Generalized Vector Dominance Research Articles

A two component model describing nucleon structure functions in the low-x region

A two component model describing the electromagnetic nucleon structure functions in the low-x region, based on generalized vector dominance and color dipole approaches is briefly described. The model operates with the mesons of ρ -family having the mass spectrum of the form m n 2 = m ρ 2 ( 1 + 2 n ) and takes into account the nondiagonal transitions in meson-nucleon scattering. The special cut-off factors are introduced in the model, to exclude the γ − q q ¯ − V transitions in the case of narrow q q ¯ -pairs. For the color dipole part of the model the well known FKS-parameterization is used.

Corrections to the generalized vector dominance due to diffractive ρ3 production

The idea of the vector dominance is still in use in various analyses of the experimental data of photon–hadron reactions. It makes sense, therefore, to recast results of microscopic calculations of such reactions in this language. Here we present the diffractive DIS ρ3 production as a specific correction to generalized vector dominance. We perform a coupled channel analysis of spin–orbital excitations in diffractive photoproduction and reiterate the point that ρ3 in diffractive DIS will be sensitive to a novel aspect of diffraction.

Electron–positron annihilation into three pions and the radiative return

The Monte Carlo event generator PHOKHARA, which simulates hadron and muon production at electron-positron colliders through radiative return, has been extended to final states with three pions. A model for the form factor based on generalized vector dominance has been employed, which is consistent with presently available experimental observations.

Probing the dynamics of the charmonium interactions in the coherent photoproduction off nuclei at moderate energies

We demonstrate how the elementary amplitudes $\gamma N\to \Psi N$, the amplitude of the nondiagonal $J/\psi N\Leftrightarrow \psi' N$ transition, and the total $J/\psi N$ and $\psi' N$ cross sections can be determined from measurements of the coherent $J/\psi$ and $\psi'$ photoproduction off light nuclei at moderate energies. For this purpose we provide a detailed numerical analysis of the coherent charmonium photoproduction off silicon within the generalized vector dominance model (GVDM) adjusted to account for the physics of charmonium models and color transparency phenomenon.

Predictions of the generalized Glauber model for the coherentρproduction at relativistic and ultrarelativistic energies

We calculate the rapidity distribution and the total cross section of coherent and incoherent $\ensuremath{\rho}$ production in heavy ion ultraperipheral collisions at ${\sqrt{s}}_{\mathrm{NN}}=130\mathrm{GeV}$ using the generalized vector dominance model and the Gribov-Glauber approach. We find the coherent cross section of \ensuremath{\rho}-production ${\ensuremath{\sigma}}_{\mathrm{coh}}=490\mathrm{mb}$ compared to ${\ensuremath{\sigma}}_{\mathrm{coh}}=370\ifmmode\pm\else\textpm\fi{}170\ifmmode\pm\else\textpm\fi{}80\mathrm{mb}$ recently reported by the STAR collaboration at RHIC. The predicted cross section inside the acceptance of the experiment, $|y|<~1,$ agrees with the data within half a standard deviation. It is emphasized that measurements of the rapidity distribution will provide a much more stringent test of the used approach.

Photonuclear interaction of high energy muons and tau leptons

General formalism for a two-component description of the inelastic lepton-nucleon scattering in the diffractive region is proposed. Nonperturbative contribution to electromagnetic structure functions of a nucleon is described by the modified generalized vector dominance model containing special cut-off factors restricting the phase volume of initial qq-pairs of virtual photon's fluctuations. Perturbative QCD contribution is described by the phenomenological model suggested (in nonunitarized form) by Forshaw, Kerley and Shaw. Formulae needed for a numerical calculation of photonuclear cross sections integrated over Q^2 are presented. It is argued that in a case of the photonuclear cross sections at superhigh energies of leptons (E>=10^6GeV), integrated over Q^2, the following two-component scheme is good enough: the nonperturbative contribution is approximated by the old parameterization of Bezrukov and Bugaev, and perturbative one is described by the model of Forshaw, Kerley and Shaw with parameters determined from DESY data. Corresponding results of numerical calculations of the perturbative part, for the cases of muon and tau-lepton scattering at superhigh energies, are given.

Coherence effects in diffractive electroproduction from nuclei

We argue that study of the cross section of coherent photo(electro) production of $\rho,\rho^{\prime}, ...$-mesons provides an effective method to probe onset of black body limit(BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model(GVDM). We show that this model describes very well $\rho$-meson coherent photoproduction at $6 \leq E_{\gamma} \leq 10$ GeV. The advantages of the process of coherent dijet production and hard diffractive processes in general for probing the onset of BBL and measuring the light-cone wave function of the photon in a hard scattering regime where decomposition over twists becomes inapplicable are explained. We argue also that the regime of color fluctuations manifests itself at intermediate energies in exclusive and semiexclusive photoproduction of vector mesons and that the color transparency(CT) should reveal itself in these processes at $Q^2$ of the order of a few GeV$^2$.

Diffractive production and the total cross section in deep inelastic scattering

We explore the consequences for diffractive production, gamma* p --> X p, in deep inelastic scattering at low values of x\sim Q^2/W^2 <<1 that follow from our recent representation of the total photoabsorption cross section, sigma_{gamma* p}, in the generalized vector dominance/ color dipole picture(GVD/CDP) that is based on the generic structure of the two-gluon-exchange from QCD. Sum rules are derived that relate the transverse and the longitudinal (virtual) photoabsorption cross section to diffractive forward production of q q-bar states that carry photon quantum numbers ("elastic diffraction"). Agreement with experiment in the W^2 and Q^2 dependence is found for M_X^2/Q^2<<1, where M_X is the mass of the produced system X. An additional component ("inelastic diffraction"), not actively contributing to the forward Compton amplitude, is needed for diffractive production at high values of M_X. Our previous theoretical representation of the total photoabsorption cross section sigma_{gamma* p}=sigma_{gamma* p}(eta), in terms of the scaling variable eta=(Q^2+m_0^2)/Lambda^2(W^2) is extended to include the entire kinematic domain, x=<0.1 and all Q^2 with Q^2>=0, where scaling in eta holds experimentally.

Signals for black body limit in coherent ultraperipheral heavy ion collisions

We argue that study of total cross section of photoabsorption and coherent photoproduction of ρ, ρ′-mesons in ultraperipheral heavy ion collisions (UPC) is effective method to probe onset of black body limit (BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model. We show that this model describes very well ρ-meson coherent photoproduction at 6⩽ E γ ⩽10 GeV. In the case of ρ-meson production we find a UPC cross section which is a factor ∼1.5 larger than the one found by Klein and Nystrand. The advantages of the process of coherent dijet production to probe onset of BBL in hard scattering regime where decomposition over the twists becomes inapplicable are explained and relative importance of the γ+pomeron and γ+ γ mechanisms is estimated.

Deep inelastic scattering at low x: generalized vector dominance and the color dipole picture

We summarize recent work on low-x deep inelastic scattering. The generalized vector dominance/color-dipole picture (GVD/CDP) implies a scaling behavior for σ γ* p ( W 2, Q 2) ≅ σ γ* p (η), with η = ( Q 2 + m 2 0)/Λ 2( W 2) and yield an excellent representation of the experimental results on σ γ* p (η).

A possible two-component structure of the non-perturbative Pomeron

We propose a QCD-inspired two-component Pomeron form which gives an excellent description of the pp, πp, Kp, γp and γγ total cross-sections. Our fit has a better χ 2/ dof for a smaller number of parameters as compared with the PDG fit. Our 2-Pomeron form is fully compatible with weak Regge exchange-degeneracy, universality, Regge factorization and the generalized vector dominance model.

Deep inelastic scattering, QCD, and generalized vector dominance

Deep inelastic scattering, QCD, and generalized vector dominance

Low χbj DIS, QCD and generalized vector dominance

We give a brief overview on the present status of Generalized Vector Dominance as applied to vector-meson production and the total photoabsorption cross section in the region of small χ bj . We comment on how GVD originates from QCD notions such as color transparency.

Deep inelastic scattering, QCD, and generalized vector dominance

We provide a formulation of generalized vector dominance (GVD) for low–x deep-inelastic scattering that explicitly incorporates the ${\gamma}^{\ast} \to q{\bar q}$ transition and a QCD-inspired ansatz for the $(q{\bar q})p$ forward-scattering amplitude. The destructive interference originally introduced in off-diagonal GVD is recovered in the present formulation and traced back to the generic structure of two-gluon-exchange as incorporated into the notion of colour transparency. Asymptotically, the transverse photoabsorption cross section behaves as $(\ln Q^2)/Q^2$ , implying a logarithmic violation of scaling for $F_2$ , while the longitudinal-to-transverse ratio decreases as $1/\ln Q^2$ .

Vector-meson electroproduction from generalized vector dominance

Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section, σ T,γ ∗ p→V p , behaves asymptotically as 1/ Q 4, while R V≡σ L,γ ∗ p→V p /σ T,γ ∗ p→V p becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the level established in low-energy photoproduction and diffractive hadron–hadron interactions. The data on ρ 0-meson production from the Fermilab E665 Collaboration and on φ- and ρ 0-meson production from HERA are found to be in agreement with these predictions.

Two issues in diffractive physics at HERA

A recently proposed formulation of generalized vector dominance applicable to light vector meson electroproduction is introduced. Various models of inclusive diffractive deep-inelastic scattering are surveyed, common features discussed and differences highlighted.

Nuclear shadowing and ρ photoproduction

\ensuremath{\rho} photoproduction on complex nuclei is reexamined using a generalized vector dominance model which succesfully predicts the oberved nuclear shadowing in real photoabsorption and deep inelastic scattering. This model is shown to give a good fit to \ensuremath{\rho} photoproduction data on both nucleons and complex nuclei, in which the disagreement between the measured \ensuremath{\gamma}-\ensuremath{\rho} coupling and the \ensuremath{\gamma}-\ensuremath{\rho} coupling required by the simple vector dominance model is eliminated. The $\ensuremath{\rho}N$ total cross sections required are similar to those predicted by the additive quark model, and the magnitude of the correction to simple vector dominance is consistent with that inferred from the analysis of real photoabsorption and deep inelastic scattering.

Photoproduction off nuclei: Particle and jet production

High energy multiparticle photoproduction off nuclear targets is studied. The photon is assumed to interact in direct scattering processes or as a resolved $q\overline{q}$ state according to the generalized vector dominance model. In the description of resolved interactions multiple soft and hard processes in each $q\overline{q}$-nucleon interaction are taken into consideration. The model, formulated within the framework of the two-component dual parton model, is shown to agree with hadron production data from real and weakly virtual photon-nucleus interactions. Differences between multiparticle production in photon-nucleus and hadron-nucleus interactions and features of jet production in photon-nucleus collisions at DESY HERA energies are discussed.

Photoproduction off nuclei and pointlike photon interactions. I. Cross sections and nuclear shadowing

High energy photoproduction off nuclear targets is studied within the Glauber-Gribov approximation. The photon is assumed to interact as a $q\bar{q}$-system according to the Generalized Vector Dominance Model and as a ``bare photon'' in direct scattering processes with target nucleons. We calculate total cross sections for interactions of photons with nuclei taking into account coherence length effects and point-like interactions of the photon. Results are compared to data on photon-nucleus cross sections, nuclear shadowing, and quasi- elastic $\rho$-production. Extrapolations of cross sections and of the shadowing behaviour to high energies are given.

On the A- and Q2-dependence of shadowing in deep-inelastic scattering on heavy nuclei

We present further predictions for the A-dependence and the x- and Q 2-dependence of shadowing in deep-inelastic scattering from heavy nuclei within the framework of generalized vector dominance, which, as previously shown, accounts well for the shadowing effect which only very recently has been emperically established in inelastic muon scattering.