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We analyze the perturbative and parametric stability of the QCD predictions for the Callan-Gross ratio R(x,Q2) = FL/FT and azimuthal cos(2φ) asymmetry in heavy-quark leptoproduction. Our analysis shows that large radiative corrections to the structure functions cancel each other in their ratio R(x,Q2) and azimuthal asymmetry with good accuracy. As a result, the NLO contributions to the Callan-Gross ratio and cos(2φ) asymmetry are less than 10% in a wide region of the variables x and Q2. We provide compact analytic predictions for R(x,Q2) and asymmetry in the case of low x≪ 1. Simple formulae connecting the high-energy behavior of the Callan-Gross ratio and azimuthal asymmetry with the low-x asymptotics of the gluon density in the target are derived. It is shown that the obtained hadron-level predictions for R(x,Q2) and azimuthal asymmetry are stable at x≪ 1 under the DGLAP evolution of the gluon distribution function.

Perturbative stability of the QCD predictions for the ratio R=F_L/F_T and azimuthal asymmetry in heavy-quark leptoproduction