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.     

Diffractive resonance production and f-dominance of the pomeron

A model for predicting diffractive production of meson resonances is presented. From a study of the s and t dependence of the available cross section and density matrix data, we deduce that the assumption of f, f′ dominance of the pomeron is violated. Relative to elastic scattering, where f, f′ dominance works well, the pomeron component in these inelastic processes appears to be about one half as strong in amplitude.     

Photon contribution to the “super-hard” pomeron

It is shown that direct photons provide a leading twist mechanism for diffractive jet production in which the jets carry away all of the momentum lost by the proton. Two-photon processes are thus expected to asymptotically dominate “super-hard” pomeron events in ep collisions. We report the expected rates from these events for recent ZEUS and H1 data cuts. We also estimate the direct photon contribution to the “super-hard” pomeron events observed by the CERN UA8 group for collisions. It is again argued that direct photons are the leading mechanism for these events. We find that direct photons are an appreciable fraction of the events seen by UA8.     

Probing the structure of the pomeron

We suggest that the pseudo-rapidity cut dependence of diffractive deep-inelastic scattering events at HERA may provide a sensitive test of models of diffraction. A comparison with the experimental cross section shows that the Donnachie-Landshoff model and a simple two-gluon exchange model of the pomeron model are disfavoured. However a model with a direct coupling of the pomeron to quarks is viable for a harder quark–pomeron form factor, as is a model based on the leading-twist operator contribution. We also consider a direct-coupling scalar pomeron model. We comment on the implications of these results for the determination of the partonic structure of the pomeron. Received: 5 March 1999 / Published online: 3 August 1999     

Heavy quarkonium production in double pomeron exchange processes in perturbative QCD

We calculate the heavy quarkonium production in double pomeron exchange processes in perturbative QCD by using two-gluon exchange model. For the P-wave χ_J productions, we find χ₁ and χ₂ production amplitudes which vanish in the forward scattering limit. We also calculate direct J/ψ()+γ production in the same approach, and these direct contributions are much smaller than the feeddown contributions from the P-wave states.     

A QCD analysis of diffractive deep-inelastic scattering data

We perform a novel type of analysis of diffractive deep-inelastic scattering data, in which the input parton distributions of the pomeron are parameterised using the perturbative QCD expressions. In particular, we treat individually the components of the pomeron of different size. We are able to describe simultaneously both the recent ZEUS and H1 diffractive data. In addition to the usual two-gluon model for the perturbative pomeron, we allow for the possibility that it may be made from two sea quarks.Received: 12 July 2004, Revised: 3 September 2004, Published online: 24 September 2004     

Bethe-Salpeter equations forq $$\bar q$$ andqqq systems in the instantaneous approximation

We consider a model for elastic scattering and inelastic diffractive production at high energy, which is inspired by Quantum Chromodynamics. The pomeron arises in our model from gluon exchange between quark constituents. The color-neutrality of each hadron implies strong cancellations among the gluon exchanges. Hence our model is “subtractive”, in contrast to the old “additive” quark model. The subtractive model provides a natural explanation for the large cross section which is observed for diffractive dissociation. We show that multiple gluon exchange contributes significantly, alongside two gluon exchange, in building the pomeron.     

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.     

Physics with tagged forward protons at the LHC

We emphasize the importance of tagging the outgoing forward protons to sharpen the predictions for New Physics at the LHC (such as the diffractive production of a light Higgs boson). The rescattering effects lead to a rich distinctive structure of the cross section as a function of the transverse momenta of the protons. We show that a study of the correlations between the proton transverse momenta for double-diffractive production of central dijets will provide a detailed check of the whole diffractive formalism. Adopting a perturbative two-gluon structure of the Pomeron, we emphasize that quarkonium production, via Pomeron-Pomeron fusion, is strongly suppressed. This offers a favourable production mechanism for non- states, such as glueballs. Received: 15 March 2002 / Published online: 21 June 2002     

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon () and pomeron () momentum participating in the production of the dijet system. The observed dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section increases as increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron. Received: 20 April 1998 / Published online: 2 July 1998     

A theoretical approach to low multiplicity diffractive dissociation

We investigate the dynamics of low mass inelastic diffractive production in the framework of the “1/N dual unitarization” scheme. The smallness of inelastic diffractive dissociation is explicitly demonstrated by incorporating a Deck type mechanism with the crucial planar bootstrap equation. Although both inelastic and elastic pomeron couplings are of the same order in 1/N, the origin for their smallness, however, is not identical. Our work further confirms the validity of the iterative procedure, where the elastic amplitude is first generated from only non-diffractive intermediate states (except possibly for central collisions). Using a previous study of the “Cylinder” strength, we present also a semi-quantitative results for the integrated cross-section for low multiplicity diffractive production and compare it with the elastic cross-section at very high energies.     

Shadowing effects in nuclear parton distributions for small values ofx

The shadowing corrections to gluon and quark distributions in nuclei in the region of small values ofx are discussed. They are related to parton distributions in a pomeron which are in principle measurable in hard diffractive processes on the nucleon target. Multiple scattering corrections to shadowing are considered in a model dependent way. The perturbative QCD evolution of shadowing is also taken into account. Various possibilities of the partonic content of a pomeron are considered. It is shown in particular that the conventional parametrizations of parton distributions in a pomeron which are based on the assumption that it consists mostly of gluons imply substantial nuclear shadowing in gluon distributions in heavy nuclei. Possible phenomenological implications of shadowing corrections in nuclear parton distributions for various semi-hard processes with nuclear targets are briefly discussed.     

Validity of double scaling analysis in semi-inclusive processes –J/\psi production at HERA

In this paper we check the validity of the ideas of double scaling as given by Ball and Forte in a semi inclusive process like production at HERA, in different kinematical regions, for low values of the Bjorken variable . In particular, we study production in the inelastic and diffractive (elastic) regimes using the double scaling form of the gluon distribution functions. We compare these predictions with data (wherever available) and with other standard parameterisations. We find that double scaling holds in the inelastic regime over a larger kinematic region than that given by the analysis of the proton structure function . However, in the diffractive region, double scaling seems to suggest an admixture of hard pomeron boundary conditions for the gluon distribution, while predicting a steeper rise in the cross section than suggested by present data. Received: 17 December 1997 / Published online: 10 March 1998     

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.     

Self-consistent diffraction in the multiperipheral model (II)

We present a self-consistent solution to a multi-peripheral model in which the triple pomeron vertex vanishes. This solution possesses all of the good features of our previous model, having naturally diffractive and multiperipheral terms in the total cross section and in particle production. In addition, its region of validity has been increased to include the normal Regge region and a smooth transition from Regge-like behavior to the diffractive limit is obtained.     

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.     

Diffractive Scattering Factorization from J/ψ and γ Associated Production in HERA ep Collisions

Using three sets of Pomeron structure functions, the cross sections of J/ψ and γ associated productionvia resolved photon and proton diffractive scatting in ep collision are investigated. It is found that the cross sectionscalculated with various gluon distribution functions of Pomeron are different. The discrepancy is about 1.8 times fordifferential cross sections and 1.7 times for total cross sections. The experimental studies of the process could give valuableinsight in the diffractive production mechanism and test the color-octet mechanism for heavy quarkonium production ina new environment.