The ratio R=FL/FT in DIS as a probe of the charm content of the proton

We analyze the Callan-Gross ratio R(x,Q2)=FL/FT in heavy-quark leptoproduction as a probe of the charm content of the proton. To estimate the charm-initiated contributions, we use the ACOT(χ) variable-flavor-number scheme. Our analysis shows that charm densities of the recent CTEQ sets of parton distributions have sizeable impact on the Callan-Gross ratio in a wide region of x and Q2. In particular, the ACOT(χ) predictions for the quantity R(x,Q2) are about half as large as the corresponding expectations of the photon-gluon fusion mechanism for x∼10−2-10−1 and Q2≫m2. This is because the structure functions FT(x,Q2) and FL(x,Q2) have different dependences on the mass logarithms of the type αsln(Q2/m2). On the other hand, our recent studies indicate that, contrary to the production cross sections, the Callan-Gross ratio is sufficiently stable under radiative corrections to the photon-gluon fusion component for x≳10−4. We conclude that the quantity R(x,Q2) in heavy-quark leptoproduction is perturbatively stable but sensitive to resummation of the mass logarithms. For this reason, in contrast to the structure functions, the ratio R(x,Q2)=FL/FT could be good probe of the charm density in the proton.