Generalized Vector Meson Dominance Research Articles

Upsilon Photo-Production off Proton in QCD Inspired Model

Based on the generalized vector meson dominance model in QCD, we study photoproduction of vector meson γ off the proton by use of the QCD inspired model in which the contributions from quark-quark, gluon-gluon, and quark-gluon interference term to observable are taken into consideration. Calculations are performed for total cross section σtot, differential cross section dσ/dt, ratio of the real part to imaginary part of forward scattering amplitude ρ, and nuclear slop parameter function β. The mediators of interactions between projectiles (the quark and antiquark pair fluctuated from the real the photon) and the proton target (three-quark system) are the tensor Glueball and Odderon instead of using the usual Pomeron exchange. The theoretical predictions for σtot (s) are consistent with the experimental data within error bars of the data. The data for dσ/dt, ρ, and β are urgently needed.

Diffractive upsilon photo-production off the proton in the QCD inspired model

Based on the generalized vector meson dominance model in QCD, we study the photoproduction of vector meson ϒ off the proton by use of the QCD-inspired model in which the contributions from quark-quark, gluon-gluon and quark-gluon interference term to observable are taken into consideration. Calculations are performed for total cross section σ tot, differential cross section dσ/dt, ratio of the real part to imaginary part of forward scattering amplitude ρ and nuclear slop parameter function β. We analyze the individual contributions from quark gluon degrees of freedom and the QCD Odderon to the total cross section σ tot (s), differential cross section dσ/dt, ratio of the real part to imaginary part of the forward scattering amplitude ρ, and nuclear slop parameter function β. The mediators of interactions between projectiles (the quark and antiquark pair fluctuated from the real photon) and the proton target (three quark system) are the tensor Glueball and Odderon instead of using the usual Pomeron exchange. The theoretical predictions for σ tot(s) are consistent with the experimental data within error bars of the data. The data for dσ/dt, β and ρ are urgently needed. The gluon-gluon interaction makes a significant contribution to the observables while the Odderon contribution is negligibly small. Therefore, we may conclude that it is impossible to find the QCD Odderon in the γ+p → ϒ+p process as suggested before.

Deeply Virtual Compton Scattering on nucleons and nuclei in the Generalized Vector Meson Dominance model

We consider Deeply Virtual Compton Scattering (DVCS) on nucleons and nuclei in the framework of the Generalized Vector Meson Dominance (GVMD) model. We demonstrate that the GVMD model provides a good description of the HERA data on the dependence of the proton DVCS cross-section on Q2, W (at Q 2 = 4 GeV^2) and t . At Q 2 = 8 GeV^2, the soft W -behavior of the GVMD model somewhat underestimates the W -dependence of the DVCS cross-section due to the hard contribution not present in the GVMD model. We estimate 1/Q 2 power-suppressed corrections to the DVCS amplitude and the DVCS cross-section and find them large. We also make predictions for the nuclear DVCS amplitude and cross-section in the kinematics of the future Electron-Ion Collider. We predict significant nuclear shadowing, which matches well predictions of the leading-twist nuclear shadowing in DIS on nuclei.

Charm production in antiproton–nucleus collisions at the [formula omitted] and the [formula omitted] thresholds

We discuss the production of charmonium states in antiproton–nucleus collisions at the ψ′ threshold. It is explained that measurements in p¯A collisions will allow to get new information about the strengths of the inelastic J/ψN and ψ′N interaction, on the production of Λc and D¯ in charmonium–nucleon interactions and for the first time about the nondiagonal transitions ψ′N→J/ψN. The inelastic J/ψ-nucleon cross section is extracted from the comparison of hadron–nucleus collisions with hadron–nucleon collisions. We extract the total J/ψ-nucleon cross section from photon–nucleon collisions by accounting for the color transparency phenomenon within the frame of the GVDM (generalized vector meson dominance model). We evaluate within the GVDM the inelastic ψ′-nucleon cross section as well as the cross section for the nondiagonal transitions. Predictions for the ratio of J/ψ to ψ′ yields in antiproton–nucleus scatterings close to the threshold of ψ′ production for different nuclear targets are presented.

Interpretation of electron–proton scattering at low [formula omitted

Low-Q2 photons do not resolve partons in the proton, which gives problems when applying the deep inelastic scattering formalism, such as an unphysical, negative gluon density extracted from data. Considering instead hadronic fluctuations of the photon, we show that the generalised vector meson dominance model (GVDM) gives a good description of the measured cross section at low Q2, i.e., reproduces F2(x,Q2), using only few parameters with essentially known values. Combining GVDM and parton density functions makes it possible to obtain a good description of F2 data over the whole range of x and Q2.

Unified many-body approach to mesons, Glueballs and hybrids

A unified quark and gluon description of mesons, glueballs and exotic hybrids is presented for a relativistic field theoretical Hamiltonian in the Coulomb gauge. The effective QCD Hamiltonian entails a linear confining interaction with slope predetermined by lattice theory and is approximately diagonalized using different many-body techniques: BCS for the vacuum ground state; TDA and RPA for the excited hadron spectrum. Chiral symmetry rigorously emerges and is spontaneously broken by the model BCS vacuum which contains dynamically generated constituent quark and gluon condensates described by respective gap equations. The observed low energy meson spectrum and the quenched lattice glueball measurements are reproduced along with a Regge trajectory consistent with the Pomeron. Using generalized vector meson dominance, glueball decays are predicted and a glueball photoproduction experimental signature is predicted. Finally, a variational relativistic three quasiparticle calculation for hybrid mesons is reported which concurs with alternative model predictions that the lightest hybrid states are near but above 2 GeV. This strongly indicates that the recently observed J PC = 1 −+ exotics at 1.4 and 1.6 GeV are more likely four quark states.

Spin structure function g 1 (x, Q) and the DHGHY integral I(Q) at low Q: predictions from the GVMD model

Theoretical predictions for polarized nucleon structure function $g_1(x,Q^2)$ at low $Q^2$ are obtained in the framework of the Generalized Vector Meson Dominance model. Contributions from both light and heavy vector mesons are evaluated. In the photoproduction limit the first moment of $g_1$ is related to the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto sum rule. This property is employed to fix the magnitude of the light vector meson contribution to $g_1$, using the recent measurements in the region of baryonic resonances. Results are compared to the data on $g_1(x,Q^2)$. Finally, the DHGHY moment function $I(Q^2)$ is calculated, and our theoretical predictions are confronted with the recent preliminary data obtained at the Jefferson Laboratory.

Study of the processe+e−→π+π−π0in the energy regionsfrom 0.98 to 1.38 GeV

The cross section of the process $e^+e^-\to \pi^+\pi^-\pi^0$ was measured in the Spherical Neutral Detector experiment at the VEPP-2M collider in the energy region $\sqrt[]{s} = 980 \div 1380$ MeV. The measured cross section, together with the $e^+e^-\to \pi^+\pi^-\pi^0$ and $\omega\pi^+\pi^-$ cross sections obtained in other experiments, was analyzed in the framework of the generalized vector meson dominance model. It was found that the experimental data can be described by a sum of $\omega$, $\phi$ mesons and two $\omega^\prime$ and $\omega^{\prime\prime}$ resonances contributions, with masses $m_{\omega^\prime}\sim 1490$,$m_{\omega^{\prime\prime}}\sim 1790$ MeV and widths $\Gamma_{\omega^\prime}\sim 1210$, $\Gamma_{\omega^{\prime\prime}}\sim 560$ MeV. The analysis of the $\pi^+\pi^-$ invariant mass spectra in the energy region $\sqrt[]{s}$ from 1100 to 1380 MeV has shown that for their descriptionone should take into account the $e^+e^-\to\omega\pi^0\to\pi^+\pi^-\pi^0$ mechanism also. The phase between the amplitudes corresponding to the $e^+e^-\to\omega\pi$ and $e^+e^-\to\rho\pi$ intermediate states was measured for the first time. The value of the phase is close to zero and depends on energy.

Double-spin asymmetry in the cross section for exclusive ρ0 production in lepton–proton scattering

Evidence for a positive longitudinal double-spin asymmetry <A_1^rho> = 0.24 +-0.11 (stat) +-0.02 (syst) in the cross section for exclusive diffractive rho^0(770) vector meson production in polarised lepton-proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon-proton system has a value of <W> = 4.9 GeV, while the average negative squared four-momentum of the virtual photon is <Q^2> = 1.7 GeV^2. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector meson dominance model.

ZEUS results on the measurement and phenomenology of $F_2$ at low x and low $Q^2$

Measurements of the proton structure function $F_2$ for $0.6 < Q^2 < 17 {GeV}^2$ and $1.2 \times 10^{-5} < x <1.9 \times 10^{-3}$ from ZEUS 1995 shifted vertex data are presented. From ZEUS $F_2$ data the slopes $dF_2/d\ln Q^2$ at fixed $x$ and $d\ln F_2/d\ln(1/x)$ for $x < 0.01$ at fixed $Q^2$ are derived. For the latter E665 data are also used. The transition region in $Q^2$ is explored using the simplest non-perturbative models and NLO QCD. The data at very low $Q^2$ $\leq 0.65 {GeV}^2$ are described successfully by a combination of generalised vector meson dominance and Regge theory. From a NLO QCD fit to ZEUS data the gluon density in the proton is extracted in the range $3\times 10^{-5} < x < 0.7$. Data from NMC and BCDMS constrain the fit at large $x$. Assuming the NLO QCD description to be valid down to $Q^2\sim 1 {GeV}^2$, it is found that the $q\bar{q}$ sea distribution is still rising at small $x$ and the lowest $Q^2$ values whereas the gluon distribution is strongly suppressed.

Gluon versus sea quark shadowing

We calculate the shadowing of sea quarks and gluons and show that the shadowing of gluons is not simply given by the sea quark shadowing, especially at small x. The calculations are done in the lab frame approach by using the generalized vector meson dominance model. Here the virtual photon turns into a hadronic fluctuation long before the nucleus. The subsequent coherent interaction with more than one nucleon in the nucleus leads to the depletion σ(γ ∗A)<Aσ(γ ∗N) known as shadowing. A comparison of the shadowing of quarks to E665 data for 40 Ca and 207 Pb shows good agreement.

Cross section fluctuations of a photon projectile in the generalized vector meson dominance model

We explain that the generalized vector meson dominance (GVMD) models successful in the theoretical description of photoproduction and electroproduction in high energy processes effectively implement color coherent phenomena expected in QCD. Within a model, which reproduces Bjorken scaling and nuclear shadowing for deep inelastic processes, we calculate ${P}_{\ensuremath{\gamma}}(\ensuremath{\sigma}{,Q}^{2}),$ the probability for a photon to interact with a target with the cross section \ensuremath{\sigma}. We find that within the GVMD model the virtual photon has two main components: the ``hard'' one with a small cross section of the order of ${1/Q}^{2}$ and the high probability and the ``soft'' one with a typical hadronic cross section with the probability suppressed by the factor of ${1/Q}^{2}.$ Thus the GVDM model produces ${P}_{\ensuremath{\gamma}}(\ensuremath{\sigma}{,Q}^{2})$ similar to that within the parton model and suggests the interplay between hard and soft QCD at ${Q}^{2}&gt;1{\mathrm{GeV}}^{2}.$ We study the limit $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\sigma}}0$ and show that ${P}_{\ensuremath{\gamma}}(\ensuremath{\sigma}{,Q}^{2})\ensuremath{\propto}1/\ensuremath{\sigma}$ in both the GVMD model and QCD. Thus similarly to the parton model the GVMD model predicts the color transparency phenomenon. This is an indirect indication that Bjorken scaling, color transparency and shadowing in nuclei in DIS are closely related.

ZEUS results on the measurement and phenomenology of

Measurements of the proton structure function \(F_2\) for \(0.6< Q^2 < 17 {\rm GeV}^2\) and \(1.2\times 10^{-5} < x < 1.9\times 10^{-3}\) from ZEUS 1995 shifted vertex data are presented. From ZEUS \(F_2\) data the slopes \(dF_2/d\ln Q^2\) at fixed x and \(d\ln F_2/d\ln(1/x)\) for \(x < 0.01\) at fixed \(Q^2\) are derived. For the latter, E665 data are also used. The transition region in \(Q^2\) is explored using the simplest non-perturbative models and NLO QCD. The data at very low \(Q^2 \leq 0.65{\rm GeV}^2\) are described successfully by a combination of generalised vector meson dominance and Regge theory. From a NLO QCD fit to ZEUS data the gluon density in the proton is extracted in the range \(3\times 10^{-5} < x < 0.7\). Data from NMC and BCDMS constrain the fit at large x. Assuming the NLO QCD description to be valid down to \(Q^2\sim 1{\rm GeV}^2\), it is found that the \(q\bar{q}\) sea distribution is still rising at small x and the lowest \(Q^2\) values whereas the gluon distribution is strongly suppressed.

Phenomenology of nuclear shadowing in deep-inelastic scattering

We investigate shadowing effects in deep-inelastic scattering from nuclei at small valuesx < 0.1 of the Bjorken variable. Unifying aspects of generalized vector meson dominance and color transparency we first develop a model for deep-inelastic scattering from free nucleons at smallx. In application to nuclear targets we find that the coherent interaction of quark-antiquark fluctuations with nucleons in a nucleus leads to the observed shadowing atx < 0.1. We compare our results with most of the recent data for a large variety of nuclei and examine in particular the Q2 dependence of the shadowing effect. While the coherent interaction of low mass vector mesons causes a major part of the shadowing observed in the Q2 range of current experiments, the coherent scattering of continuum quark-antiquark pairs is also important and guarantees a very weak overall Q2 dependence of the effect. We also discuss shadowing in deuterium and its implications for the quark flavor structure of nucleons. Finally we comment on shadowing effects in high-energy photon-nucleus reactions with real photons.

$q\bar q$ PAIRS IN THE GENERALIZED VECTOR MESON DOMINANCE MODEL FOR THE NUCLEAR SHADOWING EFFECT IN DEEP INELASTIC SCATTERING

The [Formula: see text] pair component of the virtual photon in deep inelastic scattering is considered in the context of the generalized vector meson dominance model in order to explain the shadowing effect within the conventional nuclear model. The [Formula: see text]-nucleon interaction is estimated by the additive quark model. The nuclear effect is taken into account by the detailed evaluation of the microscopic passage of both hardonic and electromagnetic components of the virtual photon through the nuclear medium. Good predictions of the structure function ratio are obtained from the shadowing region to the usual EMC effect in terms of the conventional nuclear model, but the finer target dependence in the shadowing region is not obtained.

Studies of multihadronic final states in photon-photon interactions

We have measured the cross section andQ2-dependence of the reaction γγ→ multihadrons, using the CELLO detector at DESY, for visible final-state masses between 4 and 9 GeV. The data are well fitted by an incoherent sum of Generalised Vector Meson Dominance and Quark Parton Model terms, but at lowQ2 require the addition of a third component which resembles the QPM but is less two-jet-like. With and without this term, the GVDM cross section atQ2=0 is found to be 200±20 and 250±25 nb, respectively.

Landau ghost and the Källén-Lehmann representation in the relativistic many-body problem

We extend a method proposed by Redmond for the elimination of the Landau ghost in meson propagators to meson-nucleon field theories at finite density. This method is based on the perturbative calculation under the requirement of the correct analytic properties of the propagators expressed by the Källén-Lehmann representation. Using the dispersion relation technique based on the general framework of perturbation theory, the analytic structure of the meson propagators is clarified, and it is proved that the modified propagators obtained by our method are in fact free of the Landau ghost. We also construct the effective action incorporating our ghost free propagators and obtain the modified energy density of nuclear matter. Examples in the 1-loop case are presented. As an application, we derive the electromagnetic response function from our modified effective action, employing the generalized vector meson dominance principle. The RPA Coulomb response function using the ghost free meson propagators is calculated in nuclear matter and the effects of the ghost elimination are discussed.

Analysis of the electroproduction structure functions in the lowQ 2 region, combining the vector meson dominance and the parton model with possible scaling violation

The nucleon structure functionF 2 is constructed and analysed for low values ofQ 2 using the generalised vector meson dominance representation with the largeQ 2 spectral function calculated from the analytic continuation of the parton model structure function. Various parametrisations of the parton distributions are considered. Possible effects of the largeQ 2 scaling violation on the lowQ 2 part of the structure function are investigated. The magnitude of the total contribution given by this asymptotic part can be as high as 50% of the vector meson contribution in the low-Q 2, low-χ region. The contribution of the valence quarks alone to the structure function at lowQ 2 turns out to be at least as important as the corresponding non-Pomeron Regge-pole-like terms coming from the vector meson part. Increase of the structure function with ν coming from the increase of the quark sea in the limit of small χ implied by QCD turns out to be relatively weak at lowQ 2. Predictions of the model are compared with available experimental data. The photoproduction cross section and the nuclear effects in the structure function are also briefly discussed.

Photon Shadowing in Nuclei

Photonuclear Shadowing is studied using the Glauber - generalized vector meson dominance approach. The calculation is performed using both diagonal and off - diagonal generalized vector dominance (GVMD) models. The results are compatible with experiments within the large systematic uncertainties of the available data.

Smallx behaviour of deep inelastic scattering structure functions within and beyond perturbative QCD

The bound on the behaviour of deep inelastic scattering structure functions in the limitx≃0 is derived for arbitraryQ2. To this aim the generalised vector meson dominance representation is used with the total cross-sections assumed to obey the Froissart bound and constrained to follow perturbative QCD at largeQ2. Semiphenomenological eikonal prescription of implementing unitarity upon the leading QCD contribution is formulated and possible effect of absorptive corrections upon structure functions is estimated.