Elastic production of vector mesons $\gamma^{*} N\to V N$ is the pomeron-exchange dominated diffractive reaction with much potential of probing the BFKL pomeron. The BFKL pomeron can conveniently be described in terms of the dipole cross section which is a solution of the generalized BFKL equation. In this paper we discuss, how the energy and $Q^{2}$ dependence of elastic production of vector mesons at HERA will allow scanning the dipole cross section as a function of dipole size $r$. We show that determinaton of the intercept of the BFKL pomeron requires measuring the $\rho^{0}$ and $J/\Psi$ production at $Q^{2} \sim (100-200)GeV^2$ and/or the quasireal photoproduction of the $\Upsilon$. We present predictions for the effective intercept in the kinematic range of the forthcoming HERA experiments, which can shed much light on the nonperturbative component of the pomeron.Comment: 15 pages, $ 6 figures (to be requested

Recent data from RHIC for high-pT hadrons in gold-gold collisions raised again the long standing problem of quantitatively understanding the Cronin effect, i.e. nuclear enhancement of high-pT hadrons due to multiple interactions in nuclear matter. In nucleus-nucleus collisions this effect has to be reliably calculated as baseline for a signal of new physics in high-pT hadron production. The only possibility to test models is to compare with available data for pA collisions, however, all existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows to explain available data without fitting to them and to provide predictions for pA collisions at RHIC and LHC. We point out that the mechanism causing Cronin effect drastically changes between the energies of fixed target experiments and RHIC-LHC. High-pT hadrons are produced incoherently on different nucleons at low energies, whereas the production amplitudes interfere if the energy is sufficiently high. PACS: 24.85.+p, 13.85.Ni, 25.40.Qa It was first observed back in 1975 [1] that high-p T hadrons are not suppressed in proton-nucleus collisions, but produced copiously. This effect named after James Cronin demonstrates that bound nucleons cooperate producing high-p T particles. Indeed, it has been soon realized that multiple interactions which have a steeper than linear A-dependence lead to the observed enhancement. An adequate interpretation of the Cronin effect has become especially important recently in connection with data from RHIC for high-p T hadron production in heavy ion collisions [2,3]. The observed suppression factor can be understood as a product of two terms. One is due to multiple interactions within the colliding nuclei, analogous to the Cronin effect. The second factor arises from final state interaction with the produced medium, the properties of which are thus probed. This second factor, the main goal of the experiment, can be extracted from data only provided that the Cronin effect for nuclear collisions can be reliably predicted. However, in spite of the qualitative understanding of the underlying dynamics of this effect, no satisfactory quantitative explanation of existing pA data has been suggested so far. Available models contain parameters fitted to the data to be explained (e.g. see [4,5,6]) and miss important physics. In this paper we suggest a comprehensive description of the dynamics behind the Cronin effect resulting in parameterfree predictions which agree with available data.First of all, the mechanism of multiple interactions significantly changes with energy. At low energies a highk T parton is produced off different nucleons incoherently, while at high energies it becomes a coherent process. This is controlled by the coherence lengthwhere k T is the transverse momentum of the parton produced at mid rapidity and then hadronizing into the detected hadron with transverse momentum p T . For a coherence length which is...

Recent measurements by the BRAHMS collaboration of high-pT hadron production at forward rapidities at RHIC found the relative production rate (d − Au)/(p − p) to be suppressed, rather than enhanced. Examining other known reactions (forward production of light hadrons, the DrellYan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large xF . Since this is the region where x2 is minimal, it is tempting to interpret this as a manifestation of coherence, or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with data.