Difraktivno foto- i leptonski proizvedeni vektorski mesoni ρ, ρ ′ I ρ ′′

In the framework of non-perturbative QCD, we calculate high-energy diffractive production of vector mesons ρ, ρ ′ and ρ ′′ by real and virtual photons on a nucleon. The initial photon dissociates into a qq¯-dipole and transforms into a vector meson by scattering off the nucleon which, for simplicity, is represented as quark-diquark. The relevant dipole-dipole scattering amplitude is provided by the non-perturbative model of the stochastic QCD vacuum. The wave functions result from considerations in the frame of light-front dynamics; the physical ρ ′ - and ρ ′′-mesons are assumed to be mixed states of an active 2S-excitation and some residual rest (2Dand/or hybrid state). We obtain good agreement with the experimental data and get an understanding of the markedly different π +π −-mass spectra for photoproduction and e+e –annihilation.U okviru neperturbativne QCD, izračunali smo difraktivnu tvorbu vektorskih mezona ρ, ρ ′ and ρ ′′ na nukleonu na visokim energijama s realnim i virtualnim fotonima. Početni se foton razdvaja na qq¯-dipol i pretvara u vektorski mezon pri raspršenju na nukleonu, koji se, radi jednostavnosti, predstavlja kao kvarkdikvark. Tražena se dipolno-dipolna amplituda dobije neperturbativnim modelom stohastičkog QCD vakuuma. Valne se funkcije dobiju razmatranjem u sustavu dinamike valne fronte. Pretpostavlja se da su fizički mezoni ρ ′ - i ρ ′′ miješana stanja aktivne 2S-uzbude i ostatka (2D i/ili hibridnih stanja). Dobiva se dobro slaganje s eksperimentalnim podacima i postiže razumijevanje izrazito različitih masenih spektara π +π − za fototvorbu i poništenje e+e −

Difraktivno foto- i leptonski proizvedeni vektorski mesoni ρ, ρ ′ I ρ ′′

In the framework of non-perturbative QCD, we calculate high-energy diffractive production of vector mesons ρ, ρ ′ and ρ ′′ by real and virtual photons on a nucleon. The initial photon dissociates into a qq¯-dipole and transforms into a vector meson by scattering off the nucleon which, for simplicity, is represented as quark-diquark. The relevant dipole-dipole scattering amplitude is provided by the non-perturbative model of the stochastic QCD vacuum. The wave functions result from considerations in the frame of light-front dynamics; the physical ρ ′ - and ρ ′′-mesons are assumed to be mixed states of an active 2S-excitation and some residual rest (2Dand/or hybrid state). We obtain good agreement with the experimental data and get an understanding of the markedly different π +π −-mass spectra for photoproduction and e+e –annihilation.U okviru neperturbativne QCD, izračunali smo difraktivnu tvorbu vektorskih mezona ρ, ρ ′ and ρ ′′ na nukleonu na visokim energijama s realnim i virtualnim fotonima. Početni se foton razdvaja na qq¯-dipol i pretvara u vektorski mezon pri raspršenju na nukleonu, koji se, radi jednostavnosti, predstavlja kao kvarkdikvark. Tražena se dipolno-dipolna amplituda dobije neperturbativnim modelom stohastičkog QCD vakuuma. Valne se funkcije dobiju razmatranjem u sustavu dinamike valne fronte. Pretpostavlja se da su fizički mezoni ρ ′ - i ρ ′′ miješana stanja aktivne 2S-uzbude i ostatka (2D i/ili hibridnih stanja). Dobiva se dobro slaganje s eksperimentalnim podacima i postiže razumijevanje izrazito različitih masenih spektara π +π − za fototvorbu i poništenje e+e −

Universality in the Electroproduction of Vector Mesons

We study universality in the electroproduction of vector mesons using a unified nonperturbative approach which has already proved to reproduce extremely well the available experimental data. In this framework, 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 calculations suggest a finite infrared behavior for the strong coupling constant.Comment: 22 pages, 10 figure

Exclusive vector meson production at HERA from QCD with saturation

Following recent predictions that the geometric scaling properties of deep inelastic scattering data in inclusive gamma*-p collisions are expected also in exclusive diffractive processes, we investigate the diffractive production of vector mesons. Using analytic results in the framework of the BK equation at non-zero momentum transfer, we extend to the non-forward amplitude a QCD-inspired forward saturation model including charm, following the theoretical predictions for the momentum transfer dependence of the saturation scale. We obtain a good fit to the available HERA data and make predictions for deeply virtual Compton scattering measurements.Comment: 10 pages, 5 figures, full analysis including the charm contribution and J/PSI production. Conclusions confirme

Exclusive production of vector mesons ρ,ρ′\rho, \rho' and ρ′′\rho'' by real and virtual photons

We present a non-perturbative QCD calculation of high-energy diffractive photo- and leptoproduction of vector mesons $\rho$, $\rho'$ and $\rho''$ on a nucleon. The initial photon splits up in a $q\bar{q}$-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 $\rho'$- and $\rho''$-states to be mixed states of an active 2S-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 $\pi^+\pi^-$-invariant mass for photoproduction and $e^+e^-$-annihilation.Comment: Talk given in Montpellier, QCD Euroconference 98, to be published in the proceedings Nucl.Phys. B (Proc.Suppl.). 5 pages, LaTeX2e, 13 ps/eps figures included, needs style file espcrc2MOD.sty (appended

Production of Polarized Vector Mesons off Nuclei

Using the light-cone QCD dipole formalism we investigate manifestations of color transparency (CT) and coherence length (CL) effects in electroproduction of longitudinally (L) and transversally (T) polarized vector mesons. Motivated by forthcoming data from the HERMES experiment we predict both the A and Q^2 dependence of the L/T- ratios, for rho^0 mesons produced coherently and incoherently off nuclei. For an incoherent reaction the CT and CL effects add up and result in a monotonic A dependence of the L/T-ratio at different values of Q^2. On the contrary, for a coherent process the contraction of the CL with Q^2 causes an effect opposite to that of CT and we expect quite a nontrivial A dependence, especially at Q^2 >> m_V^2.Comment: Revtex 24 pages and 14 figure

Extracting the rho meson wavefunction from HERA data

We extract the light-cone wavefunctions of the rho meson using the HERA data on diffractive rho photoproduction. We find good agreement with predictions for the distribution amplitude based on QCD sum rules and from the lattice. We also find that the data prefer a transverse wavefunction with enhanced end-point contributions.Comment: 13 pages, 7 figures, significant improvements over the original version with a new section on distribution amplitudes adde

Decomposition of the QCD String into Dipoles and Unintegrated Gluon Distributions

We present the perturbative and non-perturbative QCD structure of the dipole-dipole scattering amplitude in momentum space. The perturbative contribution is described by two-gluon exchange and the non-perturbative contribution by the stochastic vacuum model which leads to confinement of the quark and antiquark in the dipole via a string of color fields. This QCD string gives important non-perturbative contributions to high-energy reactions. A new structure different from the perturbative dipole factors is found in the string-string scattering amplitude. The string can be represented as an integral over stringless dipoles with a given dipole number density. This decomposition of the QCD string into dipoles allows us to calculate the unintegrated gluon distribution of hadrons and photons from the dipole-hadron and dipole-photon cross section via kT-factorization.Comment: 43 pages, 14 figure