Model calculation of nucleon structure functions

Deep inelastic polarized and unpolarized structure functions for a free nucleon are obtained in a modified center-of-mass bag model, which includes the symmetry-breaking effects from spin-dependent interactions. The quark distribution functions, calculated at $Q_0^2\simeq {\left(0.9\mathrm{} \frac{\mathrm{GeV}}{c}\right)}^2$, are evolved to a higher $Q^2$ region and compared with the data and other models. The model gives a reasonable description for the valence part of the structure functions at $x>\mathrm{}0.3\mathrm{}$. For the small-$x$ region, the contributions from the sea are necessary. The spin-dependent effects are important in describing the existing data.