Violations of the Callan-Gross relation as function of X,Q2 from QCD

The Callan-Gross relation, originally based on the parton model, predicts that, in (say) electroproduction, sigmasub(L)/sigmasub(T) = 0. When QCD corrections are taken into account this relation gets modified by terms of order αsub(c)(Q2). We define R approximately sigmasub(L)/sigmasub(T) to measure the violation of the Callan-Gross relation. One can obtain exactly the behaviour of R(x,Q2) at the endpoints x = 0,1 from QCD. This will incidentally allow us to write a simple and explicit parametrization, which is exact at x = 0,1 and a good approximation at all other x. We will carry out the analysis for proton targets and e, μ projectiles; the extension to ν scattering or other targets is straightforward