Spin effects in diffractive J/\Psi leptoproduction and the structure of Pomeron coupling

We calculate the cross section and asymmetry of the diffractive J/ leptoproduction for a simple model of a Pomeron coupling with a proton, which contains the term and a spin-dependent part. It is shown that the asymmetry caused by the Pomeron coupling does not vanish for nonzero . The sensitivity of the polarized diffractive J/ production to the spin-dependent part of the Pomeron coupling is found to be rather weak. As a result, it is difficult to study the structure of the Pomeron coupling with the proton in future polarized diffractive experiments on the J/ production. Received: 27 April 1999 / Revised version: 29 June 1999 / Published online: 14 October 1999     

An investigation of a possible hard contribution to φ photoproduction

We investigate the possibility that the process of φ photoproduction may have a significant hard perturbative QCD component. This suggestion is based on a study of the energy dependence of the forward φ photoproduction cross section followed by a calculation where we show that a coherent sum of the pQCD and conventional soft Pomeron contributions provides an excellent reproduction of the experimental data. Our results suggest that the transition from the predominantly soft photoproduction of light ρ and ω vector mesons to the predominantly hard photoproduction of heavy J/Ψ and is smooth and gradual, similar to the transition observed in deep inelastic scattering studies of the proton structure function in the small x limit. Our predictions for higher HERA energies are presented.     

\gamma^* \gamma^* reactions at high energies

The total hadronic cross sections at high energy are calculated as a function of energy and photon virtuality in a model combining Reggeon exchange, the quark box diagram (a fixed pole in Regge language) and soft and hard pomeron exchanges evaluated in the context of dipole-dipole scattering. Good agreement is obtained with the data for the real cross section and for the real photon structure function . However the model prediction for the cross section is too small. This is attributed to an incorrect extrapolation of the dependence of the hard pomeron adopted here. Parametrising it independently shows that the hard part of the cross section can be well represented by a simple Regge pole with intercept . Received: 19 August 1999 / Published online: 3 February 2000     

Soft multihadron production from partonic structure and fragmentation functions

We describe a two-chain model for soft multiparticle production in hadronic collisions. The model is formulated in a parton framework and is consistent with the dual topological scheme for the Pomeron. The sole inputs are valence quark structure functions in the colliding hadrons and parton fragmentation functions, both of which are known from “hard” processes. Our model, which contains no adjustable parameters, reproduces the shape, the energy dependence, and the normalization of inclusive spectra both in the central region and in the fragmentation region. The model provides a natural explanation for the ratio of πp topp cross sections.     

New possibilities of old soft pomeron in DIS

A traditional Regge model with a -independent Pomeron intercept closed (or equal) to one is constructed in order to describe the available data on the proton structure function. A Dipole Pomeron model which does not explicitly violate unitarity is developed and investigated. An excellent agreement with the 1209 data is found () in the whole kinematical domain investigated by experiments. A comparison of the model with already existing ones is made. The x-, slopes and the effective intercept are discussed as and x functions. Received: 9 June 1998 / Revised version: 27 August 1998 / Published online: 14 January 1999     

Solutions of independent DGLAP evolution equations for the gluon distribution and singlet structure functions in the next-to-leading order at low x

We present a set of independent formulas to extract the gluon distribution and the singlet structure function from its derivatives with respect to lnQ ² in the next-to-leading order of perturbation theory at low x based on a hard Pomeron exchange. In this approach, both singlet quarks and gluons have the same high-energy behavior at small x. This approach requires the QCD input parameterizations for independent DGLAP evolutions, which we calculated numerically and compared with the MRST, GRV, and DL models. The Pomeron has a hard nature. Its evolution gives a good fit to the experimental data. The values obtained are in the range 10⁻⁴ ≤ x ≤ 10⁻² at Q ² = 20 GeV². The text was submitted by the author in English.     

Charmonium production in deep inelastic scattering at HERA

The electroproduction of and mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers and photon-proton centre of mass energies GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic production is measured as a function of and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic and meson production is measured as a function of . The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions. Received: 10 March 1999 / Published online: 8 September 1999     

Photoproduction of heavy vector mesons at HERA – a test field for diffraction

Exclusive diffractive photoproduction of heavy vector mesons (V=, and ) at HERA is studied in a model employing a dipole Pomeron exchange (P) with an inelastic vertex. The model is fitted to the data on , B, and for and beyond the threshold region. The elastic cross sections for both and photoproduction show a moderate increase within the HERA energy region. The flattening of the slope B(s) (indicating little or no shrinkage) for is not correlated with the slope of the Pomeron trajectory. Estimates for photoproduction at HERA are given. Received: 11 January 1999 / Published online: 8 September 1999     

Can a pomeron be really “supersoft”?

The low energy J/gf photoproduction cross-section has been studied on the basis of the Pomeron model. To incorporate the discrepancy between experimental data and predictions by conventional models, i.e. the sum of the soft Pomeron with intercept 1.08 and the hard Pomeron with intercept 1.418, a Regge trajectory associated with a scalar meson (f,a) exchange which we call “supersoft” Pomeron, is introduced additionally. To distinguish between the conventional model and this new additional Pomeron, observations related to other polarization observables in upcoming polarized experiments are discussed.     

关于软玻密子的结构

本文修改了Landshoff-Nachtmann的软玻密子(Pomeron P)场论模型.从强子组分夸克具有结构的观点出发,根据高能强作用软过程中的最大非微扰强作用反应假定,提出了有关软P新的结构图像.对撞强子中的一对组分夸克被分解为裸夸克和一系列非微扰胶子(和夸克对).与此相应,软P的结构是由胶子形成的一系列切割梯形图之和所代表.在系统能量s很大而动量转移|t|很小的多重雷吉(Regge)运动学区间,并在保留lns的领头阶近似下,计算了这组切割梯形图之和所对应的散射振幅和总截面.它们的表达式出现了对s的Regge型幂次因子,得出了软P轨迹的简洁表达式.简单说明了按本文提出的方案可以讨论那些相关的强作用过程.     

Deep-inelastic inclusive ep scattering at low x and a determination of \alpha_s

A precise measurement of the inclusive deep-inelastic scattering cross section is reported in the kinematic range GeV and . The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb. The double differential cross section, from which the proton structure function and the longitudinal structure function are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative is observed to rise continuously towards small x for fixed . The cross section data are combined with published H1 measurements at high for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range to within an experimental accuracy of about 3% for GeV. A fit of the H1 measurements and the data of the BCDMS collaboration allows the strong coupling constant and the gluon distribution to be simultaneously determined. A value of is obtained in NLO, with an additional theoretical uncertainty of about , mainly due to the uncertainty of the renormalisation scale. Received: 21 December 2000 / Revised version: 10 May 2001 / Published online: 6 July 2001     

Azimuthal dependence of forward-jet production in DIS in the high-energy limit

As a signal for the BFKL Pomeron in small-x deep inelastic ep scattering, we calculate the azimuthal dependence of the inclusive cross section of forward jets relative to the outgoing electron. For not very large differences in rapidity between the current jet and the forward jet the cross section peaks at π/2. For increasing rapidity BFKL dynamics predicts a decorrelation in the azimuthal dependence between the electron and the forward jet.     

J/\psi-photoproduction and the gluon structure of the nucleon

Using short distance QCD methods based on the operator product expansion, we calculate the photoproduction cross section in terms of the gluon distribution function of the nucleon. Comparing the result with data, we show that experimental behaviour of the cross section correctly reflects the x-dependence of the gluon distribution obtained from deep inelastic scattering. Received: 3 March 1999 / Published online: 28 May 1999     

<Emphasis Type="Italic">N</Emphasis>=4 SYM and QCD motivated approach to soft interactions at high energies

In this paper we construct a model that satisfies the theoretical requisites of high energy soft interactions, based on two ingredients: (i) the results of N=4 SYM, which at present is a unique theory that allows one to deal with a large coupling constant; and (ii) the requirement of matching with high energy QCD. In accordance with these ideas, we assume that the soft Pomeron intercept is relatively large, and the slope of the Pomeron trajectory is equal to zero. We derive analytical formulae that sum both enhanced and semi-enhanced diagrams for elastic and diffractive amplitudes. We fit the available experimental data, and predict the value for cross sections at the energies accessible at the LHC and beyond. The main corrections to the model are studied and evaluated.     

Inclusive Single- and Dijet Rates in Next-to-Leading Order QCD for γ*p and γ*γ Collisions

We present one- and two-jet inclusive cross sections for γ*γ scattering and virtual photoproduction in ep collisions. The hard cross sections are calculated in next-to-leading order QCD. Soft and collinear singularities are extracted using the phase-space-slicing method. The initial state singularity of the virtual photon depends logarithmically its’ virtuality. This logarithm is large and has to be absorbed into the parton distribution function of the virtual photon. We define for this purpose an factorization scheme similar to the real photon case. We numerically study the dependence of the inclusive cross sections on the transverse energies and rapidities of the outgoing jets and on the photon virtuality. The ratio of the resolved to the direct cross section in ep collisions is compared to ZEUS data.     

The triple regge limit of diffractive dissociation in deep inelastic scattering

We derive, within perturbative QCD, a formula for the inclusive cross section of the diffractive dissociation of the deep inelastic photon γ^*+q→X+q in the triple Regge limits?M ²?Q ²?Λ ². We use the leading ln(s/M ², ln(M ²/Q ²) approximation and derive an, expression for the triple Pomeron vertex. The Pomeron above this vertex is found to be of higher order than the BFKL Pomeron. The resulting formula for the cross section is infrared finite. We show that the Abramovsky-Gribov-Kanchelli cutting rules are satisfied, and we discuss implications for the Pomeron structure function.     

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction where PS denotes a pseudoscalar meson , , , or and X either a proton or resonance or continuum state into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in pp and forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering. Received: 2 February 1999 / Revised version: 22 March 1999 / Published online: 28 May 1999