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, Q(2)) = F-L/F-T in heavy-quark leptoproduction as a probe of the charm content of the proton. To estimate the charm-initiated contributions, we use the ACOT(chi) 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(X) predictions for the quantity R(x, Q2) are about half as large as the corresponding expectations of the photon-gluon fusion mechanism for x similar to 10(-2)-10(-1) and Q(2) >> m(2). This is because the structure functions F-T(x, Q(2)) and F-L(x, Q(2)) have different dependences on the mass logarithms of the type alpha(s) ln(Q(2)/m(2)). 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 greater than or similar to 10(-4). We conclude that the quantity R(x, Q(2)) 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, Q(2)) = F-L/F-T could be good probe of the charm density in the proton. (C) 2009 Elsevier B.V. All rights reserved.