On the perturbative stability of the QCD predictions for the ratio R=FL/FT in heavy-quark leptoproduction

We analyze the perturbative and parametric stability of the QCD predictions for the Callan-Gross ratio, R(x,Q2)=FL/FT, in heavy-quark leptoproduction. We consider the radiative corrections to the dominant photon-gluon fusion mechanism. In various kinematic regions, the following contributions are investigated: exact NLO results at low and moderate Q2>m2, and both NLO and NNLO soft-gluon (or threshold) corrections at large Bjorken variable x. Our analysis shows that large radiative corrections to the structure functions FT(x,Q2) and FL (x,Q2) cancel each other in their ratio R(x,Q2) with good accuracy. As a result, the NLO contributions to the Callan-Gross ratio are less than 10% in a wide region of the variables x and Q 2. We provide compact LO predictions for R(x,Q2) in the case of low x<<1. A simple formula connecting the high-energy behavior of the Callan-Gross ratio and low-x asymptotics of the gluon density is derived. It is shown that the obtained hadron-level predictions for R(x→0,Q2) are stable under the DGLAP evolution of the gluon distribution function. Our analytic results simplify the extraction of the structure functions F2c(x,Q2) and F2b(x,Q2) from measurements of the corresponding reduced cross sections, in particular at DESY HERA. (orig.)