The effect of shadowing on the early state of ultrarelativistic heavy ion collisions is investigated along with transverse energy and hard process production, specifically Drell-Yan, J/ψ, and Υ production. We choose several parton distributions and parameterizations of nuclear shadowing, as well as the spatial dependence of shadowing, to study the influence of shadowing on relevant observables. Results are presented for Au+Au collisions at √ s N N = 200 GeV and Pb+Pb collisions at √ s N N = 5.5 TeV.

Production of electron-positron pairs with invariant masses less than 300 MeV from thermalized hadronic matter in relativistic heavy ion collisions is calculated using a soft virtual photon approximation. The general theoretical framework is reviewed and extended to include arbitrarily massed and charged reaction partners, which we apply to pions and quarks. This result, exact within the soft photon approximation, is compared with a widely used approximate result which uses an electromagnetic amplitude limited in validity to momentum transfers less than 4m 2 π(q) . This momentum-restricted method works very well for pions, constituent quarks and medium quarks; whereas, it fails when applied to current quarks. A field theory calculation is performed for the ππ elastic cross section which gives excellent agreement with data. Quark-antiquark annihilation diagrams in the Born approximation are estimated and compared with π 0 and η Dalitz decay contributions to the e + e − invariant mass spectra. A comparison is made between the rate of production of zero total momentum soft dileptons obtained using resummation techniques in QCD perturbation theory to that which we calculate using this soft photon approximation. Then a Bjorken picture for the evolution is adopted allowing an integration over the history of the colliding nuclei. Using initial conditions likely to be found at RHIC or LHC we conclude even if Dalitz decays can be identified and subtracted from the experimental data it will be difficult to distinguish quark from pion degrees of freedom in the low-mass e + e − spectra.

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We explore how nuclear modifications to the free nucleon structure functions (also known as shadowing) affect heavy quark production in collisions at different impact parameters. We assume that the nuclear modifications arise from a density dependent effect such as gluon recombination and are thus proportional to the local density. We calculate the dependence of charm and bottom quark production on impact parameter and show that density dependent modifications can lead to significant reductions in the heavy quark production cross sections in central relative to peripheral interactions.[S0031-9007 (98)06967-1] PACS numbers: 25.75.Dw, 21.65. + f, 24.85. + p Experiments [1]have shown that the proton and neutron structure functions are modified by a nuclear environment. For momentum fractions x , 0.1 and 0.3 , x , 0.7, a depletion is observed in the nuclear parton distributions. The low x, or shadowing, region and the larger x, or EMC, region are bridged by an enhancement known as antishadowing for 0.1 , x , 0.3. Recently, the entire characteristic modification as a function of x has been referred to as shadowing. Many theoretical explanations of the nuclear effect have been proposed. In most, the degree of modification depends on the local nuclear density. For example, if the modification is due to gluon recombination [2], then the degree of modification should depend directly on the local gluon density and hence on the spatial position of the interaction within the nucleus. Nuclear binding and rescaling models also predict that the structure function depends on the local density [3]. Recently, first corrections to the Glauber calculation of shadowing corrections at very low x, based on shadowing in the nucleon, were shown to be large in the nucleus [4].Most measurements of structure functions have used lepton or neutrino beams. Typically these experiments are insensitive to the location of the interaction within the nucleus so that the resulting structure functions are averaged over the entire nucleus. The Fermilab E745 collaboration studied nN interactions with dark tracks in a bubble chamber where the dark tracks indicated that the interaction occurred deep within the nucleon. They found that structure function does vary spatially, but had little direct sensitivity to the impact parameter [5].This Letter discusses the effects of spatially dependent nuclear parton distributions on charm and bottom production cross sections as a function of impact parameter. Assuming that shadowing is proportional to the local nucleon density, we study the dependence of heavy quark production on the nucleon structure functions and the shadowing parametrization.We show that the heavy quark production cross section changes significantly when this spatial dependence is considered. The variation with impact parameter is especially important because relatively peripheral collisions are often used as a baseline in searches for new phenomena in more central collisions [6]. Large impact parameter collisions tend to probe the nuclear surfac...

The effect of gluon shadowing on charm quark production in large impact parameter ultrarelativistic heavy ion collisions is investigated. Charm quark production cross sections are calculated for a range of noncentral impact parameters which can be determined from the global transverse energy distribution. We show that charm quark production is a good probe of the local parton density which determines the effectiveness of shadowing. We discuss why shadowing may have a spatial dependence and show that this spatial dependence may be detected in noncentral heavy ion collisions.