We calculated the single spin asymmetries for the reaction $P+P(\uparrow)\rightarrow l \overline{l}+X$ in the framework of twist-3 QCD for HERA energies. The necessary imaginary phase is produced by the on-shell contribution of the quark propagator, while the long distance part is analogous to that providing the direct photon asymmetry calculated by J. Qiu and G. Sterman. The asymmetry turns out to be generally of the order percent.Comment: 7 pages, 4 figures, LaTe

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.

The extend to which geometrical effects contribute to the production and suppression of the J/ψ and qq minijet pairs in general is investigated for high energy heavy ion collisions at SPS, RHIC and LHC energies. For the energy range under investigation, the geometrical effects referred to are shadowing and anti-shadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; f A = Af N . The strength of the shadowing/anti-shadowing effect increases with the mass number. Therefore it is interesting to see the difference between cross sections for e.g. S + U vs. P b+ P b at SPS. The recent NA50 results for the survival probability of produced J/ψ's has attracted great attention and are often interpreted as a signature of a quark gluon plasma. This publication will present a fresh look on hard QCD effects for the charmonium production level. It is shown that the apparent suppression of J/ψ's must also be linked to the production process. Due to the uncertainty in the shadowing of gluons the suppression of charmonium states might not give reliable information on a created plasma phase at the collider energies soon available. The consequences of shadowing effects for theGeV, √ s = 200 GeV and √ s = 6 TeV are calculated for some relevant combinations of nuclei, as well as the p T distribution of minijets at midrapidity for N f = 4 in the final state.1 Work supported by BMBF, DFG, GSI 1 1. Introduction Since the advent of QCD in the 70's great emphasis was laid on the existence of a phase transition of, yet unknown, order, being typical for nonabelian gauge field theories. From lattice calculations it was emphasized that, at zero chemical potential, a phase transition should show up at some temperature T c ≈ 150 −200 MeV when explicitly taking quarks into account. The value for T c is slightly higher for a pure gauge theory. Also, at non-zero chemical potential, as suggested in the MIT bag model, one should access a phase transition due to the increasing outward pressure of the partons inside the bag finally leading to a deconfined phase. Due to the difficulties emerging when considering dynamical fermions the work on non-zero chemical potential has not yet reached the same level of success as that for µ = 0 in lattice QCD. Now, in actual high energy heavy ion collisions the following scenario can occur. Two streams of initially cold nuclear matter collide and may result in a plasma phase, which is created within the transverse dimension of approximately the size of the overlapping nuclei. The plasma cools down to form hadronic degrees of freedom in the subsequent expansion. If one has this phase transition in mind one also has to confront the question of its experimenatal detection. Typical signatures under discussion are leptonic (dilepton [1] and photon [2] production due to the interactions among the quasi free partons via the different QCD processes qq → γg, gq → γq, ...) and hadronic ones, such as the suppression of J/ψ's....

The transverse momentum distribution of prompt photons coming from the very early phase of ultrarelativistic heavy-ion collisions for the ultrarelativistic energies is calculated by means of perturbative QCD. We calculate the single photon cross section (AϩB→␥ϩX) by taking into account the partonic subprocesses q ϩq →␥ϩg and qϩg→␥ϩq as well as the bremsstrahlung corrections to those processes. We choose a lower momentum cutoff k 0 ϭ2 GeV separating the soft physics from perturbative QCD. We compare the results for those primary collisions with the photons produced in reactions of the thermalized secondary particles, which are calculated within scaling hydrodynamics. The QCD processes are taken in leading order. Nuclear shadowing corrections, which alter the involved nuclear structure functions are explicitly taken into account and compared to unshadowed results. Employing the GRV parton distribution parametrizations we find that at ultrarelativistic energies prompt QCD photons dominate over the thermal radiation down to transverse momenta k T Ϸ2 GeV. At LHC, however, thermal radiation from the quark-gluon plasma dominates for photon transverse momenta k T р5 GeV, if nuclear shadowing effects on prompt photon production are taken into account. ͓S0556-2813͑98͒05406-5͔PACS number͑s͒: 24.85.ϩp, 25.75.Ϫq, 12.38.Mh

We calculate the shadowing of sea quarks and gluons and show that the shadowing of gluons is not simply given by the sea quark shadowing, especially at small x. The calculations are done in the lab frame approach by using the generalized vector meson dominance model. Here the virtual photon turns into a hadronic fluctuation long before the nucleus. The Ž ) . Ž ) . subsequent coherent interaction with more than one nucleon in the nucleus leads to the depletion s g A -As g N known as shadowing. A comparison of the shadowing of quarks to E665 data for 40 Ca and 207 Pb shows good agreement.