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L. Frankfurt

Publications and source records attributed to L. Frankfurt.

At least 91 records · Page 5Linked to original sources
arXiv2000

Suppression of Quarkonium Production in Heavy Ion Collisions at RHIC and LHC

A model for the production of quarkonium states in the midrapidity region at RHIC and LHC energy range is presented which explores well understood properties of QCD only. An increase of the quarkonium hadronisation time with the initial energy leads to a gradual change of the most important phenomena from fixed target- to collider-energies. We evaluate nuclear effects in the quarkonium production due to medium modification of the momentum distribution of the heavy quarks produced in the hard interactions, i.e. due to the broadening of the transverse momentum distribution. Other nuclear effects, i.e. nuclear shadowing and parton energy loss, are also evaluated.

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arXiv2000

Unitarity and the QCD-improved dipole picture

As a consequence of QCD factorization theorems, a wide variety of inclusive and exclusive cross sections may be formulated in terms of a universal colour dipole cross section at small $x$. It is well known that for small transverse size dipoles this cross section is related to the leading-log gluon density. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transverse size region. We point out that the observed magnitude and small $x$ rise of the gluon density from conventional fits implies that the DGLAP approximation has a restricted region of applicability. We found that `higher twist' or unitarity corrections are required in, or close to, the HERA kinematic region, even for small `perturbative' dipoles for scattering at central impact parameters. This means that the usual perturbative leading twist description, for moderate virtualities, $1 < Q^2 < 10$ GeV$^2$, has rather large `higher twist' corrections at small $x$. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles ($b \gsim 0.4$ fm) to $F_2$, and also to $F_L$. This also leads to deviations from the standard leading twist DGLAP results, at small $x$ and moderate $Q^2$. Our model also describes the low $Q^2$ data very well without any further tuning. We generalize the Gribov unitarity limit for the structure functions of a hadron target to account for the blackening of the interaction at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.

hep-ph↗
arXiv2000

The Transparency of 12C for Protons

Existing cross-section data for 1p-shell knockout in the reaction 12C(e,e'p) - as obtained under different kinematic conditions - are shown to be mutually consistent, apart from a recent measurement performed in Mainz. New data have been collected at the Amsterdam Pulse Stretcher that confirm the normalization of the older measurements. An analysis of the world's 12C(e,e'p) data has yielded precise values of the spectroscopic factor for 1p-shell and 1s-shell knockout from 12C. These values have been used to evaluate the transparency of the 12C nucleus for 1p-shell and 1s-shell protons separately on the basis of recent high-energy 12C(e,e'p) data taken at a four-momentum transfer squared of 1.1 GeV/c^2. As the resulting average value of the nuclear transparency, 0.81(0.04), is considerably higher than the value obtained from previous analyses and theoretical estimates, the high Q^2 data were used instead for an independent determination of the spectroscopic strength for 1p + 1s knockout. Combining these results with the low Q^2 data the spectroscopic factors appear to be momentum-transfer dependent. Possible explanations of these surprising results in terms of reaction-mechanism effects or a possible breakdown of the quasi-particle concept at high Q^2 are discussed as well.

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arXiv1999

Charmonium Suppression - Interplay of Hadronic and Partonic Degrees of Freedom

Last year the E866-group of the Fermilab measured the $x_F$ dependence of $J/Ψ$ and $Ψ'$ suppression in $pA$ collisions. We discuss two of the effects found in that experiment with regard to color coherence effects: the different suppression of the $J/Ψ$ and the $Ψ'$ at $x_F<0$ and the significant suppression of both at large $x_F$. The small $x_F$ regions is dominated by fully formed charmonium states and thus enables us to discuss the formation time and the cross section of the different charmonium states. In the large $x_F$ region the interaction of the charmonium states with nuclear matter has to be described by partonic degrees of freedom, because in that kinematic domain the formation time is much larger than the nuclear radii. The understanding of this region will be crucial for the interpretation of the data of the future heavy ion colliders RHIC and LHC.

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arXiv1999

Diffraction at HERA, Color Opacity and Nuclear Shadowing

The QCD factorization theorem for diffractive processes in DIS is used to derive formulae for the leading twist contribution to the nuclear shadowing of parton distributions in the low thickness limit. Based on the current analyzes of diffraction at HERA we find that the average strength of the interactions which govern diffraction in the gluon sector at $x \le 10^{-3}, Q_0=2 GeV$ is ~50 mb. This is 3 times larger than in the quark sector and suggests that applicability of DGLAP approximation requires significantly larger $Q_0$ in the gluon sector. We use this information to estimate quark and gluon shadowing for heavy nuclei and to calculate its$Q^2$ depen- dence. For A~200 the amount of the gluon shadowing: $G_A/AG_N\sim 0.25-0.4$ at $x<10^{-3},Q=2GeV$ is sensitive to the probability of the small size configurations within wave function of the gluon "partonometer" at the $Q_0$ scale. At this scale for $A\sim 200$ the nonperturbative contribution to the gluon density is reduced by a factor of 4-5 at $x \le 10^{-3}$ unmasking PQCD physics in the gluon distribution of heavy nuclei. Such shadowing would strongly modify the first stage of the heavy ion collisions at LHC, and would lead to large color opacity effects in $eA$ collisions at x<0.001. The leading twist contribution to the cross section of the coherent $J/ψ$ production off $A\ge 12$ nuclei at $\sqrt s \ge 70$ GeV is strongly reduced as compared to the naive color transparency expectations. The Gribov black body limit for $F_{2A}(x,Q^2)$ is extended to the case of the gluon distributions in nuclei and shown to be relevant for the HERA kinematics of $eA$ collisions. Properties of the final states are also briefly discussed.

hep-ph↗
arXiv1999

Diffractive Exclusive Photon Production in DIS at HERA

We demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive exclusive production of photons at large $Q^2$ at HERA, while the aligned jet model allows one to estimate the cross section for intermediate $Q^2 \sim 2 GeV^2$. Furthermore, we find that the imaginary part of the amplitude for the production of real photons is larger than the imaginary part of the corresponding DIS amplitude by about a factor of 2, leading to the prediction of a significant counting rate for the current generation of experiments at HERA. We also find a large azimuthal angle asymmetry in $ep$ scattering for HERA kinematics which allows one to directly measure the real part of the DVCS amplitude and hence the nondiagonal parton distributions.

hep-ph↗
arXiv1999

Diffractive production of charm and gluon nuclear shadowing

We evaluate nuclear shadowing of the total cross section of charm particles production in DIS within the framework of the Gribov theory of nuclear shadowing using as input the recent QCD Pomeron parton density analysis of the HERA diffractive data. Assuming that the QCD factorization theorem is applicable to the charm production off nuclei we also calculate shadowing of the gluon densities in nuclei and find it sufficiently larg for heavy nuclei: $G_{A\sim 200}(x,Q^2)/AG_N(x,Q^2) \sim 0.45-0.5 \cdot (A/200)^{-0.15}$ for $x \sim 10^{-3÷-4}, Q^2 \sim 10 ÷40 GeV^2$ to influence significantly the physics of heavy ion collisions at LHC. We also discuss some properties of the final states for $γ^*A $ processes dominated by the scattering off small $x$ gluons like the high $p_t$ jet and charm production.

hep-ph↗
arXiv1999

Deeply Virtual Compton Scattering at HERA - A Probe of Asymptotia

We demonstrate that the measurement of an azimuthal angle asymmetry in deeply virtual Compton scattering (DVCS) at HERA energies, is experimentally feasible and allows one to determine for the first time the ratio $η$, of the real to imaginary part of the DIS amplitude. We further show that such a measurement would discriminate between different scenarios of the energy dependence of $F_2(x,Q^2)$ at energies beyond those reachable at HERA.

hep-ph↗
arXiv1998

J/psi suppression in heavy ion collisions - interplay of hard and soft QCD processes

We study J/psi suppression in AB collisions assuming that the charmonium states evolve from small, color transparent configurations. Their interaction with nucleons and nonequilibrated, secondary hadrons is simulated using the microscopic model UrQMD. The Drell-Yan lepton pair yield and the J/psi/Drell-Yan ratio are calculated as a function of the neutral transverse energy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data.

hep-ph↗
arXiv1998

J/Ψproduction, $χ$ polarization and Color Fluctuations

The hard contributions to the heavy quarkonium-nucleon cross sections are calculated based on the QCD factorization theorem and the nonrelativistic quarkonium model. We evaluate the nonperturbative part of these cross sections which dominates at $\sqrt{s_{NN}}\approx 20$ GeV at the Cern Super Proton Synchrotron (SPS) and becomes a correction at $\sqrt{s_{NN}}\approx 6$ TeV at the CERN Large Hadron Collider (LHC). $\J$ production at the CERN SPS is well described by hard QCD, when the larger absorption cross sections of the $χ$ states predicted by QCD are taken into account. We predict an $A$-dependent polarization of the $χ$ states. The expansion of small wave packets is discussed.

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arXiv1998

QCD - looking forward

We outline theoretical ideas on the soft and hard dynamics of strong high-energy interactions and discuss promising directions for future high-energy experimental investigations including the ones which would allow one to reveal the three-dimensional structure of QCD bound states, investigate the onset of regime of large parton densities, and observe the violation of the DGLAP evolution equation. We emphasize that many qualitatively new phenomena should be present for the forward kinematics both in electron-nucleon(nucleus) collisions at HERA and in $pp, pA$ collisions at LHC.

hep-ph↗
arXiv1998

DVCS at HERA

We demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive exclusive production of photons at large $Q^2$ at HERA, while the aligned jet model allows one to estimate the cross section for intermediate $Q^2 \sim 2 GeV^2$. Furthermore, we find that the imaginary part of the amplitude for the production of real photons is larger than the imaginary part of the corresponding DIS amplitude by a factor of about 2, leading to predictions of a significant counting rate for the current generation of experiments at HERA. We also find a large azimuthal angle asymmetry in $ep$ scattering for HERA kinematics which allows one to directly measure the real part of the DVCS amplitude and hence the nondiagonal parton distributions.

hep-ph↗
arXiv1998

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 < nu < 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. In addition to 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.

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arXiv1997

Cross Section Fluctuations of Photon Projectile in Generalized Vector Meson Dominance Model

We explain that the generalized vector meson dominance (GVMD) model is successful in describing hard high energy processes because it contains expected in QCD Color Coherent Phenomena. Within the model, which reproduces Bjorken scaling and nuclear shadowing for deep inelastic processes, we calculate $P_γ(σ,Q^2)$, the probability for the photon to interact with a target with the cross section $σ$. We find that within the GVMD model the virtual photon has two main components: the ''hard'' one with the 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 $P_γ(σ,Q^2)$ similar to that within the parton model and suggests the conspiracy between hard and soft QCD at $Q^2 > 1$ GeV$^2$. We study the limit $σ\to 0$ and show, that $P_γ(σ,Q^2) \propto 1/ σ$ in both 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.

hep-ph↗
arXiv1997

Diffractive Heavy Quarkonium Photo- and Electroproduction in QCD

Hard diffractive photo- and electroproduction of heavy vector mesons ($J/ψ$ and $Υ$) is evaluated within the leading $α_s\ln{Q^2 \overΛ_{QCD}^2}$ approximation of QCD. In difference from our earlier work on that subject, also the production of transversely polarized vector mesons is calculated. Special emphasis is placed on the role of the vector meson's $q\bar q$ light-cone wave function. In that context, conventional non-relativistic quarkonium models and a light-front QCD bound state calculation are critically examined and confronted with QCD expectations. Our numerical analysis finds a significant high momentum tail in the latter wave functions and a deviation from the expected asymptotic behavior of $ϕ_V(z,b=0)\propto z(1-z)$. We then design an interpolation to match the quarkonium models at large inter-quark separations with QCD expectations at small distances. We use these results to compare our predictions for the forward differential cross section of $J/ψ$ photo- and electroproduction with recent experimental results from HERA. In addition, our earlier discussion of $ρ^o$ electroproduction is updated in light of recent experimental and theoretical enhancements.

hep-ph↗
arXiv1997

Soft-hard interplay and factorization for baryon production in the target fragmentation region in ep collisions

We discuss baryon production in the nucleon fragmentation region in deep inelastic scattering. The dependence of the nucleon spectra on Bjorken-$x$ is evaluated within the parton model and in QCD, and ways to look for the break-down of the DGLAP approximation via baryon production are suggested. We argue also that the leading neutron production in these small $x$ processes is rather insensitive to the pion parton densities of the nucleon.

hep-ph↗