Studies of unitarity at small x using the dipole formulation

Mueller's dipole formulation of onium-onium scattering is used to study unitarity corrections to the BFKL power growth at high energies. After a short discussion of the spatial distribution of colour dipoles in a heavy quarkonium and the associated fluctuations, results are presented showing that the one- and two-pomeron contributions to the total cross section are the same at a rapidity Y ≅ 14. Above this rapidity the large fluctuations in the onium wave function cause the multiple pomeron series to diverge. Resumming the series allows one to show that unitarity corrections set in gradually for the total cross section, which is dominated by rare, large, configurations of the onia. The elastic cross section comes mostly from much smaller impact parameters and has significant unitarity corrections starting at a rapidity Y ≅ 8.     

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     

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.     

坡密子和胶子球

讨论了高能强子 -强子散射过程中的坡密子以及坡密子的 QCD内容 .认为坡密子有可能就是具有量子数 IG=0 + ,JPC=2 + +的张量胶子球 ξ( 2 2 30 ) .用雷其化胶子球模型计算了高能质子 -质子散射截面和坡密子 -核子的耦合参数 β.理论结果与实验的要求一致. The pomeron in high energy hadron hadron scattering as well as the QCD nature of the pomeron are discussed. We claimed that the pomeron may be the tenser glueball ξ(2 230) with quantum number I G =0 +, J PC =2 ++ . Under this reggeized glueball model the cross section of high energy proton proton scattering and the coupling parameter of the pomeron nucleon, β , are calculated. The theoretical results of the present model are in good agreement with experimental data.     

\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     

On the QCD pomeron at large momentum transfer

We investigate the asymptotics of the QCD scattering amplitude with vacuum quantum number exchange in the region where the energy is large and the momentum transfer is much smaller than the energy but large compared to the hadronic scale. The appropriate modification of the QCD pomeron partial wave equation is derived. The singularities in the angular momentum which arise at relatively large momentum transfer are studied in double logarithmic approximation.     

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     

The first three pomerons...

A model of a three pomeron contribution to high energy elastic pp and scattering is proposed. The data are well described for all momenta ( GeV) and energies ( GeV) (). The model predicts the appearance of two dips in the differential cross-section which will be measured at LHC. The parameters of the pomeron trajectories are . Received: 11 July 2001 / Published online: 20 December 2001     

Double Diffractive Dissociation Process p + p(p) → Xp + Xp(p) + Π+ + Π- at High Energy and the Pomeron Model of Donnachie-Landshoff

Large p_γ charged pion pair produced in Double Diffractive Dissocjation (DDD) process at high energy is studied with double pomeron (IP) exchange mechanism. Using DonnachieLandshoff pomeron model we have calculated the cross-section of the process at the energy interval from ISR to LHC when the rapidity of the charged pions is approximately equal to zero. It is shown that this process is the favorite in experiments which could effectively be used to examine Donnachie-Landshoff pomeron model and provide a possibility to check the value of the suppression coefficient N².     

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.     

Predictions for high-energy real and virtual photon–photon scattering from color dipole BFKL–Regge factorization

High-energy virtual photon–virtual photon scattering can be viewed as an interaction of small size color dipoles from the beam and target photons, which makes scattering at high energies (LEP, LEP200 and NLC) an indispensable probe of the short distance properties of the QCD pomeron exchange. Based on the color dipole representation, we investigate the consequences for the scattering of the incorporation of asymptotic freedom into the BFKL equation which makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL–Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to scattering and DIS off protons. Numerical predictions based on our early works on the color dipole BFKL phenomenology of DIS on protons are in good agreement with the experimental data on the photon structure function and the most recent data on the cross section from the OPAL and L3 experiments at LEP200. We discuss the role of non-perturbative dynamics and predict a pronounced effect of the Regge-factorization breaking due to large unfactorizable non-perturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL–Regge expansion for scattering including the issue of the decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance. Received: 9 January 2001 / Revised version: 25 September 2001 / Published online: 7 December 2001     

From color glass to color dipoles in high-energy onium–onium scattering

Within the Color Glass formalism, we construct the wavefunction of a high energy onium in the BFKL and large-N_c approximations, and demonstrate the equivalence with the corresponding result in the Color Dipole picture. We propose a simple factorization formula for the elastic scattering between two non-saturated “color glasses” in the center-of-mass frame. This is valid up to energies which are high enough to allow for a study of the onset of unitarization via multiple pomeron exchanges. When applied to the high energy onium–onium scattering, this formula reduces to the Glauber-like scattering between two systems of dipoles, in complete agreement with the dipole picture.     

Double pomeron exchange in high-energy pp collisions

The double pomeron exchange cross section in high-energy pp collisions is analyzed in detail for different phase-space cuts. The ratio of double pomeron exchange signal to diffraction dissociation background is examined as a function of s. The theoretical predictions are found to be in excellent agreement with the recent results of the experiments at the CERN ISR. The properties of the central cluster as well as the predictions for high energies are discussed.     

Pomeron as a Reggeized Tensor Glueball

We study gluonic content of the pomeron and propose that the pomeron could be a reggeized tensor glueball ζ(2230) with quantum numbers I^GJ^Pc = 0⁺2⁺⁺.This conjecture is examined in high energy proton-proton elastic scattering,and the calculations lend a favorable support to our physical idea.     

Scattering from non-overlapping potentials. I. General formulation

The problem of scattering from an assembly of non-overlapping spherical potentials is solved in partial-wave basis for each of the constituent potentials. The resulting scattering operator is a quotient of two infinite matrices and depends on “on-shell” partial wave amplitudes of the individual potentials. It suggests in general a truncation scheme which essentially considers only those partial waves effective for each collision at the given energy. The multiple-scattering series is recovered and limiting cases of low energy and high energy are considered. Applications to high-energy scattering of elementary particles on nuclei are briefly discussed.     

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.     

Exploiting the equivalence of reggeon field theory in zero transverse dimensions and reaction–diffusion processes

The reggeon field theory in zero transverse dimensions is investigated. Two versions of the theory are considered: one that allows for at most triple pomeron interactions and the other that embodies an additional 2→2 quartic reggeon coupling. The behavior of the scattering amplitude at asymptotic rapidities is obtained in both cases. In an s-channel picture of the high energy scattering both models can be viewed as reaction–diffusion processes. We derive known results in reggeon field theory rather easily using the reaction–diffusion formalism. We find that some results which are surprising from the reggeon field theory point of view turn out to have a simple interpretation from the reaction–diffusion point of view.