The Charm Quark Contribution to the Proton Structure Function

The charm quark structure function $F^c_2$ and the longitudinal structure function $F_l^p$ are directly sensitive to the gluon content of proton and therefore are crucial in understanding of proton structure function, in particular at low momentum transfer $Q^2$ and low Bjorken x. In the framework of perturbative QCD the charm structure function is calculated in the leading order (LO) and the proton structure function is investigated in the next leading order (NLO) at small x region. The valence quark distribution is obtained from the relativistic quark-exchange model. The calculated $F_2^c(x,Q^2)$, $F_2^p(x,Q^2)$ and $F_L^p(x,Q^2)$, are compared with the present available experimental data.