Index
Various shadowing aspects of nuclear parton distributions are discussed in a parton-model framework. We explain existing $x$ dependent data of $F_2^A/F_2^D$, then the model is applied to a flavor asymmetry $\bar u-\bar d$ in a nucleus, to shadowing in valence-quark distributions, and to nuclear dependence of $Q^2$ evolution. First, we find that a finite nuclear $\bar u-\bar d$ distribution could be possible due to nuclear interactions even in the flavor symmetric case in the nucleon ($\bar u -\bar d$=0). Second, it is indicated that valence-quark shadowing could be used for discriminating among various shadowing models. Third, we find that nuclear dependence of $Q^2$ evolution $\partial [F_2^{Sn}/F_2^C]/ \partial [\ln Q^2]$, which was found by NMC, is essentially understood by modification of parton distributions in nuclei. However, higher-twist effects in nuclear interactions could be tested by studying the details of the nuclear $Q^2$ evolution.
We give a brief overview of nuclear parton distributions. First, the EMC effect is discussed together with possible interpretations such as nuclear binding and $Q^2$ rescaling. Next, we explain shadowing descriptions: vector-meson-dominance-type and parton-recombination models. Nuclear dependence of $Q^2$ evolution should be interesting in testing whether or not DGLAP equations could be applied to nuclear structure functions. Status of nuclear sea-quark and gluon distributions is discussed. The structure function $b_1$, which will be measured at HERMES and possibly at ELFE, could shed light on a new aspect of high-energy spin physics.
In order to test the NMC finding of flavor asymmetry $\overline u - \overline d$ in the nucleon, existing Drell-Yan data for the tungsten target are often used. However, we have to be careful in comparing nuclear data with the nucleon ones. We investigate whether there exists significant nuclear modification of the $\overline u - \overline d$ distribution in a parton-recombination model. It should be noted that a finite $\overline u - \overline d$ distribution is theoretically possible in nuclei even if the sea is symmetric in the nucleon. In neutron-excess nuclei such as the tungsten, there exist more $d$-valence quarks than $u$-valence quarks, so that more $\overline d$-quarks are lost than $\overline u$-quarks due to parton recombinations. Our results suggest that the nuclear modification in the tungsten is a 2--10 \% effect on the NMC $\overline u - \overline d$ distribution. The nuclear modification of the flavor asymmetry should be an interesting topic in connection with ongoing Drell-Yan experiments.
Q**2 dependence of nuclear structure functions is important for understanding perturbative QCD in nuclear environment. However, current experimental data are not sufficient for studying nuclear dependence in the Q**2 evolution because the data are limited in the small Q**2 region at small x. The future HERA program can study the evolution in the large Q**2 region, and it provides us crucial information on recombination effects and on higher-order alpha_s effects in the nuclear structure functions.
Numerical solution of DGLAP $Q^2$ evolution equations is studied for polarized parton distributions by using a ``brute-force" method. NLO contributions to splitting functions are recently calculated, and they are included in our analysis. Numerical results in polarized parton distributions and in the structure function $g_1$ are shown. In particular, we discuss how numerical accuracy depends on number of steps in the variable $x$ and in $Q^2$.
We give a brief summary of the Gottfried sum rule and a flavor asymmetric distribution $\bar u-\bar d$ in the nucleon. First, experimental history of the sum-rule studies is discussed. Second, future prospects for studying the asymmetry at Fermilab and RHIC are discussed. We also comment on possible nuclear modification.
$Q^2$ evolution of structure functions in the nucleon and nuclei is investigated by using usual DGLAP equations and parton-recombination equations. Calculated results for proton's $F_2$ and for the ratio $F_2^{Ca}/F_2^D$ are compared with various experimental data. Furthermore, we study nuclear dependence of $Q^2$ evolution in tin and carbon nuclei: $\partial [F_2^{Sn}/F_2^C]/ \partial [\ln Q^2]\ne 0$, which was found by NMC.