arXiv subjects
Publications and source records attributed to J. Nemchik.
We present the first evaluation of the color dipole diffraction slope from the data on diffractive photo- and electroproduction of vector mesons. The energy and dipole size dependence of the found dipole diffraction slope are consistent with the color dipole gBFKL dynamics.
In the color dipole gBFKL dynamics we predict a strikingly different Q^2 and energy dependence of the diffraction slope for the elastic production of ground state V(1S) and radially excited V'(2S) light vector mesons. The color dipole model predictions for the diffraction slope for ρ^0 and ϕ^0 production are in a good agreement with the data from the fixed target and collider HERA experiments. We present how a different form of anomalous energy and Q^2 dependence of the diffraction slope for V'(2S) production leads to a different position of the node in radial wave function and discuss a possibility how to determine this position from the fixed target and HERA data.
Within the color dipole gBFKL dynamics applied to the diffraction slope we predict an anomalous t dependence of the differential cross section as a function of energy and Q^2 for production of radially excited V'(2S) light vector mesons in contradiction with a well known standard monotonous t- behaviour for V(1S) mesons. The origin of this phenomenon is based on the interplay of the nodal structure of V'(2S) radial wave function with the energy and dipole size dependence of the color dipole cross section and of the diffraction slope. We present how a different position of the node in V'(2S) wave function leads to a different form of anomalous t- behaviour of the differential cross section and discuss a possibility how to determine this position from the low energy and HERA data.
We develop the color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction $γ^{*}N \to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments. We predict a substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$. The $Q^{2}$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^{2}$ (which is supported by the available experimental data). We predict an approximate flavor independence of the diffraction slope in the scaling variable $Q^{2}+m_{V}^{2}$. For diffractive production of the radially excited $2S$ states ($Ψ',Υ'$) the counterintuitive inequality of diffraction slopes $B(2S) \lsim B(1S)$ is predicted, which defies the common wisdom that diffraction slopes are larger for reactions with larger size particles.
We present a space-time description of hadronization of highly virtual quarks originating from a deep-inelastic electron scattering (DIS). Important ingredients of our approach are the time- and energy--dependence of the density of energy loss for gluon radiation, the Sudakov's suppression of no radiation, and the effect of color transparency, which suppresses final state interaction of the produced colorless wave packet. The model is in a good agreement with available data on leading hadron production off nucleons and nuclei. The optimal energy range for study of the hadronization dynamics with nuclear target is found to be a few tens of GeV, particularly energies available in the experiment HERMES.
The integrated cross section for the incoherent photoproduction of vector mesons on nuclei $γ^* A \to VX$, $X\not=A$, is calculated within Glauber theory and as a function of the photon energy. The inverse of the longitudinal momentum transfer is called coherence length $l_c$ and depends on the virtuality and the energy of the photon. Nuclear transmission factors strongly depend on $l_c/R_A$ ($R_A$ is the nuclear radius) and this effect may interfere with the search for color transparency effects.
We develop the color dipole phenomenology of diffractive photo- and electroproduction $γ^{*}\,N\rightarrow V(V')\,N$ of light vector mesons ($V(1S) = ϕ^0, ω^0, ρ^0$) and their radial excitations ($V'(2S) = ϕ', ω', ρ'$). The node of the radial wave function of the $2S$ states in conjunction with the energy dependence of the color dipole cross section is shown to lead to a strikingly different $Q^2$ and $ν$ dependence of diffractive production of the $V(1S)$ and $V'(2S)$ vector mesons. We discuss the restoration of flavor symmetry and universality properties of production of different vector mesons as a function of $Q^{2}+m_{V}^{2}$. The color dipole model predictions for the $ρ^{0}$ and $ϕ^{0}$ production are in good agreement with the experimental data from the EMC, NMC, ZEUS and H1 collaborations. We present the first direct evaluation of the dipole cross section from these data.
We present the color dipole phenomenology of diffractive photo- and electroproduction $γ^{*} N\to V(V') N$ of vector mesons ($V(1S) = ϕ^0, ω^0, ρ^0, J/Ψ, Υ$) and their radial excitations ($V'(2S) = ϕ', ω', ρ', Ψ', Υ'$). The main emphasis is related to light vector mesons. We discuss how the energy dependence of the color dipole cross section in conjuction with the node of the radial wave function of the $2S$ states can lead to an anomalous $Q^2$ and energy dependence of diffractive production of $V'(2S)$ vector mesons. The color dipole model predictions for $V(1S)$ light vector meson production are compared with the experimental data from the EMC, NMC, ZEUS and H1 collaborations.
We present the first evaluation of color dipole cross section from experimental data on diffractive photo- and electroproduction of vector mesons. The dipole-size and energy dependence of the found dipole cross section is consistent with expectations from the BFKL dynamics.
Study of color transparency (CT) effects at moderate energies is more problematic than is usually supposed. Onset of CT can be imitated by other mechanisms, which contain no explicit QCD dynamics. In the case of the $(e,e'p)$ reaction the standard inelastic shadowing well known in the pre-QCD era, causes a substantial growth of nuclear transparency with $Q^2$ and a deviation from the Glauber model, analogous to what is assumed to be a signal of CT. In the case of exclusive virtual photoproduction of vector mesons, CT is expected to manifest itself as an increase of nuclear transparency with $Q^2$ in production of the ground states and as an abnormal nuclear enhancement for the radial excitations. We demonstrate that analogous $Q^2$-dependence can be caused at moderate energies by the variation of so called coherence length, which is an interference effect, even in the framework of the vector dominance model. One should disentangle the real and the mock CT effects in experiments planned at CEBAF, at the HERMES spectrometer, or at the future electron facility ELFE.
Radiative energy loss of a highly virtual quark originating from a deep-inelastic electron scattering plays a crucial role in production of leading hadrons off nuclei. The density of energy loss for gluon radiation turns out to be time- and energy--dependent in inclusive hadron production. Important phenomena involved are Sudakov's suppression of no radiation of that part of gluon spectrum, which is forbidden by energy conservation, and color transparency, which suppresses the final state interaction of the produced colorless wave packet. We model the soft part of hadronization, which usually is supposed to be due to the color strings, using also the gluon radiation and nicely reproduce the string parameters. Our parameter-free calculations provide a good agreement with available data on $z_h$-dependence of the quark fragmentation function in vacuum, as well as $ν$-, $z_h$- and $Q^2$-dependence of nuclear effects. We come to the conclusion that the energy range of ELFE - HERMES is most sensitive to the underlying dynamics of hadronization provided that nuclear targets are used.
The variation of the coherence length $l_c=2ν/(Q^2+m_V^2)$ in virtual photoproduction of vector mesons off nuclei as a function of the photon energy $ν$ or virtuality $Q^2$ results in dramatic changes in the values of the nuclear transparency. Color transparency effects can be easily mixed up with the effects of the coherence length in incoherent photoproduction on nuclei.
The standard first-order inelastic correction (IC) well known in the pre-QCD era, causes a rising $Q^2$-dependence of nuclear transparency in the quasielastic electron scattering, $A(e,e'p)A'$, at moderate $Q^2$, similar to what is supposed to be the onset of color transparency (CT). Although IC is a part of the whole pattern of CT, it contains no explicit QCD dynamics. Evaluation of this correction is based on experimental data on diffraction dissociation and is independent of whether CP phenomenon exists or not. The growth of nuclear transparency is numerically comparable with the expected signal of CT up to about $Q^2\approx 20\ GeV^2$. Analogous effect in $A(p,2p)A'$ reaction is estimated as well.
CEBAF is a high-luminocity factory of virtual photons with variable virtuality $Q^{2}$ and transverse size. This makes CEBAF, in particular after the energy upgrade to (8-12)GeV, an ideal facility for uncovering new phenomena, and opening new windows, at the interface of the perturbative and nonperturbative QCD. We discuss color transparency as the case for a broad program on electroproduction of vector mesons $ρ^{0},\,ω^{0},\,ϕ^{0}$ and their radial excitations $ρ',\,ω',\,ϕ'$ at CEBAF. We also comment on the second generation of experiments on color transparency in $^{4}He(e,e'p)$ scattering, which are also feasible at CEBAF. In 1994, we can make more reliable projections into future because our understanding of the onset of color transparency has greatly been augmented by two experiments completed in 1993:\\ i) no effect of CT was seen in the SLAC NE18 experiment on $A(e,e'p)$ scattering at virtualities of the exchanged photon $Q^{2} \lsim 7$ GeV$^{2}$, \\ ii) strong signal of CT was observed in the FNAL E665 experiment on exclusive $ρ^{0}$- meson production in deep inelastic scattering in the same range of $Q^{2}$. \\ We discuss the impact of these observations on the CEBAF experimental program. We argue they both are good news, both were anticipated theoretically, and both rule in the correct QCD mechanism of the onset of CT.
Elastic production of vector mesons $γ^{*} N\to V N$ is the pomeron-exchange dominated diffractive reaction with much potential of probing the BFKL pomeron. The BFKL pomeron can conveniently be described in terms of the dipole cross section which is a solution of the generalized BFKL equation. In this paper we discuss, how the energy and $Q^{2}$ dependence of elastic production of vector mesons at HERA will allow scanning the dipole cross section as a function of dipole size $r$. We show that determinaton of the intercept of the BFKL pomeron requires measuring the $ρ^{0}$ and $J/Ψ$ production at $Q^{2} \sim (100-200)GeV^2$ and/or the quasireal photoproduction of the $Υ$. We present predictions for the effective intercept in the kinematic range of the forthcoming HERA experiments, which can shed much light on the nonperturbative component of the pomeron.
We predict a strikingly different $A$ and $Q^{2}$ dependence of quasielastic leptoproduction of the $ρ^{0}(1S)$-meson and its radial excitation $ρ'(2S)$ on nuclei. Whereas for the $ρ^{0}$ production nuclear transparency $T_{A}$ decreases monotonically with $A$, for the $ρ'$ nuclear transparency $T_{A}$ can have the counterintuitive nonmonotonic $A$-dependence, having the minimum for light nuclei, and increasing with $A$ for medium and heavy nuclei. Strong enhancement of the $ρ' /ρ^{0}$ cross section ratio makes nuclear targets the $ρ'$-factory. The origin of the anomalous $A$-dependence is in the interplay of color transparency effects with the nodal structure of the $ρ'$ wave function. The predicted effects take place at moderate $Q^{2}\lsim (2-3)$GeV$^{2}$, which can be explored in high statistics experiments at CEBAF.