Single Spin Asymmetry in Heavy Flavor Photoproduction as a Test of pQCD

We analyze in the framework of pQCD the properties of the single spin asymmetry in heavy flavor production by linearly polarized photons. At leading order, the parallel- perpendicular asymmetry in azimuthal distributions of both charm and bottom quark is predicted to be about 20% in a wide region of initial energy. Our analysis shows that the next-to-leading order corrections practically do not affect the Born predictions for the azimuthal asymmetry at energies of the fixed target experiments. Both leading and next-to-leading order predictions for the asymmetry are insensitive to within few percent to theoretical uncertainties in the QCD input parameters: $m_{Q}$, $\mu_{R}$, $\mu_{F}$, $\Lambda_{QCD}$ and in the gluon distribution function. We estimate also nonperturbative contributions to azimuthal distributions due to the gluon transverse motion in the target and the final quark fragmentation. Our calculations show that nonperturbative corrections to a $B$-meson azimuthal asymmetry are negligible. We conclude that measurements of the single spin asymmetry would provide a good test of pQCD applicability to heavy flavor production at fixed target energies.Comment: 15 pages, ReVTeX, 13 .eps figures. The subsection added to correspond to journal versio

Baryon stopping and hyperon enhancement in the improved dual parton model

We present an improved version of the dual parton model which contains a new realization of the diquark breaking mechanism of baryon stopping. We reproduce in this way the net baryon yield in nuclear collisions. The model, which also considers strings originating from diquark-antidiquark pairs in the nucleon sea, reproduces the observed yields of p and Lambda and their antiparticles and underestimates cascades by less than 50 %. However, Omega's are underestimated by a factor five. Agreement with data is restored by final state interaction, with an averaged cross-section as small as 0.14 mb. Hyperon yields increase significantly faster than antihyperons, in agreement with experiment.Comment: 40 pages, 18 postscript figure

Non-saturation of the J/psi suppression at large transverse energy in the comovers approach

We show that, contrary to recent claims, the $J/\psi$ suppression resulting from its interaction with comovers does not saturate at large transverse energy $E_T$. On the contrary, it shows a characteristic structure - change of curvature near the knee of the $E_T$ distribution - which is due to the $E_T$ (or multiplicity) fluctuation, and agrees with recent experimental results.Comment: 12 pages, latex, 2 postscript figure

Baryon Number Transfer in Hadronic Interaction

The process of baryon number transfer due to string junction propagation in rapidity is considered. It has a significant effect in the net baryon production in $pp$ collisions at mid-rapidities and an even larger effect in the forward hemisphere in the cases of $\pi p$ and $\gamma p$ interactions. The results of numerical calculations in the framework of the Quark-Gluon String model are in reasonable agreement with the data.Comment: Latex, 17 figs. in ps files, 1 table. Submitted to EPJ C The title of paper were changed, some proovings in the text, references and in figs were mad

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

Nuclear Shadowing and Diffraction

The relation between diffraction in lepton-proton collisions and shadowing of nuclear structure functions which arises from Gribov inelastic shadowing, is described. A model realizing such relation, which produces a parameter-free description of experimental data on nuclear structure functions at small $x$, is presented. The application to the description of multiplicities in nuclear collisions is discussed and related to other approaches.Comment: LaTeX, 6 pages, 5 eps figures, uses enclosed ws-ijmpa.cls; invited talk given by N. Armesto at the Eighth Workshop on Non-Perturbative Quantum Chromodynamics, Paris, France, June 7th-11th 200

Anomalous suppression of π0\pi^0 production at large transverse momentum in Au + Au and d + Au collisions at sNN=\sqrt{{\bf s}_{\bf NN}} = 200 GeV

We propose a model of suppression of large $p_T$-pions in heavy ion collisions based on the interaction of the large $p_T$ pion with the dense medium created in the collision. The model is practically the same as the one previously introduced to describe $J/\psi$ suppression. Both the $p_T$ and the centrality dependence of the data are reproduced. In deuteron-gold collisions, the effect of the final state interaction with the dense medium turns out to be negligibly small. Here the main features of the data are also reproduced both at mid and at forward rapidities.Comment: 20 pages, latex, 3 postscript figures. Last version, to be published in EJP

Fluctuations and Correlations in STAR

The study of correlations and fluctuations can provide evidence for the production of the quark-gluon plasma (QGP) in relativistic heavy ion collisions. Various theories predict that the production of a QGP phase in relativistic heavy ion collisions could produce significant event-by-event correlations and fluctuations in, transverse momentum, multiplicity, etc. Some of the recent results using STAR at RHIC will be presented along with results from other experiments at RHIC. The focus is on forward-backward multiplicity correlations, balance function, charge and transverse momentum fluctuations, and correlations.Comment: 6 pages, 13 figures, Invited talk at QNP06, Madrid, Spain,June 200

Hard diffraction in hadron--hadron interactions and in photoproduction

Hard single diffractive processes are studied within the framework of the triple--Pomeron approximation. Using a Pomeron structure function motivated by Regge--theory we obtain parton distribution functions which do not obey momentum sum rule. Based on Regge-- factorization cross sections for hard diffraction are calculated. Furthermore, the model is applied to hard diffractive particle production in photoproduction and in $p\bar{p}$ interactions.Comment: 13 pages, Latex, 13 uuencoded figure