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.     

Regge Models of Proton Diffractive Dissociation Based on Factorisation and Structure Functions

Recent results by the authors on proton diffractive dissociation (single, double and central) in the low-mass resonance region with emphasis on the LHC kinematics are reviewed and updated. Based on the previous ideas that the contribution of the inelastic proton–Pomeron vertex can be described by the proton structure function, the contribution of the inelastic Pomeron–Pomeron vertex appearing in central diffraction is now described by a Pomeron structure function.     

J/\psi +{\mathrm{jet}} diffractive production in the direct photon process at HERA

We present a study of the diffractive production in direct photon processes at HERA based on the factorization theorem for lepton-induced hard diffractive scattering and the factorization formalism of the non-relativistic QCD (NRQCD) for quarkonia production. Using the diffractive gluon distribution function extracted from HERA data on diffractive deep inelastic scattering and diffractive dijet photon production, we demonstrate that this process is measurable at DESY HERA. The experimental study of this process can give valuable insight in the diffractive production mechanism and test the color-octet mechanism for heavy quarkonia production in a new environment. Received: 13 September 1999 / Revised version: 31 December 1999 / Published online: 6 April 2000     

Unitarity bounds for inelastic diffraction

Unitarity in the s-channel is invoked to derive an upper bound for the inelastic diffractive cross-section as a function of impact parameter. The application of this bound to high-energy proton-proton scattering strongly suggests that inelastic diffraction should be peripheral in the impact parameter space.     

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states. Received: 17 September 1997 / Published online: 23 February 1998     

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.     

Large-angle scattering of heavy ions: (II). General structure of inelastic cross section

Closed-form expressions are derived for the differential cross section of inelastic heavy-ion scattering at large angles. The inelastic transition amplitude given by the distorted-waves theory for excitation of low-lying collective states is evaluated by an extension of analytic methods developed in the preceding paper for elastic large-angle scattering. The very close relation between the inelastic and elastic cross sections is displayed. In particular it is shown that the angular distributions have a universal form, and that the backward-angle inelastic excitation function has, aside from a slowly varying overall energy dependence, an oscillatory gross structure of diffractive origin which is nearly identical to that of the elastic excitation function, irrespective of the physical mechanism involved.     

Rydberg molecules in external fields: a semiclassical analysis

We analyze the spectra of simple Rydberg molecules in static fields within the framework of closed/periodic-orbit theories. We conclude that in addition to the usual classical orbits one must consider classically forbidden diffractive paths. Further, the molecule brings in a new type of "inelastic" diffractive trajectory in addition to the usual "elastic" diffractive orbits encountered in systems with point scatterers. The relative importance of inelastic versus elastic diffraction is quantified by merging the usual closed orbit theory framework with molecular quantum defect theory.     

Surface Corrugation in Rotational and Diffractive Scattering of O2 from LiF (001)

A quantum dynamic calculation on a five-dimensional O₂/LiF (001) model system is performed using the multi-configuration time-dependent Hartree method. The obtained results show that the mechanism of rotational and diffractive excitation in details: Comparison with the rotational excited state, the initially non-rotational state is seen to favor the inelastic scattering in the rotational excitation process. The surface corrugation can damp the quantum interferences and produce a greater amount of rotational inelastic scattering at the expense of the elastic process in the rotational excitation process. The diffraction process and the average energy transferred into the rotational and diffractive mode are also discussed.     

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.     

Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity neutral meson production in virtual photon proton collision at high energies. Due to the opposite -parities of photon and meson () this process probes the -channel odderon exchange which is described here as noninteracting three–gluon exchange. Estimates for the cross section of inelastic diffractive process are presented. The total cross section of diffractive meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons () in collisions are of the same order if the photon virtuality is . Received: 30 July 1997     

Fragmentation models and unitarity

We use the diffractive excitation model to evaluate the inelastic part of the proton-proton unitarity equation. Conventional assumptions about the phase of the vacuum singularity, and an experimentally motivated choice of mass dependence for the diffractive slope, lead to an upper bound for the proportion of diffractively excited inelastic processes. We also discuss the apparent breakdown of factorization.     

Diffractive dijets with a leading antiproton in &pmacr;p collisions at sqrt

We report results from a study of events with a leading antiproton of beam momentum fraction 0.9057 GeV. Using the dijet events, we evaluate the diffractive structure function of the antiproton and compare it with expectations based on results obtained in diffractive deep inelastic scattering experiments at the DESY ep collider HERA.     

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     

Diffractive parton distributions from the saturation model

We review diffractive deep inelastic scattering (DIS) in the light of the collinear factorization theorem. This theorem allows one to define diffractive parton distributions in the leading twist approach. Due to its selective final states, diffractive DIS offers interesting insight into the form of the diffractive parton distributions which we explore with the help of the saturation model. We find Regge-like factorization with the correct energy dependence measured at HERA. A remarkable feature of diffractive DIS is the dominance of the twist-4 contribution for small diffractive masses. We quantify this effect and make a comparison with the data. Received: 22 February 2001 / Revised version: 22 March 2001 / Published online: 3 May 2001     

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.