L. Frankfurt

Factorization for hard exclusive electroproduction of mesons in QCD

We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of meson and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also ! ,

Hard nuclear processes and microscopic nuclear structure

Methods to test QCD-inspired theoretical ideas on microscopic nuclear structure, in high-energy hard lepton-nucleus reactions, are reviewed. Special attention is given to options at existing facilities and at facilities that will come into operation in the early nineties (the jet target facility at PEP and CEBAF). The importance of light-cone dynamics, the physics of large longitudinal distances, and the presence of point-like quark-gluon configurations in hadrons for the theoretical description of high-energy processes (and especially nuclear shadowing) is explained. Ways to investigate the interrelations between perturbative and nonperturbative QCD in nuclear shadowing are suggested. Options for probing the hard scattering wave functions of hadrons are analysed.

Diffractive leptoproduction of vector mesons in QCD

We demonstrate that the distinctive features of the forward differential cross section of diffractive leptoproduction of a vector meson can be legitimately calculated in perturbative QCD in terms of the light-cone [ital q[bar q]] wave function of the vector meson and the gluon distribution of the target. In particular, we calculate the [ital Q][sup 2] and nuclear dependence of the diffractive leptoproduction of vector mesons and estimate the cross section. The production of longitudinally polarized vector mesons by longitudinally polarized virtual photons is predicted to be the dominant component, yielding a cross section behaving as [ital Q][sup [minus]6]. The nuclear dependence of the diffractive cross sections, which follows from a factorization theorem in perturbative QCD, provides important tests of color transparency as well as constraints on the shadowing of the gluon structure functions and the longitudinal structure functions of nuclei.

The physics of ultraperipheral collisions at the LHC

Abstract We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon–nucleon (nucleus) interactions. The current LHC detector configurations can explore hard phenomena at small x with nuclei and nucleons at photon–nucleon center-of-mass energies above 1 TeV, extending the x range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic J / ψ and Υ production as well as in high t ρ 0 production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than those at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used as a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.

Hard diffractive electroproduction of vector mesons in QCD

Hard diffractive electroproduction of longitudinally polarized vector mesons is calculated within the leading {alpha}{sub {ital s}}ln({ital Q}{sup 2}/{Lambda}{sub QCD}{sup 2}) approximation of QCD using the leading order parton densities within the nucleon. Novel QCD features of the production of excited states and of the restoration of flavor symmetry are analyzed. At the onset of the asymptotic regime, our analysis finds an important role of quark Fermi motion within the diffractively produced vector mesons, and we suggest to use this effect to measure the high momentum tail of the wave function of the vector mesons. We deduce a kinematical boundary for the region of applicability of the decomposition of the hard amplitudes over powers of {ital Q}{sup 2} and/or a limit on the increase of the cross sections of hard processes at small {ital x}, and briefly analyze its consequences. We also estimate the nuclear attenuation of the diffractive electroproduction of vector mesons and compare with estimates of the shadowing of the longitudinal structure function. {copyright} {ital 1996 The American Physical Society.}

pQCD physics of multiparton interactions

We study production of two pairs of jets in hadron–hadron collisions in view of extracting contribution of double hard interactions of three and four partons (3→4, 4→4). Such interactions, in spite of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, 2→4, in the back-to-back kinematics when the transverse momentum imbalances of two pairing jets are relatively small.We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions, 2GPDs (Block et al., Phys. Rev. D 83, 071501, 2011), and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders.A special emphasis is given to 3→4 double hard interaction processes which, being of the same order in

Diffractive heavy quarkonium photoproduction and electroproduction in QCD

\alpha _{\mathrm {s}}

Leading Twist Nuclear Shadowing Phenomena in Hard Processes with Nuclei

as the 4→4 process, turn out to be geometrically enhanced compared to the latter and should contribute significantly to four-jet production.The framework developed here takes into systematic consideration perturbative Q2 evolution of 2GPDs. It can be used as a basis for future analysis of NLO corrections to multiparton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.

Four-jet production at LHC and Tevatron in QCD

Hard diffractive photoproduction and electroproduction of heavy vector mesons (J/{psi} and {Upsilon}) is evaluated within the leading {alpha}{sub s}ln(Q{sup 2}/{Lambda}{sub QCD}{sup 2}) approximation of QCD. Different 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{close_quote}s q{bar q} light-cone wave function. In that context, conventional nonrelativistic 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 {phi}{sub V}(z,b=0){proportional_to}z(1{minus}z). We then design an interpolation to match the quarkonium models at large interquark separations with QCD expectations at small distances. We use these results to compare our predictions for the forward differential cross section of J/{psi} photoproduction and electroproduction with recent experimental results from DESY HERA. In addition, our earlier discussion of {rho}{degree} electroproduction is updated in light of recent experimental and theoretical enhancements. {copyright} {ital 1997} {ital The American Physical Society}

Evidence for strong dominance of proton-neutron correlations in nuclei.

We present and discuss the theory and phenomenology of the leading twist theory of nuclear shadowing which is based on the combination of the generalization of the Gribov-Glauber theory, QCD factorization theorems, and the HERA QCD analysis of diffraction in lepton-proton deep inelastic scattering (DIS). We apply this technique for the analysis of a wide range of hard processes with nuclei---inclusive DIS on deuterons, medium-range and heavy nuclei, coherent and incoherent diffractive DIS with nuclei, and hard diffraction in proton-nucleus scattering---and make predictions for the effect of nuclear shadowing in the corresponding sea quark and gluon parton distributions. We also analyze the role of the leading twist nuclear shadowing in generalized parton distributions in nuclei and in certain characteristics of final states in nuclear DIS. We discuss the limits of applicability of the leading twist approximation for small x scattering off nuclei and the onset of the black disk regime and methods of detecting it. It will be possible to check many of our predictions in the near future in the studies of the ultraperipheral collisions at the Large Hadron Collider (LHC). Further checks will be possible in pA collisions at the LHC and forward hadron production at the Relativistic Heavy Ion Collider (RHIC). Detailed tests will be possible at an Electron-Ion Collider (EIC) in the USA and at the Large Hadron-Electron Collider (LHeC) at CERN.