest whose coupling to the photon is suppressed. We obtain good agreement with the experimental data and get an understanding of the markedly different spectrum in the π +π −-invariant mass for photoproduction and e + e −-annihilation.

Diffractive photo- and leptoproduction of vector mesons $\rho$, $\rho'$ and $\rho''$

We calculate diffractive photo- and leptoproduction of $\rho$ -, $\rho'$ - and $\rho''$ -mesons. The incoming photon dissociates into a $q\bar{q}$ -dipole which scatters on the nucleon and transforms into a vector meson state. The scattering amplitude is calculated in non-perturbative QCD with the model of the stochastic vacuum. Assuming that the physical $\rho'$ - and $\rho''$ -mesons are mixed states of an active 2S-excitation and some residual hybrid state which cannot be produced diffractively in lowest order QCD, we obtain good agreement with the data, especially the markedly different spectrum in the $\pi^+\pi^-$ -invariant mass for photoproduction and $e^+e^-$ -annihilation.

Diffractive photo- and leptoproduction of vector mesons

Diffractive photo- and leptoproduction of vector mesons

Spin effects in diffractive J/

We calculate the cross section and the $A_{ll}$ asymmetry of the diffractive vector meson leptoproduction for a simple model of the pomeron coupling with the proton. It is found that the sensitivity of the spin--dependent cross section of the diffractive $J/\Psi$ production to the pomeron coupling structure is rather weak. The conclusion is made that it will be difficult to study the structure of the pomeron coupling with the proton in future polarized diffractive experiments on the $J/\Psi$ production.

Coherent vector meson production from deuterons

In this paper we discuss coherent photo- and leptoproduction of vector mesons from deuterons at intermediate (virtual) photon energies, 3 ≤ν≤ 30 GeV. We consider the scattering from unpolarized and polarized targets as well as processes where the polarization of the recoil deuteron is measured. Our main motivation results from the need for a quantitative analysis of the space-time structure of photon-induced processes. In this respect we suggest several possibilities to explore the characteristic longitudinal interaction length in coherent vector meson production. Furthermore, we outline methods for an investigation of color coherence effects. Besides the presentation of benchmark values for the maximal possible color coherence effect in various kinematic regions, we illustrate the anticipated phenomena within the color diffusion model. Finally we recall that the determination of vector meson-nucleon cross sections is not a closed issue. Besides being not known to a satisfying accuracy, they can be used to determine the strength of the D-state in low mass vector mesons.

Sudakov Form Factors in Leptoproduction of Vector Mesons

It is shown that Sudakov form factors for a color dipole in a QCD-inspired model of leptoproduction of vector mesons reduce the value of the cross-section of the process by an order of magnitude. They suppress the large size quark–antiquark pairs and unequal sharing of energy among the components of the dipole. Some freedom in the choice of the model parameters is also discussed.

Coherent photo- and lepto-production of vector mesons from deuterium

We discuss the coherent photo- and lepto-production of vector mesons from deuterium at intermediate (virtual) photon energies, 3 GeV ≲ ν ≲ 30 GeV. These processes provide several options to explore the space-time evolution of small size quark-gluon configurations. Furthermore, we study the dependence of the production cross section on the energy and momentum transfer t due to variations of the finite longitudinal interaction length. Kinematic regions are determined where the production cross section is most sensitive to the final state interaction of the initially produced hadronic wave packet. For unpolarized deuteron targets this double scattering contribution can be investigated mainly at large values of the momentum transfer t. For polarized targets kinematic windows sensitive to double scattering are available also at moderate t. We suggest several methods for an investigation of color coherence effects at intermediate energies.

Behaviour of the forward peak in hard diffractive leptoproduction of vector mesons

The measured forward slope in elastic and inelastic leptoproduction of vector mesons differ by a substantial amount. In an attempt to describe this phenomenon we construct a two radii model for the target proton, and estimate the effective parameters of the hard Pomeron obtained from a pQCD dipole model with eikonal shadowing corrections (SC). We show that the SC reduce the intercept of the hard Pomeron and generate an effective shrinkage of the forward peak and a diffractive dip at | t| ≈ 1 GeV 2, which appears to affect the value of the experimentally measured slope.

Vector meson leptoproduction and nonperturbative gluon fluctuations in QCD

We present a nonperturbative QCD calculation of diffractive vector meson production in virtual photon nucleon scattering at high energy. We use the nonperturbative model of the stochastic QCD vacuum which yields linear confinement and makes specific predictions for the dependence of high-energy scattering cross sections on the hadron size. Using light cone wave functions of the photon and vector mesons, we calculate electroproduction cross sections for {rho}, {omega}, {phi}, and J/{psi}. We emphasize the behavior of specific observables such as the ratio of longitudinal to transverse production cross section and the t dependence of the differential cross section. {copyright} {ital 1997} {ital The American Physical Society}

Shadowing corrections in diffractive QCD leptoproduction of vector mesons

The formulae for shadowing corrections in deep inelastic leptoproduction of vector mesons are presented. These formulae are also applicable for photoproduction of vector mesons constituted of heavy quarks. Our results are conveniently presented by the definition of a damping factor giving the reduction of the calculated cross sections due to shadowing. Our calculated cross sections are compared with those obtained with no shadowing and with the available experimental data including the recent data from HERA. We have also investigated the importance of shadowing on the relationship between ∂F 2( Q 2, x)/ ∂ ln Q 2 and the cross section for virtual photoproduction of vector mesons. A discussion of shadowing corrections to the proton's gluon density is presented and numerical estimates are given.

Diffractive leptoproduction of vector mesons in QCD.

We demonstrate that the distinctive features of the forward differential cross section of diffractive leptoproduction of a vector meson can be legitimately calculated in perturbative QCD in terms of the light-cone $q \bar q$ wave function of the vector meson and the gluon distribution of the target. In particular, we calculate the $Q^2$ and nuclear dependence of the diffractive leptoproduction of vector mesons and estimate the cross section. The production of longitudinally polarized vector mesons by longitudinally polarized virtual photons is predicted to be the dominant component, yielding a cross section behaving as $Q^{-6}$. The nuclear dependence of the diffractive cross sections, which follows from a factorization theorem in perturbative QCD, provides important tests of color transparency as well as constraints on the shadowing of the gluon structure functions and the longitudinal structure functions of nuclei.

Production of vector mesons by muons in a hybrid bubble-chamber experiment

We present results on vector-meson leptoproduction in the SLAC fast-cycling 40-in. hydrogen bubble chamber exposed to 16-GeV/c muons. From 3644 inelastic events we obtained 495 events of ${\ensuremath{\mu}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}p$, of which approximately 160 are $\ensuremath{\rho}$ events with $W>2$ GeV and ${Q}^{2}>0.05$ ${\mathrm{GeV}}^{2}$. Analyzing these data in the same way as in our previous photoproduction experiment, we find the ${\ensuremath{\rho}}^{0}$ contribution to ${\ensuremath{\sigma}}_{\mathrm{tot}}$ decreases by about 35% between ${Q}^{2}=0 \mathrm{and} 1$ ${\mathrm{GeV}}^{2}$, and the exponential slope parameter becomes approximately 20% smaller, although within errors it is still consistent with the photoproduction value. The decay angular distribution of the ${\ensuremath{\rho}}^{0}$ shows a substantial ${cos}^{2}\ensuremath{\theta}$ component which can be interpreted as evidence for production by longitudinal photons assuming $s$-channel helicity is conserved. Data from 39 $\ensuremath{\omega}$-production events are also presented. Upper limits for $\ensuremath{\varphi}$ and ${\ensuremath{\rho}}^{\ensuremath{'}}$ production are given.