Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction

Nuclear structure functions at small x and small or moderate $Q^2$ are studied using the relation with diffraction on nucleons which arises from Gribov's Reggeon Calculus. A reasonable description of experimental data is obtained with no fitted parameters. A comparison with other models and predictions for future lepton-ion colliders are provided. Consequences for the reduction of multiplicities in nucleus-nucleus collisions at energies of RHIC and LHC are examined.Comment: LaTeX, 32 pages, 14 eps figures included using epsfig; 1 reference update

Why is the J/ψ\psi suppression enhanced at large transverse energy ?

We study the ratio of $J/\psi$ over minimum bias in $Pb$ $Pb$ collisions at SPS energy. The NA50 data exhibit a sharp turn-over at $E_T \sim 100$ GeV (close to the knee of the $E_T$ distribution) followed by a steady, steep decrease at larger $E_T$. We show that this behaviour can be explained by the combined effects of a small decrease of the hadronic $E_T$ in the $J/\psi$ event sample (due to the $E_T$ taken by the $J/\psi$ trigger), together with the sharp decrease of the $E_T$ distributions in this $E_T$ region (tail). This phenomenon does not affect the (true) ratio $J/\psi$ over $DY$ (obtained by the NA50 standard analysis), but does affect the one obtained by the so-called minimum bias analysis. A good agreement is obtained with the data coming from both analysis -- as well as with the ratios of $J/\psi$ and $DY$ over minimum bias -- in the whole $E_{T}$ region.Comment: 20 pages, 4 postscript figures One modified equation and one added table. No modifications in results and figure

Percolation Effects in Very High Energy Cosmic Rays

Most QCD models of high energy collisions predict that the inelasticity $K$ is an increasing function of the energy. We argue that, due to percolation of strings, this behaviour will change and, at $\sqrt{s} \simeq 10^4$ GeV, the inelasticity will start to decrease with the energy. This has straightforward consequences in high energy cosmic ray physics: 1) the relative depth of the shower maximum $\bar{X}$ grows faster with energy above the knee; 2) the energy measurements of ground array experiments at GZK energies could be overestimated.Comment: Correction of equation (19) and figures 3 and 4. 4 pages, 4 figure

Charmonium Suppression in Lead-Lead Collisions: Is There a Break in the J/ψJ/\psi Cross-Section?

In the framework of a model based on nuclear absorption plus comover interaction, we compute the $E_{T}$ distribution of the $J/\psi$ in $PbPb$ collisions at SPS and compare it with available NA50 data. Our analysis suggests that the existence of new physics (deconfinement phase transition) in the region E_{T} \lsim 100 GeV is unlikely and that signals of new physics should rather be searched in the region E_T \gsim 100 GeV. The $E_{T}$ dependence of the $J/\psi$ transverse momentum has been computed. At large $E_T$ it turns out to be much flatter in the comover approach than in a phase transition framework. Estimates of the $J/\psi$ suppression at RHIC and LHC energies are also given.Comment: 31 pages, LaTeX, 9 postscript figures included using epsfig; references updated and comments added; final version to appear in Phys. Rev.

Deep Inelastic Scattering Data and the Problem of Saturation in Small-x Physics

We investigate the role of unitarization effects in virtual photon-proton (\gamma^*p) interactions at small x. The q\bar{q}-fluctuation of the initial photon is separated into a small distance and a large distance component and a model for the unitarization of each component is proposed. The Born approximation for the small size component is calculated using QCD perturbation theory. Reggeon diagram technique is used in order to obtain a self-consistent scheme for both total \gamma^*p cross section and diffractive production. The model gives a good description of HERA data in the small-x region, with a single Pomeron of intercept 1.2.Comment: 27 pages, 8 postscript figure

Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach

The forward-backward correlations in the $p_T$ distributions, which present a clear signature of non-linear effects in particle production, are studied in the model of percolating colour strings. Quantitative predictions are given for these correlations at SPS, RHIC and LHC energies. Interaction of strings also naturally explains the flattening of $p_T$ distributions and increase of $$ with energy and atomic number for nuclear collisionsComment: 6 pages in LaTex, 3 figures in Postscrip

Production of Secondaries in High Energy d+Au Collisions

In the framework of Quark-Gluon String Model we calculate the inclusive spectra of secondaries produced in d+Au collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. The results of numerical calculations at intermediate energies are in reasonable agreement with the data. At RHIC energies numerically large inelastic screening corrections (percolation effects) should be accounted for in calculations. We extract these effects from the existing RHIC experimental data on minimum bias and central d+Au collisions. The predictions for p+Au interactions at LHC energy are also given.Comment: 18 pages and 10 figure