Colour transparency and scaling properties of nuclear shadowing in deep inelastic scattering

We present the perturbative QCD analysis of nuclear shadowing in the deep inelastic scattering at smallx in terms of the spatial wave function ofq \(\bar q\) fluctuations of virtual photons. The wave function formalism makes it quite obvious that shadowing is the scaling, rather than the higher twist, 1/Q ², effect, contrary to a numerous recent claims. We demonstrate explicitly how the scaling shadowing comes from the large, hadronic, size quarkantiquark pairs even in the limit ofQ ²→∞, and why it should very slowly, ∞1/log (Q ²/m ²) decrease at very largeQ ². We argue in favor of the scaling triple-pomeron contribution to the nuclear shadowing and present predictions for a cross section of diffraction dissociation of virtual photons and for the mass spectrum of diffraction excitation, which can be checked at HERA and Fermilab. We predict strikingly different scaling properties of diffraction dissociation and nuclear shadowing for the longitudinal and transverse photons. Our, numerial predictions for shadowing are in good agreement with the recent EMC data.     

On the determination of double diffraction dissociation cross section at HERA

The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processesep→e′p′ρ ⁰,e′p′J/ψ,e′p′X at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in theJ/ψ photoproduction, electroproduction of theρ ⁰ at largeQ ² and diffraction dissociation of real and virtual photons into high mass statesX the contamination of the double diffraction dissociation can be as large as ∼30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the protonproton collisions.     

On the determination of double diffraction dissociation cross section at HERA

The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processes epMe'p'A⁰,r e'p'J/O,r e'p'X at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in the J/O photoproduction, electroproduction of the A⁰ at large Q² and diffraction dissociation of real and virtual photons into high mass states X the contamination of the double diffraction dissociation can be as large as ~30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the proton-proton collisions.     

Unusual effects of diffraction dissociation for multiproduction in deep inelastic scattering on nuclei

In hadron-nucleus interactions, the stronger is nuclear shadowing in the total cross section the higher is the multiplicity of secondary hadrons. In deep inelastic scattering nuclear shadowing at smallx is associated with the hadronlike behaviour of photons as contrasted to the pointlike behaviour in the non-shadowing region of largex. In this paper we predict dilution of multiproduction of secondary hadrons, and weaker fragmentation of the target nucleus, in deep inelastic leptoptoproduction on nuclei in the shadowing region of smallx as compared to the non-shadowing region of largex. This paradoxial conclusion has its origin in nuclear enhancement of the coherent diffraction dissociation of photons which affects multiproduction in the shadowing region and does not contribute in the non-shadowing region. We present numerical predictions for multiproduction in theμXe interactions studied by the Fermilab E665 collaboration.     

Experimental results on proton diffractive dissociation

Properties of proton diffractive dissociation have been investigated for four-prong final states in proton-proton reactions at 360 GeV/c, in particular forpp→ppπ ⁺π^? mπ ⁰(m=0,1,2). Mass distributions and decay angular distributions are given. The decay of the diffractive system is seen to be very anisotropic, and largep _T is suppressed at higher masses. It is found that the “pomeron” couples with a single valence quark of the incident proton, but indications for a diquark-pomeron coupling are also found. Similarities with fragmentation in lepton-hadron deep inelastic scattering are underlined.     

Diffractive production of dijets at HERA

We investigate the production of a quark-antiquark pair in diffractive photon-proton scattering, approximating soft pomeron exchange by the exchange of two nonperturbative gluons. In deep inelastic scattering at HERA, events with two jets and the scattered proton in the final state are predicted to be observable, with an important contribution from charm production. For photoproduction of light quark jets with high transverse momentum we find that both exchanged gluons must have a large invariant mass, so that the cross section is very small, whereas for charm quarks it is quite appreciable. From our calculation we also extract the quark structure function of the pomeron for the scaling variablez no too close to 0 or 1, finding a strong flavour dependence and a behaviour somewhat harder thanz(1?z) for light quarks.     

Scaling properties of nuclear shadowing in deep inelastic scattering

We present the perturbative QCD analysis of nuclear shadowing in deep inelastic scattering and prove it to be the scaling, rather than the higher twist, 1/Q², effect, contrary to numerous recent claims. We argue in favor of the scaling triple-pomeron contribution to the nuclear shadowing and comment on the mass spectrum of diffraction excitation by highly virtual photons. Our numerical predictions for shadowing are in good agreement with the recent EMC data.     

Comparison of moments from the valence structure function with QCD predictions

We present moments (both ordinary and Nachtmann) of the nucleon valence structure function measured in high Q²_νFE scattering, supplemented by data from deep inelastic eD scattering. These data seem to agree with QCD predictions for vector gluons. The QCD parameter Λ is found to be of the order 0.5 GeV.     

Forward gluon production in hadron–hadron scattering with pomeron loops

We discuss new physical phenomena expected in particle production in hadron–hadron collisions at high energy, as a consequence of pomeron loop effects in the evolution equations for the color glass condensate. We focus on gluon production in asymmetric, ‘dilute–dense’, collisions: a dilute projectile scatters off a dense hadronic target, whose gluon distribution is highly evolved. This situation is representative for particle production in proton–proton collisions at forward rapidities (say, at LHC) and admits a dipole factorization similar to that of deep inelastic scattering (DIS). We show that at sufficiently large forward rapidities, where the pomeron loop effects become important in the evolution of the target wavefunction, gluon production is dominated by ‘black spots’ (saturated gluon configurations) up to very large values of the transverse momentum, well above the average saturation momentum in the target. In this regime, the produced gluon spectrum exhibits diffusive scaling, so like DIS at sufficiently high energy.     

The pomeron structure in DIS and gluon recombination effects

A formalism for diffractive deep inelastic scattering is presented based on the concept of a pomeron structure function. Assuming the pomeron to be essentially a gluonic object with the small size indicated by its small interaction cross-sections, we show that the QCD evolution of its structure function exhibits large screening corrections due to gluon recombinations at small-χ. With a gluon-to-quark conversion appropriate at small-χ, diffractiveep cross-sections are obtained giving large enough statistics at HERA to distinguish between QCD evolution governed by the conventional Altarelli-Parisi equations and by the non-linear Gribov-Levin-Ryskin equation.     

Glue in the pomeron from nonlinear k ⊥ factorization

We derive the nonlinear k _⊥ factorization for the spectrum of jets in high-mass diffractive deep inelastic scattering as a function of three hard scales—the virtuality of the photon Q ², the transverse momentum of the jet, and the saturation scale Q _A . In contrast to all other hard reactions studied so far, we encounter a clash between the two definitions of the glue in the pomeron from the inclusive spectrum of leading quarks and the small-β evolution of the diffractive cross section. This clash casts a further shadow on customary applications of the familiar collinear factorization to a pQCD analysis of diffractive deep inelastic scattering. The text was submitted by the authors in English.     

Two-photon collisions in deep inelastic lepton-nucleon and lepton-nucleus scattering

We have calculated the μ-pair production via photon-photon collisions in deep-inelastic charged lepton scattering on proton and nuclei, using the equivalent photon approximation. Specific parametrizations were assumed for the proton and the nuclei form factors and the QED photon structure function. Results are given for lepton energies below 300 GeV and virtual photon massesQ ² less than 50 GeV², as functions of the μ-pair mass, the scaling variablex and the electric chargeZ of the hadron target. Using a photon structure function derived from QPM and VDM we also estimate the contribution of γγ collisions to hadron production in these deep inelastic processes.     

Deep inelastic electron-pomeron scattering at HERA

The idea that the pomeron has partonic structure similar to any other hadron has been given strong support by recent measurements of the diffractive structure function at HERA. We present a detailed theoretical analysis of the diffractive structure function under the assumption that the diffractive cross section can be factorized into a pomeron emission factor and the deep inelastic scattering cross section of the pomeron. We pay particular attention to the kinematic correlations implied by this picture, and suggest the measurement of an angular correlation which should provide a first test of the whole picture. We also present two simple phenomenological models for the quark and gluon structure of the pomeron, which are consistent with various theoretical ideas and which give equally good fits to recent measurements by the H1 collaboration, when combined with the pomeron emission factor of Donnachie and Landshoff. We predict that a large fraction of diffractive deep inelastic events will contain charm, and discuss how improved data will be able to distinguish the models.     

High-mass diffraction and Regge behavior of total cross sections

We consider a modified two-component model of particle production in which diffractive excitation into high mass states gives a contribution to K^±p, π^±p and pp total cross sections of sufficient magnitude and with the proper associated threshold to be a possible explanation of deviations of σ_tot from Regge pole model fits at Serpukhov energies. We present tests of the model. for K⁺p scattering by extrapolating σ_tot to National Accelerator Laboratory energies and by calculating the high mass diffraction contribution to the forward peak in inclusive reactions. The consistency of this scheme is discussed in relation to the pomeron intercept and the form of the triple-pomeron coupling.     

Gluonic regge singularities and anomalous dimensions in QCD

The Regge calculus results on the perturbative pomeron are applied to deep inelastic scattering. Explicit expressions are given for the anomalous dimensions γ_GGⁿ and γ_GFⁿ at n ? 1 to the lowest order in α and all orders in α/(n ? 1).     

Resonant inelastic contact scattering of X-ray photons on atoms and ions

The existence of an extended resonance structure outside the X-ray emission regions is theoretically predicted in the total double differential cross section for the scattering of linearly polarized photons on free atoms (ions). This structure is almost entirely determined by inelastic photon scattering of the contact type. The amplitude of the inelastic contact scattering probability is described using an analytical expression for a non-relativistic transition operator, which was previously obtained by the author outside the dipole and momentum approximations. The resonant inelastic contact scattering of X-ray photons on a neon atom and neonlike ions of argon, titanium, and iron has been studied. Calculations were performed in a nonrelativistic approximation for the wave functions of the scattering states, with allowance for many-body effects of the radial relaxation of one-electron orbitals in the Hartree-Fock field of a deep 1s vacancy and (for neon atom) the double excitation/ionization of the ground atomic state.     

Observation of interband transitions in polarized electron energy loss spectra of Gd(0001)

We have detected the 4f ⁷(5d6s)³→4f ⁸(5d6s)² interband transition in an angular-resolved, inelastic scattering experiment with spin-polarized, low-energy electrons from ferromagnetic Gd(0001). The spectrum of the inelastic scattering asymmetry clearly reveals the dominant spin-dependent energy loss mechanism involved. Furthermore its comparison with elastic scattering data allows a characterization of the combined role of diffraction and energy loss processes in inelastic electron scattering.