Nucleon shadowing effects in $Cu+Cu$ and $Au+Au$ collisions at RHIC within the HIJING code

The centrality dependence of pseudorapidity density of charged particles ($d{N}_{ch}\mathrm{/}d\eta$) in $Cu+Cu$ ($Au+Au$) collisions at RHIC energy of $\sqrt{s_{{}_{\mathrm{N}\mathrm{N}}}}=22.4$ , $62.4$ and $200$ ($19.6$, $62.4$ and $200$) GeV, is investigated within an improved HIJING code. The standard HIJING model is enhanced by a prescription for collective nucleon-nucleon ($NN$) interactions and more modern parton distribution functions. The collective $NN$-interactions are used to induce both cascade and nucleon shadowing effects. We find collective cascade broadens the pseudorapidity distributions in the tails (at $\mathrm{\mid }\eta \mathrm{\mid }>y_{\mathrm{b}\mathrm{e}\mathrm{a}\mathrm{m}}$) above $25-30\mathrm{\%}$ collision centrality to be consistent with the $d{N}_{ch}\mathrm{/}d\eta$ data at $\sqrt{s_{{}_{\mathrm{N}\mathrm{N}}}}=19.6$, $22.4$, $62.4$ GeV. The overall contribution of nucleon shadowing is shown to depress the whole shape of $d{N}_{ch}\mathrm{/}d\eta$ in the primary interaction region (at $ |\eta|< y_{\rm beam} $)forsemiperipheral($ 20-25 $\mathrm{\%})andperipheral($\ge 35-40$\mathrm{\%})$Cu+Cu$($Au+Au$)interactionsat$ \sqrt{s_{_{\rm NN}}}=200$ GeV, in accordance with the PHOBOS data.