The contribution of off-shell gluons to the structure functions F2 and $F_{\mathrm {L}}^c$ and the unintegrated gluon distributions

Abstract. We calculate the perturbative parts of the structure functions F2c and $F_{\mathrm {L}}^c$ for a gluon target having non-zero transverse momentum squared at order $\alpha_{\mathrm {s}}$. The results of the double convolution (with respect to the Bjorken variable $x_{\mathrm {B}}$ and the transverse momentum) of the perturbative part and the unintegrated gluon densities are compared with the HERA experimental data for F2c. The contribution from the $F_{\mathrm {L}}^c$ structure function ranges in 10-30% of that of F2c at the kinematical range of the HERA experiments.