Triple pomeron vertex in the limit N_c\rightarrow\infty

In the hard pomeron theory with the number of colours the diffractive amplitude obtained in [3] is compared with the results found for in [1] and in the dipole approach in [5]. It is shown that the double pomeron exchange contribution can be substituted by an equivalent triple pomeron interaction term. After such a substitution the triple pomeron vertices in [1,3,5] essentially coincide. It is demonstrated that, in any form, the triple pomeron vertex is conformal invariant. It is also shown that higher order densities in the dipole approach do not involve 1 to k pomeron verteces with but are rather given by a set of pomeron fan diagrams with only a triple pomeron coupling. Received: 11 December 1997 / Revised version: 6 February 1998 / Published online: 7 August 1998     

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     

The problem of the relation between double pomeron exchange in inclusive and exclusive experiments

The pionization region of the inclusive single-particle spectrum is accounted for by double pomeron exchange in the absorptive part of a six-point amplitude. In this paper a multiperipheral model for the six-point amplitude with double pomeron exchange is used for continuation by crossing and analyticity to the physical region of the exclusive two particle → four particle production process. The cross section for π^? p → π^?(π⁺π^?)p in the double-Regge region is then calculated and compared with the experimental analysis of Lipes, Zweig and Robertson which sets an upper bound to the strength of the double pomeron exchange coupling. This upper bound, coupled with the model for continuation to the inclusive cross section, is shown to give too small a magnitude for the double pomeron exchange in the pionization region. Further avenues for investigation are discussed.     

Evolution of the gluon density in x with a running coupling constant

We describe the calculation of the evolution of the gluon density in from an initial value to smaller values, up to in the hard pomeron formalism with a running coupling introduced on the basis of the bootstrap equation. The obtained gluon density is used to calculate the singlet part of the proton structure function. Comparison with experiment and the results following from the fixed coupling evolutio n is made. Received: 10 June 1997 / Revised version: 20 August 1997 / Published online: 24 April 1998     

The n-jet inclusive cross-section in the Hard Pomeron model

The inclusive production of several jets is studied in the framework of the hard pomeron model. The average jet momenta are found to be strongly ordered and growing towards the central c.m. rapidity, as expected. Strong positive correlations are found for pairs of jets neighbouring in rapidity, both in the forward-backward and forward-forward directions. Received: 29 September 1997 / Published online: 26 February 1998     

Survival probability for diffractive Higgs production in high density QCD

In this paper, the contribution of hard processes described by BFKL pomeron exchange is taken into account by calculating the first enhanced diagram. The survival probability is estimated, using the ratio of the first enhanced diagram and the single pomeron amplitude, taking into account all essential pomeron loop diagrams in the toy model of Mueller. The triple pomeron vertex is calculated explicitly in the momentum representation. This calculation is used for estimating the survival probability, and it turns out that the survival probability is small, at 0.4%. Hard pomeron rescattering processes contribute substantially to the survival probability.     

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     

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.     

On the triple-Pomeron vertex in perturbative QCD

We estimate the size of the triple-pomeron vertex in perturbative QCD and compare with the phenomenological value extracted from Regge fits to the experimental data. For simplicity, the results of the QCD analysis are taken in the large-N_c limit. We find that the perturbative triple-pomeron coupling is of the same order of magnitude as the observed one. We also estimate the size of the pomeron self-energy and its contribution to the renormalization of the pomeron intercept. The effect is very small, in agreement with previous non-perturbative estimates. Received: 9 October 2002 / Published online: 20 December 2002     

High energy proton-proton cross-section and Froissart bound

A model for high energy proton-proton system is developed which obeys several asymptotic bounds including Froissart bound. The amplitude taken has two parts, a Regge part and a pomeron part. The Regge part is described by the exchange degenerate pairw andP′. To calculate the pomeron part the Freund-Harari duality is invoked which states that the pomeron is dual to thes-channel background. Thes-channel background is estimated by parametrising the branch cuts inS through a conformal mapping method. Expressions for total cross-section, ratio of real-to-imaginary parts of forward amplitude and the slope parameter are presented. The corresponding quantities for are also evaluated. Our analysis favours strong exchange degeneracy.     

Inclusive Single Particle Distributions from the Dual Fermion Model

Inclusive single particle distributions are calculated from the discontinuity of a phenomenological 4π NN amplitude obtained from the Neveu-Schwarz-Ramond dual fermion model. In contrast to former calculations using the conventional Veneziano B₆-function we get a correct triple Regge behaviour at x ≈ 1 with correct pole position in the t-channel which is able to reproduce the data. The pomeron, not contained in the dual Born amplitude, is taken into account phenomenologically in the NN channel. Other pomeron contributions are subtracted from the data. We compare the inclusive pion distributions of the model in the beam fragmentation and central region with experimental data for reactions π + N → + X at 8 and 16 GeV/c.     

Total \gamma^{\star}\gamma^{\star} cross section and the QCD dipole picture

In the framework of the dipole picture of the BFKL pomeron we discuss two possibilities of calculating the total cross section of the virtual photons. It is shown that the dipole model reproduces the results obtained earlier from -factorization up to the selection of the scale determining the length of the QCD cascade. The choice of scale turns out to be important for the numerical outcome of the calculations. Received: 12 June 1997 / Published online: 20 February 1998     

Spin effects in proton-proton elastic scattering near the forward direction

We have analysed the several sets of experimental data on pp elastic scattering in the Coulomb interference region in the range p_lab between 9 and 500 GeV/c, in order to estimate the size of the spin dependent part of the strong amplitude. Calling β the ratio of this spin-dependent part to the spin-dependent amplitude in the forward direction, we find all the data strongly suggest but do not conclusively prove a non-negligible β whose value is around 10% and approximately constant with energy. This result cannot be explained theoretically in terms of exchange of any simple factorising Regge pole. We suggest important pomeron cut contributions. Implications of our analysis for some experiments at ISR energies which appear to give rising total cross sections are discussed.     

The pomeron colour structure and hadron scattering on the six-quark deuteron configuration

The pomeron colour structure provides some contribution to the elastic hadron-deuteron amplitude not contained in the Glauber like approach. This contribution is very sensitive to the short distance behaviour of the deuteron wave function and poses one to fix with a high precision a size of the deuteron six-quark bag.     

On the re-summation of enhanced pomeron diagrams

Dominant contributions of enhanced pomeron diagrams to elastic hadron–hadron scattering amplitude are re-summed to all orders. The formalism is applied to calculate total hadronic cross sections and elastic scattering slopes. An agreement with earlier results is obtained.     

A consistent next-to-leading-order QCD calculation of hadronic diffractive scattering

We calculate the order _s² and order _s³ QCD contributions to colour-singlet exchange in the leading log s approximation. We implement the resulting amplitude at the hadronic level and thus construct the QCD pomeron and odderon to this order of perturbation theory. We show that the structure of the hadronic form factors provides a natural mechanism through which the odderon gets suppressed at t = 0 whereas it dominates the elastic cross section at large t. We also demonstrate that the inclusion of nonperturbative effects through a modification of the gluon propagator accelerates greatly the convergence of the log s expansion, although not enough to provide agreement with the data.     

Coulomb interference in high-energy pp and $\bar p p$ scattering

An analysis of the Coulombic amplitude and its interference with the nuclear amplitude which is driven by the three-component pomeron is presented. It is shown that different approaches towards the evaluation of the Coulomb phase give approximately uniform results at all energies and the differences are negligible at RHIC and LHC energies. We show that the use of the amplitude which was fitted to accommodate the nucleon data only (in the region (GeV²)), combined with the Coulomb amplitude, reproduces the existing data in the Coulomb interference domain quite accurately without any adjustment of the parameters. As a consequence, we predict the differential cross section in the region of the Coulomb nucleon interference for both RHIC and LHC energies. Received: 3 June 2002 / Revised version: 27 February 2003 / Published online: 5 May 2003 RID="a" ID="a" e-mail: petrov@mx.ihep.su RID="b" ID="b" e-mail: predazzi@to.infn.it RID="c" ID="c" e-mail: prokudin@to.infn.it