Parton Distributions In Nuclei Research Articles

On the universality of the x and A dependence of the EMC effect and its relation to parton distributions in nuclei

It is shown that the latest results from the NMC (CERN) and E665 (Fermilab) groups on F 2 a(x) F 2 d(x) obtained in the shadowing region bring new evidence of the universal A dependence of distortions made in a free-nucleon structure function by a nuclear medium. The observed universality implies that one can consider separately hard ( A ≤ 4) and soft ( A > 4) parton distribution distortions. Soft distortions, which result in differencies between the deep-inelastic scattering cross-sections for nuclei with masses A 1, A 2 ≥ 4, can be explained as a consequence of the nuclear density variation, independent of x in the range 0.001 ≤ x ≤ 0.7. It is found that nuclear shadowing begins at x 1 = 0.0615 ± 0.0024, independent of A, which is consistent with models that allow for three-parton recombination processes.

J/ psi suppression in hadron-nucleus collisions.

We examine the production of J/$\Psi$ mesons in high energy hadron-nucleus collisions using models that account for both the initial state modification of parton distributions in nuclei and the final state interaction of the produced $c\bar c$ pairs. We show that, at the energies of current fixed target experiments, J/$\Psi$ production through quark-antiquark annihilation gives the largest contribution at $x_F > 0.5$, while gluon-gluon fusion dominates the production at smaller $x_F$. The observed J/$\Psi$ suppression at large $x_F$ is directly connected to nuclear shadowing in deeply inelastic scattering. We find that a $6\sim 8$mb $c\bar c$-nucleon cross section is needed to explain the data on J/$\Psi$ suppression if a simple incoherent multiple scattering formalism is used for the final state interactions.

Anomalous nuclear enhancement in deeply inelastic scattering and photoproduction.

We derive the anomalous nuclear dependence of jet cross sections in deeply inelastic scattering and photoproduction, in terms of twist-four parton distributions in nuclei. This paper presents details of the use of factorization at higher twist to describe multiple scattering in nuclei, and the methods described here are applicable to a variety of other high-${\mathit{p}}_{\mathit{T}}$ cross sections.

Scale dependence of nuclear gluon structure

QCD-evolution of the parton distributions in nuclei is studied by using distributions at an initial scale Q 0 = 2 GeV which conserve baryon charge and momentum. Two different ansätze for shadowing of gluons are considered. The main conclusion in both cases is that shadowing of gluons vanishes much more rapidly with increasing scales Q than shadowing of quarks and antiquarks. We also find that antishadowing of gluons for x ⋧ 0.1 is likely to be limited to ⋦ 20%. As an application, we compute the average inclusive one-jet cross sections in pA and AA collisions at s = 200 and 6500 GeV for A = 196 .

Direct-photon production in heavy-ion collisions.

To study plasma photons in relativistic heavy-ion collisions one must account for direct photons from all possible sources. The aim of this work is the examination of the effect that initial-state parton distributions in nuclei, distorted to accommodate the European Muon Collaboration (EMC) effect, will have on the direct-photon spectrum produced relative to the basic QCD subprocesses in free nucleon-nucleon collisions. Since this parton distortion has been quite satisfactorily described by an expansion of the nuclear state in multiquark color-singlet (cluster) basis states, this «quark cluster model» should give useful predictions for the reflection of the EMC effect in the nucleus-nucleus direct photons