Parton Distributions in Nuclei

We discuss two topics in this talk. One is a brief overview of nuclear parton distributions, and the other is SU(2)-flavor-symmetry breaking in nuclear antiquark distributions. First, we show that nuclear structure functions F 2 (x) could be explained in a Q rescaling model with parton recombination effects. Then, nuclear gluon distributions are discussed in this parton model. We point out other interesting distributions for future investigations. Second, ū− d̄ distributions in nuclei are discussed in order to shed light on nuclear modification of the ū− d̄ distribution. 1. Overview of nuclear parton distributions Modifications of the structure function F2(x) in nuclei were discovered by European Muon Collaboration (EMC effect). This effect has been an interesting topic because it may provide an explicit quark signature in nuclear phenomena. Most of these investigations discuss a “global” EMC effect in the sense that the effect is averaged over all constituents in a nucleus. However, it is interesting to investigate possible semiinclusive or semi-exclusive processes for finding a “local” EMC effect [1] and possible relations between a local gluonic EMC effect and the J/ψ suppression. In recent years, many experimental data of F 2 (x)/F D 2 (x) are obtained in the small x region, which is so called the shadowing region. There are theoretical attempts to explain the EMC effect in the medium-large x region and the shadowing in the small x. However, most models investigated both regions separately, and enough effort has not been made for studying the structure function in the whole x region. It is not straightforward to study a model which is valid in the wide x region. An attempt to combine the medium-x physics and the small-x one in a dynamically consistent way is made in Ref. [2]. Our model is based on a parton model and is not on macroscopic nuclear physics (nuclear binding, Fermi motion, vector-meson dominance, and so on) explicitly. We simply incorporated two mechanism in our model: Q rescaling and parton recombination. The rescaling has been used for explaining the EMC effect in the medium x region and the recombination for the shadowing in the small x. We calculated nuclear parton distributions by using these ideas at small Q and obtained distributions are evolved by using the Altarelli-Parisi equation. As a result, we obtained reasonably good agreement with the experimental