Some Problems of Diffraction at High Energies

An interplay of perturbative and nonperturbative effects in pomeron, odderon and reggeons dynamics is discussed. It is pointed out that experimental data on pion charge exchange reaction at high energies indicate to a dominance of the nonperturbative string-like dynamics up to rather large momentum transfer. Role of shadowing effects related to triple pomeron interactions is investigated. 1 Perturbative versus nonperturbative dynamics for pomeron, odderon and reggeons Interactions of hadrons at high energies and of virtual photons at small Bjorken–x are usually described in terms of reggeon exchanges. For total cross sections and diffractive processes the pomeron exchange plays the dominant role. It has the vacuum quantum numbers: signature σ = +, parity P = + and C–parity=+. In QCD the pomeron is usually related to gluonic exchanges in the t–channel. In this talk I shall discuss the dynamical origin of the pomeron in QCD and a relative role of perturbative and nonperturbative effects. Perturbative QCD in the leading logarithmic approximation leads to the pomeron singularity with an intercept above unity, which corresponds to summation of a ladder-type diagrams with exchanges of reggeized gluons [1] – BFKL pomeron. An increase with energy of total cross sections is determined by the value of ∆ ≡ αP (0)−1: σ tot ∼ s. In the leading approximation the BFKL pomeron has ∆ = 12ln2 π αs ≈ 0.5, which corresponds to a very fast increase of total cross sections and parton densities with energy (1/x). In this approximation the pomeron corresponds to a cut in j-plane. In the next to leading approximation there is a sequence of poles in j-plane concentrated at point j=1 and the value of ∆ substantially decreases to the values ∆ = 0.15÷ 0.25 [2]. The rightmost in j-plane pole strongly depends on nonperturbative region of small momentum transfer [3]. Thus, even in the perturbative approach it is important to have a nonperturbative input in order to describe asymptotic behavior of scattering amplitudes. The role of nonperturbative effects in pomeron dynamics and a connection of the pomeron with spectrum of glueballs were studied in refs. [4], using the method of vacuum correlators [5]. This method, based on general properties of correlators in QCD leads to a successful description of the spectrum of resonances for usual Regge-trajectories and leads to the spectrum of glueballs in a good agreement with lattice calculations [4]. It was shown [4] that confinement effects and mixing of gluonic and qq–Regge-trajectories are important for the pomeron dynamics. Note, that the intercept of the pomeron in this approach can be close to the one predicted in the perturbative QCD. The arising picture of the vacuum trajectories